The syllabus & chapters

The manipulated variable is the one you deliberately change in an experiment. Fixed variables are all the others you keep constant so the test is fair, so that any change in the responding variable is due only to the manipulated variable.

From: Introduction to Biology and Laboratory Rules

A good hypothesis states the expected relationship between the manipulated and responding variable in a way that can be tested, for example, 'as temperature increases, the rate of enzyme reaction increases up to an optimum temperature'. It should name both variables and describe the direction of the expected change, not simply state that 'temperature affects the reaction'.

From: Communicating in Biology

You should know the main fields and one or two careers each, as short-answer questions sometimes ask you to match a field to a career or a career to what it studies.

From: Introduction to Biology and Laboratory Rules

A biological drawing needs a title stating what is shown and its magnification, clear unbroken outlines drawn with a sharp pencil and no shading or colouring, and label lines that are straight, do not cross, and end exactly on the structure named. Proportions between the parts should match the actual specimen, and every label is written horizontally rather than along the slanted label line.

From: Introduction to Biology and Laboratory Rules

Writing the unit once in the column heading, such as 'Time / s' or 'Temperature / °C', keeps every value in that column a plain number, which is the convention examiners expect and mark against. Repeating the unit after each number in the cells is unnecessary, though it is usually still accepted as long as the unit is correct and used consistently throughout the table.

From: Introduction to Biology and Laboratory Rules

Precision describes how close a set of repeated readings are to one another, regardless of whether they are correct, while accuracy describes how close a reading is to the true or accepted value. A thermometer that reads 2 °C too high every time gives precise but inaccurate readings, because the readings agree with each other but not with reality, usually due to a fault in the apparatus or how it was calibrated.

From: Introduction to Biology and Laboratory Rules

A conclusion restates the relationship found between the manipulated and responding variable in a full sentence, states clearly whether this supports the original hypothesis, and may briefly note a limitation such as a small sample size or a possible source of error. It should refer to the actual data collected rather than repeating the hypothesis word for word without comment.

From: Introduction to Biology and Laboratory Rules

Yes, in principle. A field investigation still needs a clear aim, a sampling method that avoids bias, and a results table, but because it takes place outdoors, factors such as weather, time of day or natural variation between organisms are harder to keep fixed. Candidates are often asked to identify one such uncontrolled factor and explain how it might affect the reliability of the results.

From: Introduction to Biology and Laboratory Rules

A plant cell has a rigid cellulose cell wall, chloroplasts for photosynthesis and one large permanent vacuole. An animal cell has none of these, its outer boundary is the plasma membrane, and any vacuoles are small and temporary. Despite these differences, both cell types carry out the same basic life processes using a nucleus, cytoplasm and mitochondria.

From: Cell Biology and Organisation

From simplest to most complex: cell, tissue, organ, system and organism. Similar cells form a tissue, tissues form an organ, organs form a system, and systems together form the whole organism. The blood, for example, is a tissue, the heart is an organ built from several tissues, and the circulatory system links the heart to every blood vessel in the body.

From: Cell Biology and Organisation

Specialised cells have shapes and structures suited to one job, which makes multicellular organisms efficient. For example, a nerve cell is long to carry impulses and a red blood cell is biconcave to carry more oxygen. Without specialisation, every cell would need to perform every function poorly, rather than one function very well.

From: Cell Biology and Organisation

A unicellular organism such as Amoeba or Paramecium has only one cell, so that single cell must carry out nutrition, respiration, excretion, growth, movement and response by itself. It absorbs food and oxygen directly from its surroundings, releases energy from respiration inside itself, and removes waste straight through its own cell surface, with no other cell to share the work.

From: Cell Biology and Organisation

The plasma membrane surrounds the cytoplasm of every cell and is selectively permeable, allowing some substances through while blocking others. It lets in useful molecules such as oxygen, water and glucose, and lets waste products such as carbon dioxide pass out, keeping the cell's internal conditions suitable for the reactions taking place inside it.

From: Cell Biology and Organisation

Osmosis is the net movement of water molecules across a partially permeable membrane, from a solution with more water (less concentrated) to one with less water (more concentrated). No energy is needed, so it is a form of passive transport.

From: Movement of Substances Across a Plasma Membrane

Diffusion moves particles down the concentration gradient, from high to low concentration, and needs no energy. Active transport moves substances against the gradient, from low to high concentration, and needs energy from respiration together with carrier proteins in the membrane. This is how cells absorb substances that are already more concentrated inside them.

From: Active transport

When the surrounding solution is more concentrated than the cell sap, water leaves the cells by osmosis. The cells lose turgor and become flaccid, and if it continues the cell membrane pulls away from the wall (plasmolysis), so the plant wilts.

From: Movement of Substances Across a Plasma Membrane

Water potential measures the tendency of water to move out of a solution; pure water has the highest water potential, and dissolving a solute in it lowers that value. Water always moves by osmosis from a region of higher water potential to a region of lower water potential across a partially permeable membrane, which is simply another way of saying it moves from a less concentrated solution to a more concentrated one.

From: Movement of Substances Across a Plasma Membrane

A channel protein forms a fixed, water-filled pore that lets specific ions or small polar molecules diffuse straight through the membrane down their concentration gradient. A carrier protein instead binds the particle, changes shape and releases it on the other side of the membrane; carrier proteins can work passively during facilitated diffusion or use energy from ATP to pump a substance against its concentration gradient during active transport.

From: Movement of Substances Across a Plasma Membrane

Diffusion is the general term for the net movement of any particle from a region of higher to lower concentration. Osmosis is essentially the same idea applied specifically to water molecules crossing a partially permeable membrane, from a region of higher water potential to a region of lower water potential, so every osmosis question can also be explained correctly using diffusion language.

From: Movement of Substances Across a Plasma Membrane

Benedict's test for reducing sugar (heated in a water bath, turns brick-red), iodine test for starch (turns blue-black at room temperature), Biuret test for protein (turns purple without heating), and the emulsion or Sudan III test for lipids (a milky-white layer or red colouration). You should know the reagent, method, whether heating is needed, and the positive result for each.

From: Chemical Composition in a Cell

Denaturation is a permanent change in a protein's three-dimensional shape, usually caused by high temperature or extreme pH breaking the bonds that hold the folded structure together. The protein loses its function, for example an enzyme can no longer bind its substrate, or an antibody can no longer recognise its antigen, but the sequence of amino acids in the chain is not broken apart.

From: Chemical Composition in a Cell

Carbohydrates are made of monosaccharides such as glucose, proteins are made of amino acids joined by peptide bonds, and lipids are made of one glycerol molecule joined to three fatty acid molecules. Nucleic acids are made of nucleotides, each built from a sugar, a phosphate group and a nitrogenous base.

From: Chemical Composition in a Cell

Condensation is a reaction that joins two smaller molecules (monomers) into a larger one (a polymer), releasing a water molecule at the new bond. Hydrolysis is the reverse reaction: a water molecule is added to break the bond, splitting the polymer back into its monomers. Digestion in the gut relies on hydrolysis to break down large food molecules into small units the body can absorb. This same pair of reactions explains why a food molecule that took years to build in a plant or animal can be broken down for energy or absorption within hours inside the digestive system.

From: Chemical Composition in a Cell

All three are polysaccharides built entirely from glucose monomers, but the type of bond linking the units and the way the chains coil or branch differ. Starch and glycogen coil compactly for efficient energy storage in plants and animals, while cellulose forms long straight chains that give plant cell walls their strength, which is also why humans can digest starch and glycogen but not cellulose. Recognising this pattern, same monomer, different bond and different shape, is a useful way to answer comparison questions across the whole biomolecules chapter, not only for polysaccharides.

From: Chemical Composition in a Cell

As temperature rises, activity increases up to an optimum. Beyond that, the heat changes the three-dimensional shape of the enzyme's active site, so the substrate no longer fits. The enzyme is denatured and the reaction rate falls sharply, usually permanently.

From: Metabolism and Enzymes

The lock-and-key model describes how an enzyme's active site has a specific shape, like a lock, that only a substrate with a complementary shape, like a key, can fit into. This is why each enzyme is specific and normally works on only one type of substrate.

From: Enzyme action

No. Enzymes are catalysts, so they are not used up or permanently changed by the reaction they speed up. A single enzyme molecule can catalyse the same reaction repeatedly, which is why cells need only small amounts.

From: Metabolism and Enzymes

At low substrate concentration, increasing it gives more collisions between substrate and the enzyme's active sites, so the rate rises steeply. Once every active site is occupied at a given instant, however, adding more substrate cannot increase the rate any further, since the enzyme itself has become the limiting factor. The graph therefore rises then levels off into a plateau.

From: Metabolism and Enzymes

Biological detergents contain enzymes such as protease and lipase, which break down protein-based stains, like blood or egg, and fat-based stains respectively into smaller, more soluble molecules that rinse away more easily. Because these enzymes work well at body temperature, the detergent is effective at lower washing temperatures than a non-biological one, which can also save energy.

From: Metabolism and Enzymes

Pepsin works in the stomach, where hydrochloric acid keeps conditions strongly acidic, so its active site is shaped to function best at a low pH. Trypsin works in the small intestine, where bicarbonate neutralises the acid from the stomach and conditions become mildly alkaline, so its active site instead functions best at a higher pH. Each enzyme's optimum pH reflects the environment it evolved to work in, not a universal rule for all enzymes.

From: Metabolism and Enzymes

An immobilised enzyme is fixed onto or trapped within a support material, such as alginate beads, instead of being free in solution. This allows the enzyme to be reused many times and separated easily from the product, which lowers cost and avoids contaminating the product with enzyme; immobilised lactase, for example, is used industrially to produce lactose-free milk.

From: Metabolism and Enzymes

Rate is calculated as the change in the amount of product formed or substrate used divided by the time taken for that change, giving units such as cm³ of gas per minute. On a graph of amount against time, this corresponds to the gradient of the curve; because the curve is usually steepest near the start, the initial rate is normally measured over the first section of the graph, before the substrate becomes limiting.

From: Metabolism and Enzymes

Anabolism refers to reactions that build larger, more complex molecules from smaller ones, such as joining amino acids into a protein during protein synthesis, and this process requires an input of energy. Catabolism refers to reactions that break large molecules down into smaller ones, such as respiration breaking down glucose, and this process releases energy that the cell can then use for anabolic reactions or other cellular work.

From: Metabolism and Enzymes

Mitosis produces two genetically identical diploid cells and is used for growth and repair. Meiosis produces four genetically different haploid cells (gametes) and halves the chromosome number, which keeps the species number constant at fertilisation and adds variation. Both processes begin from the same parent cell, but they are used for entirely different biological purposes.

From: Cell Division

If gametes were diploid, fertilisation would double the chromosome number every generation. Because meiosis makes haploid gametes, the diploid number is restored, not doubled, when the sperm and egg fuse, keeping it constant across generations. This restoration of the diploid number at fertilisation is the whole reason meiosis exists.

From: Cell Division

Cancer arises when the normal controls on the cell cycle fail and cells divide continuously without stopping. This uncontrolled mitosis forms a tumour, and a malignant tumour can spread to other tissues. Not every tumour is cancerous, only a malignant one that invades and spreads is classified as cancer.

From: Cell Division

Interphase is the longest part of the cell cycle, taking place before any visible division occurs. During this stage the cell grows, carries out its normal metabolic activities, and replicates its DNA so that each chromosome is copied into two identical sister chromatids, ready to be shared out equally when the nucleus divides.

From: Cell Division

Meiosis involves two successive divisions rather than one, which is why it produces four cells instead of the two made by mitosis. The first division separates homologous chromosome pairs, and because the two chromosomes in each pair carry slightly different versions of the same genes, the resulting cells are genetically different from one another.

From: Cell Division

Breathing (ventilation) is the physical movement of air in and out of the lungs. Respiration is a chemical process inside every cell that releases energy from glucose. Breathing supplies the oxygen that aerobic respiration needs and removes the carbon dioxide it produces.

From: Cellular Respiration

In humans, glucose is broken down to lactic acid with a small energy release. In yeast, glucose is broken down to ethanol and carbon dioxide, also with a small energy release. Both happen without oxygen, but the products are different.

From: Cellular Respiration

Aerobic respiration uses oxygen to break glucose down completely into carbon dioxide and water, releasing nearly all the chemical energy stored in the glucose molecule. Anaerobic respiration, without oxygen, can only partially break down glucose, leaving most of its chemical energy still locked inside the end product (such as ethanol or lactic acid), so far less usable energy is released.

From: Aerobic Respiration

During vigorous exercise, the muscles' demand for energy can exceed what the oxygen supply allows, so cells switch partly to anaerobic respiration and produce lactic acid. Lactic acid builds up in the muscle tissue, lowers the local pH, and interferes with normal muscle contraction, contributing to the fatigue, soreness and cramp felt during and shortly after hard exercise, until the lactic acid is broken down. Rest and light activity afterwards allow the extra oxygen taken in through faster breathing to break the lactic acid down, which is why the soreness eases over the following day or two rather than disappearing instantly.

From: Cellular Respiration

The first stage, glycolysis, takes place in the cytoplasm of every cell and does not need oxygen. If oxygen is available, the products then move into the mitochondria, where the aerobic stages take place on and within the folded inner membrane, releasing far more energy than glycolysis alone could provide. This is also why yeast and plant cells that photosynthesise still respire continuously in every living cell, using the mitochondria whenever oxygen is present, regardless of whether the organism is also carrying out another process such as photosynthesis at the same time.

From: Cellular Respiration

During inhalation, the external intercostal muscles contract to raise the ribcage outward and upward, while the diaphragm contracts and flattens from its usual dome shape. Together these actions increase the volume of the thoracic cavity, which lowers the air pressure inside it below atmospheric pressure, so air flows in from the higher-pressure atmosphere until the pressures equalise. This step-by-step understanding lets a student answer 'explain' questions on breathing fully rather than partially.

From: Respiratory System in Humans and Animals

The alveolus has a wall only one cell thick, giving a short diffusion distance, and millions of alveoli together give the lungs an enormous total surface area. Its lining is moist, so gases dissolve before crossing, and a dense network of capillaries constantly carries gases away, keeping the concentration gradient steep and diffusion fast.

From: Respiratory System in Humans and Animals

Fish exchange gases across gills instead of alveoli. Water flows over thin, blood-rich gill filaments, usually in the opposite direction to blood flow, so oxygen diffuses from the water into the blood and carbon dioxide diffuses out along the whole length of the gill. This countercurrent arrangement keeps the concentration gradient favourable for almost the entire gill surface.

From: Respiratory System in Humans and Animals

Breathing rate is controlled mainly by the carbon dioxide concentration in the blood. Chemoreceptors detect even small rises in carbon dioxide and signal the medulla oblongata in the brain, which sends nerve impulses to the diaphragm and intercostal muscles to increase both the rate and depth of breathing until the carbon dioxide concentration falls back to normal. This feedback system works without any conscious control, so a student only needs to understand the sequence of cause and effect.

From: Respiratory System in Humans and Animals

Chemicals in cigarette smoke paralyse and destroy the cilia that normally sweep mucus and trapped particles out of the airways, allowing infection and mucus build-up that cause chronic bronchitis. The same chemicals can break down alveolus walls, causing emphysema and reducing the surface area for gas exchange, and can also trigger the uncontrolled cell growth of lung cancer. This damage usually builds up gradually over years before symptoms become noticeable.

From: Respiratory System in Humans and Animals

Digestion is breaking large food molecules into small soluble ones using enzymes. Absorption is taking those small molecules into the blood, mainly through the villi of the small intestine. Assimilation is when body cells use the absorbed nutrients to build tissues and release energy.

From: Nutrition and Human Digestive System

Bile is made by the liver and stored in the gall bladder. It emulsifies fats, breaking large fat droplets into many small ones, which greatly increases the surface area for the enzyme lipase to work on. It also neutralises the acidic food from the stomach.

From: Nutrition and Human Digestive System

Each villus has a wall one cell thick for a short diffusion distance, a large surface area (helped by microvilli), a dense capillary network to carry away absorbed sugars and amino acids, and a lacteal to absorb fatty acids and glycerol. Millions of villi together make absorption very efficient.

From: Nutrition and Human Digestive System

Defaecation is the removal of undigested food material, mainly fibre, together with dead cells and bacteria, through the anus as faeces; this material never entered the bloodstream. Excretion is the removal of waste products such as urea, carbon dioxide and excess water that were actually made by the body's own cells during metabolism, mainly through the kidneys, lungs and skin.

From: Nutrition and Human Digestive System

Iodine solution is added to a food sample and turns blue-black if starch is present. Benedict's solution is added and heated; it forms a brick-red precipitate if a reducing sugar is present. Biuret solution is added at room temperature and turns purple if protein is present. Each test targets one nutrient, so a food can give a positive result in more than one test.

From: Nutrition and Human Digestive System

Peristalsis is the wave-like contraction and relaxation of the muscle layers in the wall of the alimentary canal, which squeezes food along from the oesophagus to the rectum. Because this muscular action pushes food forward regardless of body position, food still moves through the gut even when a person eats lying down, which is why gravity is not the main force behind digestion.

From: Nutrition and Human Digestive System

Vitamin C reduces the blue dye DCPIP to a colourless form. In the practical, fruit juice is added drop by drop from a syringe or burette to a fixed, small volume of DCPIP solution until the blue colour just disappears. A juice that needs a smaller volume to decolourise the dye contains a higher concentration of vitamin C, while a juice that needs a larger volume contains less.

From: Nutrition and Human Digestive System

Blood passes through the heart twice for each complete circuit of the body: once through the right side to the lungs (pulmonary circulation) and once through the left side to the rest of the body (systemic circulation). This keeps oxygenated and deoxygenated blood separate and maintains high pressure to the body.

From: Transport in Humans and Animals

Arteries carry blood away from the heart, usually at high pressure, and have thick, muscular, elastic walls. Veins return blood to the heart at low pressure, have thinner walls and a wider lumen, and contain valves to stop blood flowing backwards. Most arteries carry oxygenated blood and most veins deoxygenated blood, except the pulmonary vessels.

From: Transport in Humans and Animals

When a blood vessel is damaged, platelets gather and release chemicals. With clotting factors and calcium, these convert the soluble protein fibrinogen into insoluble fibrin threads, which form a mesh that traps blood cells and seals the wound, stopping bleeding and blocking entry of microorganisms.

From: Transport in Humans and Animals

Plasma is the liquid that carries dissolved nutrients, wastes, hormones and carbon dioxide around the body. Red blood cells contain haemoglobin and transport oxygen. White blood cells defend the body against pathogens by engulfing them or producing antibodies. Platelets are cell fragments that trigger clotting to seal a wound and stop bleeding.

From: Transport in Humans and Animals

Red blood cells carry A and/or B antigens, and plasma carries antibodies against whichever antigen a person lacks. If incompatible blood is transfused, the recipient's antibodies bind to the donor's antigens and make the red blood cells clump together, or agglutinate, which can block blood vessels and be fatal, so ABO and Rhesus groups are always checked first.

From: Transport in Humans and Animals

The lymphatic system collects excess fluid that leaks from blood capillaries into the tissues, called tissue fluid, and carries it through lymph vessels as lymph. Lymph passes through lymph nodes, where white blood cells filter out pathogens, before the cleaned fluid rejoins the bloodstream near the heart, helping both fluid balance and the body's defence.

From: Transport in Humans and Animals

Filariasis is a disease caused by thread-like parasitic worms that are transmitted between people through mosquito bites and that settle in the lymphatic system. The worms block lymph vessels, preventing lymph from draining properly, which causes severe swelling of the limbs known as lymphoedema or elephantiasis in advanced cases. Controlling mosquitoes helps prevent its spread.

From: Transport in Humans and Animals

Active immunity comes from a person's own B-lymphocytes producing antibodies after exposure to an antigen, either through infection or vaccination; it takes time to develop but leaves memory cells for long-lasting protection. Passive immunity comes from antibodies made by another organism and given or passed to a person, so protection is immediate; however, since the person's own lymphocytes are never stimulated, no memory cells form and the protection fades as the borrowed antibodies break down.

From: Types of Immunity: Active and Passive

A vaccine contains a weakened, dead or harmless part of a pathogen, called the antigen. When injected, it triggers lymphocytes to produce antibodies and memory cells without causing the disease. If the real pathogen infects later, the memory cells produce antibodies quickly, giving long-term protection. Booster doses are sometimes needed because the level of memory cells and antibodies from a single dose can fade over time, especially for some pathogens.

From: Immunity in Humans

The first line is a barrier, skin, mucus and stomach acid, that stops pathogens entering. The second line is non-specific: phagocytes engulf any pathogens that get in. The third line is specific: lymphocytes produce antibodies against particular antigens and form memory cells. Together, these three lines work as a layered system, so a pathogen that gets past one still has to overcome the next.

From: Immunity in Humans

In an autoimmune disease, the immune system loses its ability to tell the body's own cells apart from foreign antigens, so it produces antibodies or activates lymphocytes against healthy tissue. This misdirected attack damages the organ or tissue involved, for example the joints in some forms of arthritis. Autoimmune diseases are usually long-term and are managed rather than cured outright.

From: Immunity in Humans

HIV specifically infects and destroys the T-lymphocytes that would normally coordinate the immune response and help other cells produce antibodies. As the number of functioning T-lymphocytes falls, the body becomes unable to mount an effective defence, so infections that a healthy immune system clears easily can become serious or even fatal at the stage known as AIDS.

From: Immunity in Humans

A reflex arc is the pathway of a reflex action: a receptor detects a stimulus, a sensory neurone carries the impulse to the spinal cord, a relay neurone passes it to a motor neurone, and the motor neurone triggers an effector (muscle or gland) to respond. It is fast and automatic because it does not wait for the brain to decide.

From: Coordination and Response in Humans

Nervous coordination uses electrical impulses along neurones; it is very fast, precise and short-lasting. Hormonal coordination uses chemicals (hormones) carried in the blood; it is slower to start, more widespread and longer-lasting. The body uses both, nerves for quick responses and hormones for sustained changes such as growth.

From: Coordination and Response in Humans

A synapse is a small gap between two neurones. When an impulse reaches the end of the first neurone, it releases a chemical neurotransmitter that diffuses across the gap and triggers an impulse in the next neurone. This allows impulses to pass one way between neurones.

From: Coordination and Response in Humans

Diabetes mellitus is a condition in which blood glucose level rises above the normal range because the body produces too little insulin or its cells no longer respond normally to the insulin available. Since insulin is the hormone that normally signals cells to take up glucose from the blood, when it is missing or ineffective, glucose builds up in the blood instead of being absorbed and used by cells.

From: Coordination and Response in Humans

Although they work in different ways, the two systems are linked through the brain: part of the brain called the hypothalamus monitors conditions inside the body and controls the pituitary gland, which then releases hormones that influence other glands throughout the body. This means a nervous signal can trigger a hormonal response, allowing the two systems to coordinate together rather than working in isolation.

From: Coordination and Response in Humans

Homeostasis is the maintenance of a constant internal environment in the body, such as steady body temperature, blood glucose level, water content and pH, despite changes outside. It keeps conditions ideal for enzymes and cells to work, and is controlled by organs including the kidneys, liver, skin and the nervous and endocrine systems. Almost every homeostatic mechanism in the body follows the same negative feedback pattern of detecting, responding to and correcting a change.

From: Homeostasis and Human Urinary System

First, ultrafiltration: high pressure in the glomerulus forces water, glucose, salts and urea out of the blood into the Bowman's capsule, while large proteins and blood cells stay behind. Then selective reabsorption: as the filtrate flows along the tubule, useful substances such as all the glucose, some salts and much of the water are reabsorbed into the blood. What remains, mainly urea, excess salts and water, becomes urine. This urine then flows through the ureter to the bladder, where it is stored before being released.

From: Homeostasis and Human Urinary System

Glucose is small enough to be filtered out during ultrafiltration, but it is a useful nutrient, so it is completely reabsorbed back into the blood during selective reabsorption. Healthy urine therefore contains no glucose. If glucose appears in urine, it often means blood glucose is too high, as in diabetes.

From: Homeostasis and Human Urinary System

The hypothalamus in the brain monitors blood temperature and coordinates the response through the skin. When the body is too warm, blood vessels near the skin surface dilate so more heat is lost by radiation, and sweat glands produce more sweat, which cools the skin as it evaporates. When the body is too cold, these blood vessels constrict to reduce heat loss, and muscles may contract rapidly as shivering to generate extra heat. This whole mechanism works as a single negative feedback loop running continuously throughout the day.

From: Homeostasis and Human Urinary System

After a meal, blood glucose rises, so the pancreas releases insulin, which causes cells, especially in the liver and muscles, to take up glucose and store it as glycogen, lowering blood glucose back toward normal. Between meals, blood glucose falls, so the pancreas releases glucagon instead, which causes stored glycogen to be broken down into glucose and released into the blood. Together, these two hormones keep blood glucose within a narrow, stable range.

From: Homeostasis and Human Urinary System

Muscles can only pull, not push, so they work in antagonistic pairs. To bend the arm, the biceps contracts and pulls the forearm up while the triceps relaxes. To straighten the arm, the triceps contracts and pulls the forearm down while the biceps relaxes. Because one muscle undoes the action of the other, the joint can move both ways.

From: Support and Movement in Humans and Animals

A tendon joins a muscle to a bone and transmits the pulling force produced when the muscle contracts, allowing movement to occur at a joint. A ligament instead joins one bone to another across a joint, holding it together and limiting how far the joint can move so that it does not dislocate.

From: Types of Skeleton: Hydrostatic, Exoskeleton and Endoskeleton

A hydrostatic skeleton uses fluid pressure inside the body, as in an earthworm. An exoskeleton is a hard outer covering, as in insects and crabs, which also protects the body. An endoskeleton is an internal framework of bone or cartilage, as in humans and other vertebrates, which supports the body and provides attachment for muscles.

From: Support and Movement in Humans and Animals

Arthritis is inflammation of a joint, usually affecting the cartilage or the synovial membrane, and it causes pain, swelling and stiffness, often following wear, injury or an immune reaction. Osteoporosis is a different condition in which bone density falls faster than the body can rebuild it, commonly from low calcium, low vitamin D or ageing, leaving bones fragile and more likely to fracture.

From: Support and Movement in Humans and Animals

A fish has a flexible vertebral column and myotome muscle blocks running along both sides of its body, which contract in sequence to bend the body into waves that push against water. Its fins add steering, balance and braking, while a streamlined body shape reduces drag as the fish moves through the denser medium.

From: Support and Movement in Humans and Animals

A muscle produces force by contracting, which shortens the muscle and pulls its two attachment points, the tendons, closer together; it has no mechanism for lengthening actively to push a bone away. This is why movement at a joint always needs an antagonistic pair, one muscle to pull the bone one way and a second to pull it back.

From: Support and Movement in Humans and Animals

An insect's exoskeleton is a rigid outer casing that cannot stretch or expand, so it cannot grow together with the soft tissue inside it. To increase in size, the insect must periodically shed its old exoskeleton in a process called moulting, then rapidly grow and harden a larger new one before its body can enlarge further.

From: Support and Movement in Humans and Animals

In a roughly 28-day cycle, the uterus lining first thickens ready for a possible pregnancy. Around day 14, ovulation releases a mature egg from an ovary. If the egg is not fertilised, the thickened lining breaks down and is shed as menstruation, and the cycle begins again. Hormones from the ovaries and pituitary gland control these events. Being able to place ovulation, fertilisation and menstruation correctly on a single timeline is a frequently tested skill.

From: Sexual Reproduction, Development and Growth in Humans and Animals

Identical twins form when a single fertilised egg (zygote) splits into two, so both have the same genes and are the same sex. Non-identical (fraternal) twins form when two separate eggs are released and fertilised by two different sperm, so they are genetically as different as ordinary siblings and can be different sexes.

From: Sexual Reproduction, Development and Growth in Humans and Animals

Growth is a permanent and irreversible increase in the size and dry mass of an organism. In humans it follows a sigmoid (S-shaped) curve: slow at first, rapid during infancy and adolescence, then levelling off in adulthood. It results from cell division, cell enlargement and cell differentiation. A growth curve question often also asks for the phase in which growth rate is fastest, usually adolescence in humans.

From: Sexual Reproduction, Development and Growth in Humans and Animals

Follicle-stimulating hormone (FSH) stimulates a follicle in the ovary to mature and to secrete oestrogen. Rising oestrogen thickens the uterus lining and, once it reaches a high enough level, triggers a surge of luteinising hormone (LH) that causes the mature follicle to release its egg at ovulation. Progesterone, produced afterwards by the remains of the follicle, then maintains the thickened lining until pregnancy occurs or its level falls and menstruation begins. Placing these four hormones correctly on a single labelled graph is a skill worth practising separately from just memorising their names.

From: Sexual Reproduction, Development and Growth in Humans and Animals

Incomplete metamorphosis, seen in insects such as grasshoppers, has three stages, egg, nymph and adult, where the nymph already resembles a small adult and there is no resting pupal stage. Complete metamorphosis, seen in insects such as butterflies, has four stages, egg, larva, pupa and adult, where the larva looks completely different from the adult and undergoes a dramatic change inside the pupa before emerging.

From: Sexual Reproduction, Development and Growth in Humans and Animals

Meristematic tissue is made of small, thin-walled cells that keep dividing by mitosis to produce new cells, and it is found at the tips of roots and shoots and in the cambium. Permanent tissue is made of cells that have stopped dividing and become specialised for a job, such as the epidermis for protection or xylem for transport. Meristematic tissue makes the plant grow; permanent tissue carries out functions.

From: Organisation of Plant Tissues and Growth

Just behind the root tip there are three zones. In the zone of cell division, meristematic cells divide to make new cells. In the zone of cell elongation, the new cells grow longer, pushing the root through the soil. In the zone of cell differentiation, the cells become specialised, such as into root hair cells or xylem.

From: Organisation of Plant Tissues and Growth

Xylem transports water and dissolved mineral salts from the roots upward to the leaves. Phloem transports the sugars made in photosynthesis from the leaves to other parts of the plant that need energy or storage. The two tissues together form the plant's vascular system.

From: Organisation of Plant Tissues and Growth

Primary growth is an increase in length produced by apical meristems at the tips of roots and shoots. Secondary growth is an increase in girth (thickness) produced by the vascular cambium, a ring of meristematic tissue between the xylem and phloem in many dicotyledonous stems, which divides to add secondary xylem and secondary phloem over the growing season.

From: Organisation of Plant Tissues and Growth

Growth in the exponential phase cannot continue indefinitely because a limiting factor, often space, light, water or a mineral nutrient, eventually restricts further cell division and elongation. Once that limit is reached, the rate of new growth roughly equals the rate at which older growth is balanced by senescence, so the curve levels off into the stationary phase.

From: Organisation of Plant Tissues and Growth

When light comes from one direction, auxin made at the shoot tip moves to and accumulates on the shaded side. Because auxin promotes cell elongation, the cells on the shaded side elongate more than those on the lit side, and this unequal growth curves the shoot towards the light, a response called positive phototropism.

From: Organisation of Plant Tissues and Growth

Xylem vessels are dead at maturity, they are hollow, lignified tubes with no cytoplasm, which makes them efficient at carrying water under tension without collapsing. Phloem, in contrast, contains living sieve tube elements, though these lack a nucleus, and they are supported by companion cells that help actively load and unload the sugars phloem transports.

From: Organisation of Plant Tissues and Growth

The palisade mesophyll is a layer of tall, closely packed cells just under the upper epidermis. These cells contain the most chloroplasts and are positioned near the top of the leaf, where they receive the most light. This makes them the main site of photosynthesis, while the spongy mesophyll below mainly allows gas exchange.

From: Leaf Structure and Function

Stomata are the pores that let carbon dioxide in and oxygen and water vapour out. Placing most of them on the shaded lower surface reduces the rate of water loss by transpiration, because the lower surface is cooler and less exposed to direct sunlight than the upper surface.

From: Leaf Structure and Function

The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration in a plant. At this point the carbon dioxide released by respiration is exactly used up by photosynthesis, and the oxygen matches too, so there is no net exchange of gases with the surroundings.

From: Leaf Structure and Function

Each stoma is surrounded by a pair of guard cells with cell walls that are thicker on the side facing the pore than on the outer side. When water enters the guard cells by osmosis and they become turgid, this uneven thickening makes them bow outward away from each other, opening the pore. When the guard cells lose water and become flaccid, they straighten and the pore closes, reducing both gas exchange and water loss.

From: Leaf Structure and Function

Xylem consists of dead, hollow, lignified vessels that transport water and dissolved mineral ions upward from the root through the stem into the leaf. Phloem consists of living tissue that transports the sugars produced by photosynthesis away from the leaf to other parts of the plant that need them, such as growing shoots, roots or storage organs. Both tissues run together inside the vein, but move materials in largely opposite directions.

From: Leaf Structure and Function

Water enters the roots mainly to replace what is lost by transpiration and to keep cells turgid, but only a small proportion, typically under one percent, is actually incorporated into organic molecules during photosynthesis or growth. The stomata that allow carbon dioxide to diffuse in for photosynthesis unavoidably also allow water vapour to diffuse out, so transpiration is essentially an unavoidable side effect of gas exchange rather than a wasteful process the plant could easily avoid.

From: Leaf Structure and Function

A potometer measures the rate at which a cut shoot takes up water from a reservoir, usually by timing how far an air bubble moves along a capillary tube. Because almost all the water taken up by the shoot is eventually lost as water vapour through the stomata, the rate of water uptake is used as a close estimate of the rate of transpiration under the conditions being tested.

From: Leaf Structure and Function

The midrib is the continuation of the main vein through the centre of the leaf, and the petiole is the short stalk that attaches the leaf blade to the stem. Both contain xylem and phloem for transport, but they also provide mechanical support, holding the thin, flexible lamina rigid and angled so that it can intercept as much light as possible without collapsing or overlapping with neighbouring leaves.

From: Leaf Structure and Function

Water is absorbed into root hair cells by osmosis, moving from the more dilute soil solution into the more concentrated cell sap. Mineral salts are absorbed as ions by active transport, which uses energy from respiration because the ions often move from the more dilute soil into the more concentrated cell against the concentration gradient.

From: Nutrition in Plants

Photosynthesis only makes carbohydrates from carbon dioxide and water. To make other molecules the plant needs mineral ions from the soil, for example nitrogen to make proteins and magnesium to make chlorophyll. A shortage of any of these causes a specific deficiency symptom such as yellow leaves.

From: Nutrition in Plants

A root hair cell has a long, thin extension that greatly increases the surface area in contact with the soil, so more water and ions can be absorbed at once. Its thin wall gives a short distance for water to enter, and there are very many root hairs, which together make absorption efficient.

From: Nutrition in Plants

Legumes have root nodules containing Rhizobium bacteria that fix nitrogen gas from the air into compounds the plant can use. When the legume plant later dies and decomposes, or is ploughed back into the field, this fixed nitrogen enriches the soil, which is why legumes are grown in crop rotation to restore nitrogen levels before a nitrogen-demanding crop is planted.

From: Nutrition in Plants

A parasitic plant such as dodder or Rafflesia takes water, minerals and often food directly from a living host plant, usually harming it. An epiphyte such as many orchids grows on a tree purely for physical support and obtains its own water and minerals from rain, air and debris, without harming the host. An insectivorous plant such as the pitcher plant photosynthesises normally but traps and digests insects mainly to obtain extra nitrogen from poor soil.

From: Nutrition in Plants

Magnesium is a component of chlorophyll, so a lack of it specifically prevents new chlorophyll from forming while the existing green pigment in the veins is retained a little longer, producing a leaf that is yellow between the veins while the veins stay green. Nitrogen deficiency looks different: it turns the whole of an older leaf yellow, because nitrogen is more mobile within the plant and is withdrawn from older leaves first.

From: Nutrition in Plants

Iron cannot be moved easily from older to younger parts of the plant, so when it is scarce, the youngest leaves near the growing point are affected first and turn yellow. Magnesium, in contrast, is mobile and can be withdrawn from older leaves and relocated to younger ones, so a magnesium deficiency appears first in the older leaves further down the stem.

From: Nutrition in Plants

Water is lost from the leaves as vapour through the stomata by transpiration. This creates a pull, or tension, at the top of the xylem. Because water molecules stick together (cohesion) and to the xylem walls (adhesion), this pull draws a continuous column of water up the xylem from the roots, the transpiration stream.

From: Transport in Plants

Higher light intensity opens the stomata and speeds transpiration; higher temperature increases evaporation; moving air (wind) removes water vapour and speeds it up; higher humidity slows it down because the surrounding air already holds a lot of water vapour, reducing the gradient.

From: Transport in Plants

Root pressure is a positive pressure created when root cells actively pump mineral ions into the xylem, drawing water in by osmosis; it pushes sap only partway up the stem and is strongest at night. The transpiration pull, caused by evaporation from the leaves, is a much stronger tension that draws water the full height of a tall plant.

From: Transport in Plants

At the source, companion cells actively load sugar into the sieve tubes, lowering the water potential there so water follows by osmosis and raises the pressure. This high pressure pushes the sugary sap along the sieve tubes to a sink, where sugar is unloaded for use or storage, lowering the pressure there and maintaining the flow.

From: Transport in Plants

Each stoma is bordered by two guard cells with unevenly thickened walls. When they absorb water and become turgid, they curve apart and open the stoma, allowing water vapour to escape; when they lose water and become flaccid, they straighten and close the stoma, reducing water loss and slowing transpiration.

From: Transport in Plants

Phytoextraction uses plant roots to absorb pollutants such as heavy metals and store them in the shoots, which are later harvested to remove the pollutant from the site completely. Phytostabilisation instead uses plant roots to bind pollutants within the soil, reducing their movement into groundwater without removing them, so the pollutant remains but becomes less harmful.

From: Transport in Plants

When light shines on one side of a shoot, the hormone auxin produced at the tip moves to and accumulates on the shaded side rather than the lit side. Auxin makes cells elongate, so the cells on the shaded side grow longer than those on the lit side. This unequal growth causes the shoot to bend towards the light, a positive phototropic response that helps the plant capture more light for photosynthesis.

From: Responses in Plants

Phototropism is a plant's growth response to the direction of light: shoots grow towards light (positive) and roots grow away from it (negative). Geotropism is a growth response to gravity: roots grow downwards towards gravity (positive) and shoots grow upwards away from it (negative). Both responses are controlled by the same hormone, auxin, redistributed to different effect in shoots and roots.

From: Responses in Plants

Plant hormones are applied to promote root growth in cuttings before planting, to produce seedless fruits by triggering ovary growth without fertilisation, to control the timing of fruit ripening for transport and sale, and, at higher concentrations, as selective weedkillers that make certain broad-leaved weeds grow uncontrollably until they die. These uses all come from understanding how auxin controls plant growth. Because the effective dose is so small, commercial products specify exact concentrations to avoid damaging the crop being treated.

From: Responses in Plants

Dense, starch-filled organelles called statoliths inside cells of the root cap settle towards the lower side whenever a root is not growing straight down, and this settling is thought to trigger the root to redistribute auxin so that more accumulates on the lower side. Because auxin inhibits elongation in root cells at this concentration, the lower side grows more slowly and the root curves back downward.

From: Responses in Plants

A tropism is a directional growth response, so a shoot or root bends towards or away from a stimulus depending on which direction the stimulus comes from, and it develops over hours or days through unequal cell elongation. A nastic movement, such as the folding of Mimosa pudica leaves when touched, is non-directional, happens within seconds through a rapid change in turgor pressure, and gives the same response no matter which direction the stimulus comes from. Recognising which type a described response belongs to is often the first step in answering a longer structured question correctly.

From: Responses in Plants

Pollination is the transfer of pollen grains from the anther to the stigma, either within the same flower (self-pollination) or between different plants of the same species (cross-pollination). Fertilisation happens afterwards: a pollen tube grows down to the ovule and the male gamete fuses with the female gamete to form a zygote. Pollination brings the gametes close; fertilisation is their fusion.

From: Sexual Reproduction in Flowering Plants

After fertilisation, the fertilised ovule develops into a seed, which contains the embryo and a food store. The ovary wall develops into the fruit, which protects the seeds and often helps to disperse them. The other flower parts, such as the petals and stamens, usually wither and fall off. Recognising which structure has become which part after fertilisation is often tested directly by name in a labelling question.

From: Sexual Reproduction in Flowering Plants

Insect-pollinated flowers are usually large and colourful with scent and nectar to attract insects, and have sticky stigmas and pollen. Wind-pollinated flowers are usually small and dull with no scent, and have feathery stigmas and light, smooth pollen produced in large amounts, so the wind can carry it easily.

From: Sexual Reproduction in Flowering Plants

A seed needs water to soften its seed coat and activate the enzymes stored inside it, oxygen to allow aerobic respiration that releases the energy needed for the embryo to grow, and a suitable temperature at which these enzymes can work efficiently. Light is not usually required for germination itself, although the young seedling will need it soon afterwards to begin photosynthesis.

From: Sexual Reproduction in Flowering Plants

Self-pollination transfers pollen within the same flower or between flowers on the same plant, while cross-pollination transfers pollen between different plants of the same species, usually carried by wind or an animal. Cross-pollination happens because many flowers have features, such as the anther and stigma maturing at different times or being physically separated, that reduce self-pollination and favour receiving pollen from another plant, which increases genetic variation in the offspring. A flower's specific adaptations against self-pollination are a frequent 'explain how' style question on their own.

From: Sexual Reproduction in Flowering Plants

A xerophyte lives in a dry habitat and is adapted to reduce water loss. Common features include a thick, waxy cuticle, leaves reduced to spines to lower the surface area, sunken stomata that trap moist air, fewer stomata, hairs on the leaf surface, and thick stems or tissues that store water. Together these keep the plant alive where water is scarce. Being able to state the problem behind each feature, not just list them, is what earns full marks.

From: Adaptations of Plants in Different Habitats

A hydrophyte lives in or on water and does not risk drying out, so it has a thin cuticle and can have stomata on the upper surface of floating leaves. It often has large air spaces in its tissues to help it float and to store gases, and little supporting tissue because the water holds it up. These same features would be a serious disadvantage on dry land, which shows how closely each adaptation matches its own habitat.

From: Adaptations of Plants in Different Habitats

Broad leaves have a large surface area, which would lose a lot of water by transpiration in a hot, dry habitat. By reducing its leaves to spines, a cactus greatly lowers the surface area and therefore water loss, while its thick green stem carries out photosynthesis and stores water. This trade-off between surface area and water conservation is the core idea behind almost every xerophyte adaptation.

From: Adaptations of Plants in Different Habitats

A mesophyte is a plant adapted to an ordinary habitat with a moderate, reliable water supply, such as most garden and farm plants. Because it faces neither severe water shortage nor an excess of water, it shows a balanced set of features rather than the extreme adaptations of a xerophyte or hydrophyte, and exam questions usually use it only as a comparison point.

From: Adaptations of Plants in Different Habitats

Air spaces, called aerenchyma, run through the stems and leaves of a hydrophyte and serve two purposes at once. They make the tissue buoyant enough to keep leaves and flowers near the water surface for light and pollination, and they store oxygen that diffuses slowly to underwater parts, which cannot exchange gases directly with the surrounding water.

From: Adaptations of Plants in Different Habitats

A scientific name has two parts, the genus and the species. The genus name starts with a capital letter and the species name is all lower case, and the whole name is written in italics (or underlined when handwritten). For example, the human is Homo sapiens, with a capital H for the genus and a lower-case s for the species. Even a small formatting error, such as a misplaced capital letter, still counts as a mistake even if the organism itself is correctly identified.

From: Biodiversity

A dichotomous key is a tool used to identify an organism. It presents a series of steps, each offering two contrasting statements about a feature, such as 'has wings' or 'has no wings'. You choose the statement that matches your organism and follow it to the next pair, repeating until you reach the organism's name. A good key is designed so that anyone can follow it without needing prior detailed knowledge of the organism.

From: Biodiversity

Viruses sit on the boundary between living and non-living. They are not made of cells and cannot carry out life processes or reproduce on their own. However, once inside a host cell they can take over the cell's machinery to make copies of themselves, so they show one feature of living things, reproduction, only when inside a host. This debate over a virus's status shows that the definition of 'living' in biology is not always simple or clear-cut.

From: Biodiversity

Biodiversity is described at three levels. Genetic diversity is the variation between individuals within a single species, such as different fur colours in the same species of cat. Species diversity is the number of different species living together in one habitat. Ecosystem diversity is the range of different ecosystems, such as forests, rivers and coral reefs, found across a wider region.

From: Biodiversity

A bacterium is a single, complete cell with its own cytoplasm and genetic material, so it can carry out every life process, including reproduction, independently. A virus has no cytoplasm and cannot reproduce on its own; it can only make copies of itself after invading a host cell, which is why its status as a living thing is debated.

From: Biodiversity

At each level of a food chain, only some of the energy is passed on to the next level. Much of the energy is used for the organism's own life processes and lost as heat during respiration, and some is in parts that are not eaten. Because only about a tenth is passed on each time, the energy decreases sharply, which is why food chains rarely have more than four or five levels.

From: Ecosystem

A food chain shows a single pathway of energy from a producer through a chain of consumers, for example grass to grasshopper to frog to snake. A food web is a network of interconnected food chains, showing that most organisms eat more than one type of food and are eaten by more than one predator, which gives a more realistic picture of an ecosystem.

From: Ecosystem

Nutrients such as carbon and nitrogen are chemical elements that are passed between organisms and the environment and used again and again in cycles. Energy, however, enters an ecosystem from the Sun, flows one way through the organisms, and is gradually lost as heat, so it cannot be reused and must be constantly resupplied by sunlight.

From: Ecosystem

A habitat is the physical place where an organism lives, such as a coral reef or a tree canopy, and can usually be described in one or two words. A niche is broader: it describes the organism's role within the ecosystem, including what it eats, what preys on it, when it is active, and how it interacts with other species, so two organisms can share a habitat while occupying quite different niches. In practice, exam questions often give a short description of an organism and ask a student to state its habitat in a phrase and its niche in a full sentence covering diet and interactions.

From: Ecosystem

Carrying capacity is the maximum population size that an environment can sustainably support given its available food, space, water and other resources. A population can temporarily grow above the carrying capacity if conditions are briefly favourable, but limited resources then cause the death rate to rise and the birth rate to fall, bringing the population back down towards a size the environment can sustain. A population well below its carrying capacity, by contrast, tends to grow quickly because resources are not yet limiting, which is why the growth curve is steepest in its middle section.

From: Ecosystem

Preservation means protecting a natural area and keeping it untouched by human use, for example a strict nature reserve. Conservation means using and managing natural resources carefully and sustainably so they are not used up, for example controlled fishing or selective logging. Preservation avoids use; conservation allows wise use.

From: Environmental Sustainability

Biochemical oxygen demand is the amount of oxygen that microorganisms use to break down the organic matter in a sample of water over a set time. A high BOD means there is a lot of organic pollution, because the microorganisms use a lot of oxygen, which can leave too little for fish and other aquatic life. Cleaner water has a low BOD.

From: Environmental Sustainability

Greenhouse gases such as carbon dioxide and methane in the atmosphere trap some of the heat that would otherwise escape to space, keeping the Earth warm enough for life. When human activities release extra greenhouse gases, more heat is trapped, and the average global temperature rises. This is global warming, which changes climates and sea levels.

From: Environmental Sustainability

Ozone layer depletion is mainly caused by chemicals such as chlorofluorocarbons (CFCs), once widely used in aerosol sprays, refrigerators and air conditioners. When released into the atmosphere, these chemicals break down ozone molecules in the upper atmosphere, thinning the ozone layer that normally absorbs most harmful ultraviolet radiation from the sun. A thinner ozone layer allows more UV radiation to reach the Earth's surface, raising the risk of skin damage and harm to some crops and marine organisms. International agreements have phased out most industrial uses of CFCs, which has helped slow further thinning of the ozone layer.

From: Environmental Sustainability

Eutrophication is a process that begins when excess nutrients, often from fertiliser runoff or untreated sewage, enter a river or lake. These nutrients cause algae to grow rapidly, forming a dense bloom that can block sunlight from reaching plants underwater. When the algae eventually die, decomposers break them down and use up large amounts of dissolved oxygen in the process, which can leave too little oxygen for fish and other aquatic organisms, sometimes causing large numbers of them to die.

From: Environmental Sustainability

The genotype is the pair of alleles an organism has for a characteristic, written with letters such as TT, Tt or tt. The phenotype is the characteristic that is actually shown, such as being tall or short. Two organisms can have different genotypes (TT and Tt) but the same phenotype (both tall), because T is dominant.

From: Inheritance

Write the genotypes of the two parents, work out the gametes each can produce, and combine them in a Punnett square. The square shows all the possible offspring genotypes and their proportions. From these you read off the phenotype ratio (often 3:1) and the genotype ratio (often 1:2:1) for a cross between two heterozygotes.

From: Inheritance

Sex is determined by the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y (XY). All eggs carry an X, while sperm carry either an X or a Y. If an X-carrying sperm fertilises the egg the baby is female (XX); if a Y-carrying sperm does, the baby is male (XY). There is an equal chance of each.

From: Inheritance

A test cross crosses the individual whose genotype is unknown, but which shows the dominant phenotype, with a homozygous recessive individual. If the unknown parent is homozygous dominant, every offspring will show the dominant phenotype, because the recessive parent can only contribute a recessive allele. If the unknown parent is heterozygous, roughly half the offspring are expected to show the recessive phenotype, since half its gametes carry the recessive allele. The actual ratio of offspring phenotypes therefore reveals the unknown genotype.

From: Inheritance

Colour blindness is caused by a recessive allele carried on the X chromosome. A female has two X chromosomes, so a single recessive allele on one X is usually masked by a dominant allele on the other, meaning she needs two copies to be colour blind. A male has only one X chromosome, paired with a Y, so a single recessive allele on his only X chromosome is enough to produce colour blindness, with no second allele available to mask it. This is why the condition appears far more often in males than in females.

From: Inheritance

In incomplete dominance, neither allele completely dominates the other, so the heterozygote's phenotype blends the two, producing something intermediate, such as pink flowers from red and white parents. In codominance, by contrast, both alleles are fully and separately expressed in the heterozygote rather than blending, such as a person with blood group AB showing both the A and B antigens on their red blood cells at the same time. The key difference is blending versus both phenotypes appearing fully together.

From: Inheritance

Each carrier parent has one dominant and one recessive allele and does not show the condition themselves, but each can pass on either allele to a child. Using a Punnett square for a cross between two heterozygotes, one quarter of the possible offspring genotypes are homozygous recessive, meaning that on average one in four children of two carrier parents is expected to show the condition, although each pregnancy is an independent event with that same one-in-four chance.

From: Inheritance

No. Although the ABO gene has three possible alleles across the human population, I^A, I^B and i, any single person inherits only two of them, one from each parent. A person's genotype might be I^A I^A, I^A i, I^B I^B, I^B i, I^A I^B or ii, giving blood group A, B, AB or O, but never all three alleles together in one individual.

From: Inheritance

Continuous variation shows a smooth range of values with no distinct groups, such as human height or body mass, and is usually controlled by many genes together with the environment. Discontinuous variation shows distinct categories with nothing in between, such as blood group or tongue-rolling, and is usually controlled by one or a few genes with little environmental effect.

From: Variation

Variation comes from genetic and environmental sources. Genetic variation arises during meiosis, when alleles are shuffled, and at random fertilisation, when different gametes combine; mutation adds entirely new alleles. Environmental variation comes from factors such as diet, climate and lifestyle. Many characteristics result from both genes and the environment acting together.

From: Variation

A mutation is a change in the genetic material. A gene mutation is a change in the DNA sequence of a single gene, while a chromosomal mutation changes the structure or number of whole chromosomes. Mutations happen spontaneously or are caused by mutagens such as radiation and certain chemicals. If a mutation occurs in a gamete it can be passed to offspring and become a source of new variation.

From: Variation

Discontinuous examples include blood group, tongue-rolling ability and earlobe attachment, each falling into a small number of distinct categories. Continuous examples include height, body mass and skin colour, which form an unbroken range across a population. Recognising which category an example belongs to is the first step in answering most variation questions.

From: Variation

A gene mutation is a change within the DNA of a single gene, caused by substitution, insertion or deletion of a base, and it may or may not change the protein produced. A chromosomal mutation changes the structure of a whole chromosome, through deletion, duplication, inversion or translocation, or changes the total number of chromosomes, as seen in Down syndrome.

From: Variation

Identical twins develop from a single fertilised egg that splits into two embryos, so they share an identical genotype. Any differences that appear between them, such as slight variation in height, weight or appearance, are caused entirely by environmental factors like diet, lifestyle and prenatal conditions, not by genetic differences.

From: Variation

A random mutation can, by chance, make a bacterium resistant to an antibiotic even before that antibiotic is ever used. When the antibiotic is applied, non-resistant bacteria are killed while the resistant one survives and reproduces, passing the resistance allele to its offspring, so the population becomes dominated by resistant bacteria over time.

From: Variation

Genetic engineering is the deliberate changing of an organism's genes, usually by taking a gene from one organism and inserting it into another so that the second organism gains a new characteristic. For example, the human gene for insulin can be put into a bacterium, which then produces human insulin. It is done directly in the laboratory, unlike selective breeding. This lets scientists transfer a characteristic across very different species, something impossible to achieve through natural breeding.

From: Genetic Technology

The human gene that codes for insulin is cut out and inserted into a small circle of bacterial DNA called a plasmid, forming recombinant DNA. The plasmid is put back into a bacterium, which is then allowed to reproduce. As the bacteria multiply, they read the human gene and produce human insulin, which is collected and purified for use by people with diabetes.

From: Genetic Technology

Genetic technology brings clear benefits such as medicines and improved crops, but it also raises concerns. These include possible effects on human health, unknown long-term effects on the environment and other species, and ethical questions about how far it is right to change living things. A good answer weighs the benefits against these concerns rather than taking only one side, and names a specific example for each point instead of speaking only in general terms.

From: Genetic Technology

Gene therapy is a medical technique that treats a genetic disease by inserting a working copy of a gene into a patient's cells, aiming to replace or support the function of a faulty gene. It usually targets particular cells or tissues rather than the whole body, so most current gene therapy does not change every cell a patient has or get passed on to their children. It is still an emerging treatment for a limited number of conditions.

From: Genetic Technology

Every person, other than identical twins, has a unique pattern of repeated sequences at certain points in their DNA. DNA fingerprinting extracts DNA from a sample, cuts it at specific points and compares the resulting pattern with a reference sample. A close match between two patterns strongly suggests they came from the same person or from close biological relatives, which is why the technique is used in identification and paternity testing.

From: Genetic Technology

The branches most commonly tested are botany (study of plants), zoology (study of animals), microbiology (study of microorganisms), ecology (study of organisms and their environment), genetics (study of heredity), physiology (study of body functions), taxonomy (classification of living things) and biotechnology (application of living organisms to produce useful products). Each branch has a specific focus, so exam questions often test whether you can match the correct branch to a given scenario.

From: Fields and Careers in Biology

Biology supports agriculture through better crop varieties and pest control, supports public health through medicine and vaccines, and supports industry through biotechnology, food processing and pharmaceuticals. It also underpins conservation efforts that protect Malaysia's biodiversity, which is itself an economic and ecological resource.

From: Fields and Careers in Biology

Focus on the main subject of the work described, not the job title alone. A career involving plants points to botany, one involving disease-causing microorganisms points to microbiology, and one involving heredity or breeding points to genetics. If the scenario mentions an entire environment or community of organisms, it usually points to ecology rather than a single-organism branch.

From: Fields and Careers in Biology

Rinse the affected area immediately with plenty of running water for several minutes and inform the teacher at once. Continue to follow the teacher's instructions, which may include using the eyewash station if the chemical reached the eyes, or seeking first aid. Reporting quickly, even for a small splash, is part of correct laboratory conduct.

From: Safety and Rules in the Biology Laboratory

Labels identify the hazard symbol, concentration or species of the specimen, and any special handling instructions. Handling an unlabelled or unchecked substance risks exposure to a corrosive, toxic or biohazardous material, or contamination of a living specimen, so checking the label is a basic safety step before any laboratory task.

From: Safety and Rules in the Biology Laboratory

An unidentified culture could contain a pathogenic microorganism capable of causing infection or disease. Sterilising a culture, usually by autoclaving it at high temperature and pressure, kills the microorganisms present so that the waste can be disposed of safely, without risking infection to anyone who later handles it or comes into contact with the bench.

From: Safety and Rules in the Biology Laboratory

The manipulated variable is the one factor the investigator deliberately changes, for example the temperature of a water bath. The responding variable is what is measured as a result of that change, for example the rate of an enzyme reaction. Every other factor that could affect the result, such as pH or substrate concentration, must be kept constant.

From: Communicating in Biology

An investigation is a fair test when only the manipulated variable is changed and every other variable that could affect the result is kept constant. If a controlled variable is allowed to change as well, you cannot tell whether the manipulated variable or the other factor caused the change in the responding variable, so the conclusion would not be valid.

From: Communicating in Biology

A plant cell has a cellulose cell wall, chloroplasts for photosynthesis, and a large, permanent, central vacuole for storage and maintaining turgor pressure. An animal cell lacks a cell wall and chloroplasts, and any vacuoles it has are small and temporary rather than large and permanent.

From: Cell Structure and Function

A eukaryotic cell has a true nucleus bound by a nuclear membrane and contains membrane-bound organelles such as mitochondria. A prokaryotic cell, such as a bacterium, has no nuclear membrane, its DNA lies free in the cytoplasm, and it lacks most membrane-bound organelles.

From: Cell Structure and Function

A red blood cell shows how a cell's structure is matched to its function through cell differentiation. It has a biconcave shape that gives a large surface area for absorbing and releasing oxygen, and when mature it loses its nucleus, leaving more room inside for haemoglobin. Haemoglobin binds oxygen so the cell can carry it from the lungs to the tissues. Together these features make the red blood cell specialised for transporting oxygen efficiently.

From: Cell Structure and Function

Metabolic waste such as carbon dioxide and ammonia diffuses directly out through Amoeba's cell membrane. Because Amoeba lives in fresh water, water constantly enters the cell by osmosis; a contractile vacuole collects this excess water and periodically contracts to expel it, preventing the cell from bursting.

From: Living Processes in Unicellular Organisms

A single cell is very small, giving it a large surface-area-to-volume ratio. This means every part of the cytoplasm is close to the cell membrane, so oxygen entering and carbon dioxide leaving by diffusion can meet the cell's needs quickly without any specialised respiratory structure.

From: Living Processes in Unicellular Organisms

As a cell grows, its volume increases faster than its surface area, so its surface-area-to-volume ratio falls and diffusion alone eventually becomes too slow to supply the whole cytoplasm. Instead of growing indefinitely, most unicellular organisms reproduce by binary fission once they reach a certain size, splitting into two smaller daughter cells that each restore a large surface-area-to-volume ratio suited to diffusion.

From: Living Processes in Unicellular Organisms

The order runs from smallest to largest: cell, tissue, organ, organ system, and finally the whole organism. Similar cells form a tissue, different tissues combine to form an organ, related organs form an organ system, and organ systems working together make up the complete organism.

From: Living Processes in Multicellular Organisms

A large body cannot rely on diffusion across a single cell layer to meet every need, so cells specialise into different types, each structurally adapted for one job, such as absorption, transport or contraction. This division of labour, organised into tissues and organs, allows the organism to carry out life processes far more efficiently than if every cell tried to perform every function.

From: Living Processes in Multicellular Organisms

A cell becomes specialised through a process called cell differentiation, in which a relatively unspecialised cell switches on only the specific genes needed for one function and switches off the rest, gradually developing the particular structure that suits that job. This is why, although nearly every cell in the body contains the same complete set of genes, a nerve cell and a red blood cell end up with very different structures and functions.

From: Living Processes in Multicellular Organisms

Each phospholipid has a water-attracting head and water-repelling tails. In the watery environment inside and outside a cell, the heads face outward toward the water on both sides while the tails face each other, shielded from water. This arrangement is the most stable one available, so it forms automatically and gives the membrane its characteristic bilayer structure.

From: Structure of the Plasma Membrane

Partially (selectively) permeable means the membrane allows some substances to cross freely, such as small non-polar molecules like oxygen, while restricting others, such as ions and large polar molecules, unless they pass through a specific protein channel or carrier. This selectivity comes from the hydrophobic core of the phospholipid bilayer combined with the specific transport proteins embedded in it.

From: Structure of the Plasma Membrane

Diffusion moves particles down a concentration gradient, from high to low concentration, and needs no energy because it happens naturally. Active transport moves particles against a concentration gradient, from low to high concentration, and requires energy from ATP together with specific carrier proteins, because this movement does not happen on its own.

From: Concept of Movement of Substances Across a Plasma Membrane

Osmosis follows the same basic principle as diffusion, net movement from a region of higher concentration to lower concentration, but it applies specifically to water molecules moving across a partially permeable membrane, described in terms of water potential rather than solute concentration. Unlike general diffusion, osmosis always involves a membrane that lets water through but restricts larger dissolved particles.

From: Concept of Movement of Substances Across a Plasma Membrane

Diffusion is faster when the concentration gradient is steeper, when the surface area of the membrane is larger, when the temperature is higher (because particles move faster), and when the distance the particles must travel is shorter. This is why exchange surfaces such as the alveoli and villi are large in area and only one cell thick.

From: Concept of Movement of Substances Across a Plasma Membrane

Root hair cells absorb water by osmosis because the cell sap usually has a lower water potential than the surrounding soil water, so water moves in naturally. However, mineral ions in the soil are often at a lower concentration than inside the cell, so they cannot enter by diffusion; the cell must use active transport, spending ATP, to take up these ions against their concentration gradient.

From: Movement of Substances Across a Plasma Membrane in Living Organisms

The alveolus wall is only one cell thick, keeping the diffusion distance short, and it is surrounded by a dense network of capillaries, which maintains a steep concentration gradient by constantly carrying oxygenated blood away and bringing in blood low in oxygen. Its large total surface area and moist lining further speed up the rate of diffusion of oxygen and carbon dioxide.

From: Movement of Substances Across a Plasma Membrane in Living Organisms

Water molecules are polar, with a slightly negative oxygen end and slightly positive hydrogen ends. This polarity lets water molecules surround and separate ions and other polar molecules, dissolving them. Because biologically important substances such as glucose, mineral ions and amino acids are polar or charged, water can dissolve and transport most of what a cell needs.

From: Water as a Chemical Component of the Cell

Water has a high specific heat capacity, meaning it absorbs a large amount of heat energy with only a small rise in temperature, which helps keep the internal environment of cells and the body stable. Water also has a high latent heat of vaporisation, so when sweat or water vapour from transpiration evaporates, it removes a large amount of heat from the body or leaf surface, producing a cooling effect.

From: Water as a Chemical Component of the Cell

Cohesion is the attraction between water molecules themselves, caused by hydrogen bonding, which lets them stick together and form a continuous column, such as inside a xylem vessel. Adhesion is the attraction of water molecules to a different surface, such as the inner wall of a xylem vessel or a glass tube, which lets water cling to that surface. Both properties act together to keep an unbroken thread of water moving upward through the narrow vessels of the xylem during transpiration.

From: Water as a Chemical Component of the Cell

For reducing sugar, add Benedict's solution to the sample and heat it in a water bath; a brick-red precipitate confirms a reducing sugar is present, while the solution staying blue means it is absent. For starch, add a few drops of iodine solution directly to the sample at room temperature; a blue-black colour confirms starch is present, while an orange-brown colour means it is absent.

From: Carbohydrates

Starch and cellulose are both polysaccharides made of glucose units, but the units are joined by different types of glycosidic bond. Humans produce amylase, which hydrolyses the bonds in starch into maltose and then glucose, but they do not produce an enzyme that breaks the bonds in cellulose. Cellulose therefore passes through the gut undigested as dietary fibre, which adds bulk to the food and helps move it along by peristalsis.

From: Carbohydrates

A dipeptide forms when two amino acids join by a single peptide bond. A polypeptide is a longer chain formed when many amino acids join together in sequence. A protein is one or more polypeptide chains folded into a specific three-dimensional shape that allows it to carry out a biological function, such as an enzyme or a structural fibre.

From: Proteins

A sample is mixed with Biuret reagent, or with dilute sodium hydroxide solution followed by a few drops of dilute copper(II) sulfate solution, at room temperature without heating. A colour change to purple (violet) confirms that protein is present, because peptide bonds react with the copper(II) ions; if the mixture remains blue, no protein is present.

From: Proteins

Heat energy breaks the hydrogen and other bonds that hold a protein's secondary and tertiary structure in its specific folded shape. This is denaturation, and once the shape has changed permanently, structures such as an enzyme's active site no longer fit their substrate. The sequence of amino acids (primary structure) is unaffected, but the protein's function depends on its overall shape, not on the sequence alone, so the protein stops working.

From: Proteins

Almost every reaction in a metabolic pathway is controlled by a specific enzyme, which speeds up the reaction so it can occur fast enough to sustain life at body temperature. Without enzymes, most metabolic reactions would proceed too slowly to keep a cell alive, and metabolic pathways would not be able to respond quickly to the cell's changing needs.

From: Metabolism

Beyond the optimum temperature, the heat energy breaks the bonds holding the enzyme's three-dimensional shape together, changing the shape of the active site. This is denaturation, and because the substrate can no longer fit the altered active site, the enzyme loses its function and the reaction rate drops sharply rather than gradually.

From: Enzymes

In the lock-and-key model the active site is a rigid shape that exactly matches the substrate before binding. In the induced-fit model the active site is slightly flexible: it changes shape as the substrate enters so that it moulds around the substrate, which puts strain on the substrate's bonds and helps the reaction. The induced-fit model explains enzyme action more fully, but the lock-and-key model is the one the syllabus asks you to describe.

From: Enzyme action

Denaturation permanently changes the folded three-dimensional shape of the enzyme's active site, breaking the bonds that held that shape in place. Cooling the enzyme back down does not restore the original folding pattern, so the active site remains the wrong shape for the substrate. The enzyme's chemical composition still exists, but its catalytic function is permanently lost, which is why denaturation is described as irreversible at SPM level.

From: Enzymes

Enzymes such as protease and lipase break down protein and fat stains into smaller, water-soluble molecules that rinse away easily. Because these enzymes work well at lower washing temperatures than would otherwise be needed, biological washing powders can remove stains effectively while using less energy than washing at very high temperatures.

From: Application of Enzymes in Daily Life

An immobilised enzyme is fixed to a support material rather than mixed freely into the reaction, so it can be recovered and reused repeatedly instead of being discarded after a single use. It also stays separate from the final product, avoiding contamination and reducing the cost of purifying the product afterwards.

From: Application of Enzymes in Daily Life

Denaturation means the enzyme has permanently lost its normal three-dimensional shape, including the shape of its active site. High temperatures or an extreme pH break the bonds that hold the enzyme's structure together, so the active site no longer fits the substrate. Once denatured, the enzyme cannot catalyse its reaction again, even if conditions return to normal, which is why enzyme applications are carried out at a controlled, mild temperature.

From: Application of Enzymes in Daily Life

Enzymes speed up specific reactions at mild temperatures and normal pressures, so industrial processes can run using less heat and energy than conventional chemical methods, and with less risk of damaging the product. Because each enzyme acts on one type of substrate, it produces the wanted product with few unwanted side reactions. When the enzyme is immobilised on a support, it can also be recovered and reused, which lowers the cost further and makes the process more economical.

From: Application of Enzymes in Daily Life

Cell division allows a multicellular organism to grow by increasing its number of cells, to repair tissue by replacing damaged or dead cells, and to reproduce by forming new cells or gametes. Without cell division, an organism could not develop from a fertilised egg, heal an injury, or pass genetic material to offspring.

From: Cell Division

A diploid cell (2n) contains two complete sets of chromosomes, one inherited from each parent, and is the normal chromosome number in most body cells. A haploid cell (n) contains only one complete set of chromosomes, half the diploid number, and is found only in gametes such as sperm and egg cells, so that fertilisation restores the diploid number in the offspring.

From: Cell Division

Homologous chromosomes are a matching pair, one inherited from each parent, that carry genes for the same characteristics at the same positions, although they may carry different alleles of those genes. In meiosis, the homologous chromosomes first pair up and then separate into different daughter cells. This is why each gamete ends up with only one chromosome from every pair, giving the haploid number, and why offspring inherit a mixture of genes from both parents rather than an exact copy of either one.

From: Cell Division

In prophase, chromatin condenses into visible chromosomes made of two sister chromatids. In metaphase, chromosomes align individually at the equator of the cell. In anaphase, the centromere splits and sister chromatids are pulled to opposite poles. In telophase, the separated chromatids uncoil back into chromatin at each pole, and a new nuclear membrane forms around each set.

From: The Cell Cycle and Mitosis

In an animal cell, the flexible cell membrane pinches inward at the middle, forming a cleavage furrow that deepens until the cell splits into two. In a plant cell, the rigid cell wall cannot pinch inward, so a new cell plate forms across the centre of the cell and develops into a new cell wall, dividing the cell into two separate daughter cells.

From: The Cell Cycle and Mitosis

Meiosis produces four daughter cells from one original diploid (2n) cell. Meiosis I reduces the chromosome number, producing two haploid (n) cells, and meiosis II then separates the sister chromatids in each of those cells, producing a final total of four haploid (n) cells. Each of the four cells is genetically different from the others.

From: Meiosis

Crossing over happens during prophase I of meiosis, when paired homologous chromosomes exchange segments of genetic material at points of contact called chiasmata. This creates new combinations of alleles on each chromosome that were not present in either parent chromosome, which is one of the main reasons offspring produced by sexual reproduction show genetic variation.

From: Meiosis

Meiosis involves two successive divisions and produces four genetically different haploid cells from one diploid cell, and it occurs only in reproductive organs to form gametes. Mitosis involves a single division and produces two genetically identical diploid cells from one parent cell, and it occurs throughout the body for growth, repair and asexual reproduction, without crossing over or independent assortment.

From: Meiosis

The respiratory substrate is the molecule broken down during respiration to release energy. Glucose is the most common respiratory substrate because it is readily available from digested carbohydrates or stored glycogen and starch, and its chemical structure allows it to be broken down in a series of controlled, enzyme-catalysed steps that release energy gradually rather than all at once.

From: Energy Production through Cellular Respiration

ATP is used almost as soon as it is made to power immediate cell activities such as active transport, muscle contraction and biosynthesis, so cells keep only a small working supply rather than a large store. Because energy demand can rise and fall quickly, respiration must run continuously to regenerate ATP as fast as it is used.

From: Energy Production through Cellular Respiration

Breathing is the physical process of moving air into and out of the lungs, supplying oxygen and removing carbon dioxide. Respiration is the biochemical process, occurring inside cells, that uses the oxygen supplied by breathing to break down glucose and release energy as ATP. A cell can respire without breathing, but in humans, breathing supports respiration by keeping the blood supplied with oxygen.

From: Energy Production through Cellular Respiration

The balanced equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. One molecule of glucose reacts with six molecules of oxygen to produce six molecules of carbon dioxide, six molecules of water, and energy, most of which is captured and stored in ATP for the cell to use.

From: Aerobic Respiration

The mitochondrion is where the oxygen-using stages of aerobic respiration take place, and these stages release most of the ATP a cell needs. Its inner membrane is folded into ridges (cristae), which gives a large surface area for the reactions that release energy. Cells that need a lot of energy, such as muscle and liver cells, contain large numbers of mitochondria, which is why the organelle is often called the powerhouse of the cell.

From: Aerobic Respiration

No. Aerobic respiration takes place in the cells of almost all living organisms, including plants, animals and microorganisms, whenever oxygen is available. Plants respire aerobically day and night to release energy from glucose, even though they also carry out photosynthesis during the day. Respiration and photosynthesis are separate processes: photosynthesis stores energy in glucose, while aerobic respiration releases that energy for the cell to use.

From: Aerobic Respiration

During vigorous exercise, muscle cells need energy faster than the blood can deliver oxygen to them. When oxygen supply cannot meet demand, muscle cells switch to lactic acid fermentation, an anaerobic process that provides some ATP without oxygen, but produces lactic acid as a by-product, which builds up in the muscle and contributes to fatigue and cramp.

From: Fermentation

The word equation is: glucose → ethanol + carbon dioxide + energy. Yeast carries out alcoholic fermentation, and this process is applied in baking, where the carbon dioxide gas released makes bread dough rise, and in brewing, where the ethanol produced gives beverages their alcohol content.

From: Fermentation

Leaving dough to stand gives the yeast time to carry out alcoholic fermentation, producing carbon dioxide gas that becomes trapped as bubbles within the dough, causing it to expand. This step is carried out at a warm temperature, close to the yeast's optimum, so its enzymes work quickly and enough gas is produced within a reasonable time before the dough is baked.

From: Fermentation

Every respiratory surface, whatever animal it belongs to, is thin to shorten the diffusion distance, moist so that gases dissolve before diffusing, has a large surface area so more gas molecules can cross at once, and is well supplied with the gas being exchanged, either by blood vessels or by direct contact with air or water. These four features together maximise the rate of diffusion.

From: Types of Respiratory System in Animals

An insect's tracheal system carries air directly to its tissues through a branching network of air-filled tubes, so oxygen reaches cells without entering the blood. A fish's gills instead exchange gases between water and blood at thin gill filaments, and the circulatory system then carries the oxygen around the body. Insects therefore do not rely on blood to transport oxygen, but fish do.

From: Types of Respiratory System in Animals

During inhalation, the ribcage moves upward and outward while the diaphragm contracts and flattens, increasing the volume of the thoracic cavity. This larger volume lowers the air pressure inside the thorax below the pressure of the atmosphere outside. Air therefore flows from the higher-pressure atmosphere into the lower-pressure lungs until the pressures are equal.

From: Mechanisms of Breathing: Inhalation and Exhalation

Normal, quiet exhalation is passive, the muscles relax and the natural elastic recoil of the lungs and ribcage reduces thoracic volume without active effort. Forced exhalation, such as blowing hard, is active, the internal intercostal muscles and abdominal muscles contract to push the diaphragm higher and the ribcage lower, expelling more air more quickly.

From: Mechanisms of Breathing: Inhalation and Exhalation

The alveolus has walls only one cell thick, so the diffusion distance is very short; a moist lining, so gases dissolve before diffusing; a very large total surface area, from millions of alveoli, so more gas diffuses at once; and a dense capillary network, which keeps a steep concentration gradient by constantly renewing the blood supply. Together these four features make gas exchange fast and efficient.

From: Gaseous Exchange in Humans

Oxygen is at a higher concentration in the alveolar air than in the blood, so it diffuses down this gradient across the thin, moist wall of the alveolus and the wall of the surrounding capillary, both only one cell thick. It dissolves in the moisture lining the alveolus, enters the blood plasma, and then passes into a red blood cell, where it binds to haemoglobin to form oxyhaemoglobin.

From: Gaseous Exchange in Humans

Active muscles respire faster and release more carbon dioxide into the blood, which lowers the blood pH. Chemoreceptors in the medulla oblongata and in the aorta and carotid arteries detect this change and the respiratory centre sends more frequent impulses to the diaphragm and intercostal muscles. Breathing becomes faster and deeper, so more carbon dioxide is removed and more oxygen is supplied to meet the demand.

From: Inhalation and exhalation

Food travels through the mouth, oesophagus, stomach, small intestine, large intestine, rectum and finally the anus. Each of these organs is part of the continuous tube of the alimentary canal, and food moves through them in this order, pushed along by peristalsis.

From: The Human Digestive System

The alimentary canal is the tube that food actually passes through, from the mouth to the anus. A digestive gland, such as the salivary glands, liver or pancreas, is not part of this tube, it produces digestive juices or bile and releases them into the canal to help break down food.

From: The Human Digestive System

Physical digestion, such as chewing in the mouth and churning in the stomach, breaks food into smaller pieces and increases its surface area, without changing its chemical composition. Chemical digestion uses enzymes to break the chemical bonds in large food molecules, converting starch, protein and fat into smaller, soluble molecules that the body can absorb. The two work together: physical digestion prepares food so chemical digestion can act on it faster.

From: Digestion: Physical and Chemical Breakdown of Food

Bile is produced by the liver and stored in the gall bladder before being released into the small intestine. It is not an enzyme, so it does not chemically break down fat; instead it physically emulsifies fat into tiny droplets, greatly increasing the surface area for lipase to act on. This speeds up the chemical digestion of fat by lipase.

From: Digestion: Physical and Chemical Breakdown of Food

Each villus has a wall only one cell thick, keeping the diffusion distance short, and is one of millions covering the small intestine, together with microvilli on each cell, giving an enormous surface area. A dense capillary network carries away glucose, amino acids and other water-soluble nutrients quickly, keeping a steep concentration gradient, while a central lacteal absorbs the products of fat digestion.

From: Absorption and Assimilation of Digested Food

Absorption is the physical movement of digested food molecules from the small intestine into the blood or lymph, mainly across the villi. Assimilation happens afterwards, when cells take up these absorbed nutrients from the blood and use them to build new cell materials, such as proteins and enzymes, or to release energy through respiration.

From: Absorption and Assimilation of Digested Food

The small intestine is where digestion is completed, so food is broken down into small, soluble molecules that can be absorbed. Its wall is specially adapted with folds, villi and microvilli that give a huge surface area, and a one-cell-thick lining with a rich blood supply. The stomach lacks these adaptations, and by the time material reaches the large intestine most nutrients have already been absorbed, leaving mainly water and mineral salts to be taken up there.

From: Absorption and Assimilation of Digested Food

In an open circulatory system, such as in insects, the heart pumps blood into open body spaces where it bathes tissues directly at low pressure before returning to the heart. In a closed circulatory system, such as in fish and humans, blood stays inside blood vessels throughout its journey, allowing it to be pumped at higher pressure and delivered more quickly and directly to tissues.

From: Types of Circulatory System

In single circulation, blood loses pressure as it passes through the narrow gill capillaries, so it reaches the rest of the body relatively slowly and at low pressure. In double circulation, blood returns to the heart after the lungs and is pumped a second time before travelling to the body, so it arrives at a higher pressure and speed. This delivers oxygen to tissues faster and better supports a high, constant body temperature and active lifestyle.

From: Types of Circulatory System

The left ventricle pumps oxygenated blood at high pressure through the aorta and all the way around the body, so it needs a thick, powerful muscular wall to generate this force. The right ventricle only pumps deoxygenated blood the short distance to the lungs, so it needs less force and has a thinner wall.

From: The Circulatory System of Humans

An artery has a thick, muscular and elastic wall and a narrow lumen to withstand and maintain the high pressure of blood leaving the heart, and it usually has no valves. A vein has a thinner wall, a wider lumen, and valves along its length to prevent the low-pressure blood it carries from flowing backward as it returns to the heart.

From: The Circulatory System of Humans

Blood is made of plasma, red blood cells, white blood cells and platelets. Plasma is the liquid part that carries dissolved food, carbon dioxide, hormones and heat around the body; red blood cells use haemoglobin to carry oxygen; white blood cells defend the body against pathogens; and platelets help the blood to clot at a wound so blood is not lost and pathogens cannot enter.

From: The Circulatory System of Humans

The heart muscle is myogenic, meaning the sinoatrial node generates its own electrical impulses to trigger each heartbeat, without needing a signal from the nervous system. As long as the heart muscle has a supply of oxygen and nutrients, it will continue to beat rhythmically on its own, although the nervous system is still needed to adjust the rate up or down to match the body's changing needs.

From: Mechanism of the Heart Beat

Systole is the contraction phase of the cardiac cycle, when a heart chamber squeezes and pushes blood out, first in the atria and then in the ventricles. Diastole is the relaxation phase, when a chamber relaxes and fills passively with blood. The whole heart alternates between systole and diastole roughly 60 to 100 times a minute at rest, forming one continuous cardiac cycle.

From: Mechanism of the Heart Beat

The first two lines of defence, physical/chemical barriers and phagocytes with inflammation, are non-specific, meaning they respond to any pathogen in the same general way. The third line, using lymphocytes, is specific: B-lymphocytes recognise a particular antigen and produce matching antibodies against it, and this targeted response can also create memory cells for faster protection later.

From: Body Defence Against Pathogens

Once pathogens get past the broken skin, the damaged tissue triggers inflammation, nearby blood vessels widen and become more permeable, increasing blood flow to the area, which causes redness, heat, swelling and pain. This increased blood flow brings more phagocytes to the site, which engulf and digest the pathogens by phagocytosis; if the infection persists, lymphocytes are activated to mount a specific response.

From: Body Defence Against Pathogens

Each antibody has an antigen-binding site whose three-dimensional shape is complementary to one particular antigen, in the same way a lock fits only a matching key. Because of this precise shape match, an antibody can bind tightly to its own specific antigen but not to a different, unrelated one, which is why the immune system must produce a different antibody for every new pathogen it meets.

From: Actions of Antibodies

Agglutination happens because each antibody has two antigen-binding sites, allowing it to link separate pathogens together into large clumps. This immobilises the pathogens so they cannot spread further through the body, and makes it far easier for phagocytes to engulf and destroy clumped pathogens at once instead of chasing them individually.

From: Actions of Antibodies

An antigen is a marker molecule, usually on the surface of a pathogen, that the immune system recognises as foreign. An antibody is a Y-shaped protein made by B-lymphocytes in response to that antigen. The two fit together by complementary shape, like a lock and key: the antigen is what is recognised, and the antibody is the body's specific response that binds to it and helps disable the pathogen.

From: Actions of Antibodies

A vaccine triggers artificial active immunity: it stimulates the person's own B-lymphocytes to produce antibodies and, importantly, to form memory cells, so protection lasts for years. An anti-venom injection provides artificial passive immunity: it supplies ready-made antibodies from another source without stimulating the person's own lymphocytes, so no memory cells are formed and the antibodies are gradually broken down, leaving only short-term protection.

From: Types of Immunity: Active and Passive

The nervous system sends information as electrical impulses along neurones, which travel very fast to a precise target and produce a quick, short-lived response. The endocrine system sends information as hormones released into the blood by glands; hormones travel more slowly and reach a wider range of cells, but their effects usually last much longer.

From: Coordination and Response: Nervous and Endocrine Systems

The nervous system is suited to responses that must happen almost instantly and only in one specific location, such as reflexes. The endocrine system is suited to responses that need to affect many cells at once or last for a longer period, such as growth or the body's sustained reaction to stress. Having both systems lets the body respond appropriately to both sudden, localised events and slower, body-wide changes.

From: Coordination and Response: Nervous and Endocrine Systems

A hormone is carried in the blood to every part of the body, but it only affects cells that have the matching receptor for it. Cells without that receptor do not respond, so even though the hormone reaches them, only the target cells react. This is why one hormone can produce a specific effect despite being spread throughout the body.

From: Coordination and Response: Nervous and Endocrine Systems

The central nervous system (CNS) is made up of the brain and spinal cord, and is where sensory information is processed and responses are coordinated. The peripheral nervous system (PNS) consists of the nerves that connect the CNS to the rest of the body, carrying impulses from receptors to the CNS and from the CNS to effectors.

From: The Human Nervous System

In a reflex action, the impulse from the receptor travels only as far as a relay neurone in the spinal cord, which passes it straight to a motor neurone without waiting for the brain to consciously process the information. A voluntary response, by contrast, involves the impulse travelling all the way to the brain for conscious decision-making before a signal is sent back down to an effector, which takes more time.

From: The Human Nervous System

The medulla oblongata controls involuntary, automatic actions that keep the body alive from moment to moment, such as heartbeat, breathing rate and blood pressure, so serious damage to it can disrupt these life-sustaining functions directly. Damage to the cerebrum instead affects conscious thought, memory or voluntary movement, which can be severely disabling but does not by itself stop the automatic processes that keep the body functioning.

From: The Human Nervous System

A sensory neurone carries impulses from a receptor to the central nervous system. A relay neurone, found within the brain or spinal cord, connects a sensory neurone to a motor neurone. A motor neurone carries impulses from the central nervous system out to an effector, such as a muscle, to produce a response.

From: Neurones and the Synapse

When an impulse reaches the end of the first neurone's axon, it triggers the release of a neurotransmitter into the synaptic cleft, the narrow gap between the two neurones. The neurotransmitter diffuses across this gap and binds to receptors on the next neurone, triggering a new electrical impulse there. Because the neurotransmitter is released on only one side and received on the other, the impulse can only cross the synapse in one direction.

From: Neurones and the Synapse

The myelin sheath is a fatty insulating layer, formed by Schwann cells, that wraps around many axons and speeds up the transmission of a nerve impulse, partly because gaps in it, called nodes of Ranvier, let the impulse jump quickly from node to node. Without an intact myelin sheath, an impulse must travel more slowly along the full length of the axon membrane instead of jumping between nodes, which is why damage to myelin can noticeably slow down a person's reflexes and movements.

From: Neurones and the Synapse

Negative feedback is a control mechanism in which a change away from a normal set point triggers a response that opposes the change, bringing the variable back toward that set point. It is essential to homeostasis because it keeps conditions inside the body, such as temperature and blood glucose concentration, within the narrow range needed for enzymes and cells to function properly, despite constant changes in the internal and external environment.

From: Homeostasis

The pancreas detects the rise in blood glucose and releases the hormone insulin. Insulin causes body cells to take up more glucose from the blood and causes the liver to convert excess glucose into glycogen for storage. As a result, blood glucose concentration falls back toward its normal set point, and insulin release decreases once the level returns to normal.

From: Homeostasis

No. Homeostasis keeps a variable, such as body temperature or blood glucose concentration, fluctuating within a narrow range around a normal set point, rather than holding it at one exact, unchanging value. Negative feedback constantly detects small deviations above or below the set point and triggers a response to bring the variable back, so a graph of the variable over time typically shows it rising and falling slightly around the set point rather than staying perfectly flat.

From: Homeostasis

Ultrafiltration happens at the glomerulus and Bowman's capsule, where high blood pressure forces water, glucose, urea, and salts out of the blood into the nephron, while blood cells and large proteins are held back. Selective reabsorption happens further along the renal tubule, where useful substances such as all the glucose, most of the water, and some salts are absorbed back into the blood, leaving urea and excess water and salts to be excreted as urine.

From: The Human Urinary System

Protein molecules are too large to pass out of the glomerulus capillaries during ultrafiltration, so they are never present in the filtrate that enters the nephron in the first place. Glucose molecules are small enough to be filtered out at the glomerulus, but all of it is normally reabsorbed back into the blood during selective reabsorption in the renal tubule, so none should remain in the urine that leaves the body.

From: The Human Urinary System

The kidney controls how much water and mineral salts are kept in the body or removed as urine by adjusting the rate of reabsorption along the renal tubule to match the body's needs at that moment. This ongoing control, called osmoregulation, keeps the concentration of body fluids within a range suitable for normal cell function.

From: The Human Urinary System

Kidney stones form when substances in urine, such as calcium salts, become too concentrated and crystallise into hard deposits inside the kidney. Drinking enough water throughout the day keeps urine more dilute, which reduces the chance of these substances crystallising and lowers the risk of stones forming.

From: Health Issues Related to the Urinary System

Dialysis passes a patient's blood across a partially permeable membrane, either in a machine (haemodialysis) or using the lining of the abdomen (peritoneal dialysis), which allows urea and excess substances to diffuse out of the blood in the same way a healthy kidney would remove them. Because it is an external process rather than a cure, dialysis must be repeated regularly, usually several times a week, for as long as the kidneys remain unable to filter blood on their own.

From: Health Issues Related to the Urinary System

Urea is a waste product formed when the liver breaks down excess amino acids through deamination, and it is normally removed from the blood by ultrafiltration in the nephrons of a healthy kidney. When the kidneys fail, this filtering process no longer works properly, so urea, along with excess water and salts, is not removed as urine and instead accumulates in the blood, which is why blood urea levels are used to help diagnose and monitor kidney failure.

From: Health Issues Related to the Urinary System

An exoskeleton is a hard, rigid covering that, once formed, cannot expand or grow along with the animal's body. To become larger, the animal must periodically shed its old exoskeleton in a process called moulting and produce a new, larger one underneath, during which time its body is temporarily soft and more vulnerable to predators.

From: Types of Skeleton: Hydrostatic, Exoskeleton and Endoskeleton

A hydrostatic skeleton is a fluid-filled body cavity enclosed by a muscular body wall, found in soft-bodied animals such as earthworms. Because the fluid inside cannot be compressed, contracting muscles on one side of the body changes its shape rather than its volume; alternating contractions of circular and longitudinal muscles produce the waves of movement an earthworm uses to crawl and burrow.

From: Types of Skeleton: Hydrostatic, Exoskeleton and Endoskeleton

Bones support the body by forming a rigid internal framework, protect delicate organs such as the brain, heart and lungs, and enable movement by acting as levers that muscles pull against. Bone marrow inside some bones also produces blood cells, and bones store minerals such as calcium for use elsewhere in the body.

From: The Musculoskeletal System of Humans

A hinge joint, such as the elbow or knee, allows movement in only one plane, similar to a door swinging open and shut. A ball-and-socket joint, such as the shoulder or hip, has a rounded bone end fitting into a cup-shaped socket, allowing movement in many directions, including rotation.

From: The Musculoskeletal System of Humans

A muscle can only actively pull by contracting; it cannot push to move a bone back the other way. Because of this, a joint needs two muscles with opposite actions, an antagonistic pair, so that when one contracts to move the joint one way, the other can contract later to move it back, with each muscle relaxing while its partner contracts.

From: Movement and Locomotion

Movement is any change in the position of part of an organism, such as bending an arm or blinking, and does not necessarily involve the whole organism changing location. Locomotion is specifically the movement of an entire organism from one place to another, such as walking or swimming, and relies on the same skeleton-and-antagonistic-muscle mechanism as other movement, but coordinated across the whole body.

From: Movement and Locomotion

Fertilisation normally takes place in the oviduct (fallopian tube), where an egg released from the ovary meets sperm that has travelled up from the vagina through the uterus. The resulting zygote then moves down the oviduct to implant in the uterus lining.

From: The Reproductive System of Humans

The epididymis is a long, coiled tube attached to the testis where sperm are stored and complete their maturation after being produced in the seminiferous tubules. Mature sperm leave the epididymis through the vas deferens during ejaculation.

From: The Reproductive System of Humans

Oestrogen and progesterone, produced mainly by the ovaries, control the menstrual cycle. Oestrogen causes the uterine lining to thicken after menstruation and triggers ovulation, while progesterone maintains the thickened lining in preparation for a fertilised egg. If fertilisation does not happen, both hormone levels fall, the lining breaks down, and menstrual bleeding begins the next cycle.

From: The Reproductive System of Humans

Meiosis halves the chromosome number so that each gamete is haploid. When a haploid sperm fuses with a haploid egg at fertilisation, the resulting zygote has the normal diploid chromosome number for the species, keeping the chromosome count constant across generations.

From: Gametogenesis in Humans

During oogenesis, the cytoplasm divides unequally at each meiotic division. Almost all the cytoplasm and nutrient reserves go to one large cell, the egg, while the rest form small polar bodies that do not develop further. This gives the single egg enough resources to support early development after fertilisation.

From: Gametogenesis in Humans

Oogenesis in a human egg cell is not completed at ovulation. It pauses partway through meiosis, and only resumes and completes once a sperm cell penetrates the egg. At that point, the second polar body is released and the mature egg nucleus is ready to fuse with the sperm nucleus, completing fertilisation. If no sperm penetrates the egg, oogenesis remains incomplete, and the egg is eventually shed with the uterine lining during menstruation.

From: Gametogenesis in Humans

A sharp rise in luteinising hormone (LH), known as the LH surge, triggers the release of a mature egg from the ovary, which is ovulation. This surge typically happens around day 14 of a 28-day cycle.

From: The Menstrual Cycle

If the released egg is not fertilised, the corpus luteum in the ovary breaks down and stops producing progesterone. Without progesterone, the thickened uterus lining can no longer be maintained, so it breaks down and is shed as menstrual flow.

From: The Menstrual Cycle

Ovulation timing is controlled by a surge in luteinising hormone (LH), which is itself triggered once oestrogen from a maturing follicle reaches a high enough level. Because the follicular phase (before ovulation) is more variable in length than the luteal phase (after ovulation), the exact day of ovulation can shift from cycle to cycle even when the total cycle length stays close to 28 days.

From: The Menstrual Cycle

Simple permanent tissue, such as parenchyma, collenchyma or sclerenchyma, is made of only one type of cell. Complex permanent tissue, such as xylem or phloem, is made of more than one type of cell working together to carry out a function like transport.

From: Organisation of Plant Tissues

A root's main mechanical role is anchorage, resisting the pulling (tension) forces that act on it as the plant is tugged by wind or growth, and a central star of xylem resists these pulling forces most effectively. A stem instead needs to resist bending forces from the wind acting along its length, and a ring of vascular bundles positioned near the outer edge provides this bending resistance more efficiently than a solid central core would, much as a hollow rod resists bending better than a solid rod using the same amount of material.

From: Organisation of Plant Tissues

The vascular cambium produces more, larger xylem vessels during the wet season when growth is faster, and fewer, smaller xylem vessels during the dry season when growth slows. This difference in cell size creates visible rings in a cross-section of the trunk, and each ring generally represents one year of growth.

From: Meristematic Tissues and Growth

Growth starts slowly in the lag phase as the organism adjusts, speeds up during the exponential phase when conditions are favourable, then slows and levels off in the stationary phase as resources such as food, space or nutrients become limiting. Plotting size or cell number against time under these changing conditions produces the characteristic S-shape.

From: Growth Curves

An animal's growth curve usually flattens out completely once adult size is reached and stays roughly level for the rest of its life. A plant's growth is often better represented by repeated or ongoing sigmoid curves, since active meristems allow many plants to keep producing new growth throughout their lifetime rather than growth stopping permanently at one final size.

From: Growth Curves

The death phase occurs when conditions become so unfavourable that the rate of cells or individuals dying exceeds the rate of new ones being produced, causing the population or growth measurement to decline. Common causes include the complete exhaustion of nutrients, a severe build-up of toxic waste products, or a sudden change in conditions such as temperature or pH that the population can no longer tolerate.

From: Growth Curves

The palisade mesophyll lies just below the upper epidermis, where light intensity is highest, and its cells are tall and tightly packed with many chloroplasts. This position and structure allow the palisade layer to absorb the maximum amount of light for photosynthesis.

From: The Structure of a Leaf

The spongy mesophyll has loosely arranged cells with large air spaces between them, which allow carbon dioxide, oxygen and water vapour to diffuse easily to and from the mesophyll cells and the stomata. It also contains chloroplasts and carries out some photosynthesis, though less than the palisade layer.

From: The Structure of a Leaf

Guard cells are the only epidermal cells that contain chloroplasts, so in light they photosynthesise and actively take up ions, lowering their water potential; water then enters by osmosis, making them turgid. Because their inner wall is thicker than their outer wall, turgidity makes the pair of guard cells curve apart and open the stoma. In darkness, or when the plant is short of water, the guard cells lose water and become flaccid, closing the stoma and reducing water loss.

From: The Structure of a Leaf

In the day, photosynthesis occurs alongside respiration, and photosynthesis uses more carbon dioxide and produces more oxygen than respiration alone requires, so the net exchange is oxygen out and carbon dioxide in. At night, without light, photosynthesis stops and only respiration continues, so the net exchange reverses to oxygen in and carbon dioxide out.

From: The Leaf as the Main Organ for Gaseous Exchange

Each stoma is a pore flanked by a pair of guard cells. When guard cells take up water and become turgid, they curve apart and the stoma opens, allowing carbon dioxide, oxygen and water vapour to pass through. When guard cells lose water and become flaccid, the stoma closes, limiting gas and water vapour movement.

From: The Leaf as the Main Organ for Gaseous Exchange

Evaporation is the general physical change of water from liquid to vapour, which can happen from any wet surface. Transpiration is a specific biological process in which water evaporates from the moist cell surfaces inside a leaf and then diffuses out through the stomata, and it is linked to the plant's water transport system.

From: The Leaf as the Main Organ for Transpiration

A potometer measures the rate at which a cut shoot takes up water, shown by the movement of an air bubble along a capillary tube over time. Since most of the water taken up by the shoot is lost through transpiration, the rate of water uptake gives a close estimate of the rate of transpiration under different conditions.

From: The Leaf as the Main Organ for Transpiration

Magnesium is a central component of the chlorophyll molecule. Without enough magnesium, a plant cannot make sufficient chlorophyll, so the green colour fades and the leaves, especially the tissue between the veins, turn yellow, a condition called interveinal chlorosis.

From: Main Inorganic Nutrients for Plants

Nitrogen is a key component of amino acids, which make up proteins, and of nucleic acids such as DNA and RNA. Since proteins are needed for enzymes, cell structures and growth, a nitrogen shortage leads to stunted growth and yellowing of older leaves as the plant's limited nitrogen is redirected to new growth.

From: Main Inorganic Nutrients for Plants

Growing points and root tips contain rapidly dividing cells, so they are especially sensitive to nutrients needed for cell division, but calcium and phosphorus support this in different ways. Calcium is needed to form the middle lamella that cements adjacent cell walls together, so a shortage produces weak, deformed new cell walls at the growing point. Phosphorus is needed to make ATP and DNA, so a shortage limits the energy and genetic material needed to divide cells at all, which is why both deficiencies are seen at actively growing regions even though they act through different molecules.

From: Main Inorganic Nutrients for Plants

Diffusion only moves substances from a region of higher concentration to lower concentration. Since mineral ions are often already more concentrated inside the root hair cell than in the surrounding soil solution, moving more ions in would go against the concentration gradient, so the plant must use active transport, which requires energy, instead of diffusion.

From: Uptake of Water and Mineral Salts by Plants

Root hair cells rely heavily on active transport to absorb mineral ions against their concentration gradient. Active transport requires energy in the form of ATP, which is produced by respiration in mitochondria, so root hair cells contain many mitochondria to supply this energy demand.

From: Uptake of Water and Mineral Salts by Plants

Waterlogged soil traps little air between its particles, so root cells receive far less oxygen for aerobic respiration. Since active transport of mineral ions depends on ATP produced by respiration, this shortage of oxygen sharply reduces ATP production and therefore reduces the rate at which root hair cells can absorb mineral ions, even though the surrounding water itself is plentiful.

From: Uptake of Water and Mineral Salts by Plants

Insectivorous plants grow in soil that is very poor in nitrogen, such as peat swamps, so photosynthesis alone cannot supply enough nitrogen for making proteins and nucleic acids. Trapping and digesting insects gives them an additional source of nitrogen compounds that the soil cannot provide.

From: Diversity in Plant Nutrition

An epiphyte only uses another plant as physical support to grow higher and reach more light; it does not penetrate the host's tissue or draw water, minerals or food from it. It photosynthesises for its own food and absorbs water and minerals from rain, humid air and organic debris around its roots, so it does not harm the plant it grows on.

From: Diversity in Plant Nutrition

Xylem vessels and tracheids lose their cytoplasm and become hollow, lignified tubes at maturity. This hollow structure with no living contents inside is actually an advantage, since it offers little resistance to the flow of water and mineral salts moving upward through the plant.

From: Vascular Tissues (Xylem and Phloem)

Sieve tube elements lose their nucleus and most organelles as they mature, so they cannot carry out all their own metabolic activities. The companion cell, which retains a nucleus and many mitochondria, supplies the energy and control needed to load and move sugars through the connected sieve tube.

From: Vascular Tissues (Xylem and Phloem)

Xylem forms a continuous system of dead tubes that runs only from the roots up to the leaves, so transpiration pulls water and dissolved minerals upward in one fixed direction. Phloem instead connects a 'source', such as a photosynthesising leaf, to a 'sink', such as a growing fruit or a storage root, wherever these happen to be on the plant at a given time, so the direction of sugar transport can change depending on which parts are producing food and which are using or storing it.

From: Vascular Tissues (Xylem and Phloem)

Transpiration pull is the suction created as water evaporates from the leaf and diffuses out through the stomata, which draws more water upward through the xylem to replace it. Because water molecules form a continuous, cohesive column inside the narrow xylem vessels, this pull is transmitted all the way down to the roots, making it the main force that moves water up a plant, especially a tall tree.

From: Transport of Water and Mineral Salts in Plants

Root pressure, generated by active transport of mineral ions into the xylem, can only push water up a limited height, typically no more than a few metres. Since many trees are much taller than this, transpiration pull, which is generated at the top of the plant and works through cohesion of the water column, is needed to explain how water reaches the highest leaves.

From: Transport of Water and Mineral Salts in Plants

Mineral ions absorbed by the root hair cells are actively transported into the xylem, and once inside, they simply travel dissolved in the same stream of water that transpiration pull draws upward through the stem. They do not need a separate upward transport force of their own once they are inside the xylem vessels.

From: Transport of Water and Mineral Salts in Plants

A source is a part of the plant that supplies more food than it uses, such as a mature photosynthesising leaf, while a sink is a part that uses or stores food, such as a growing fruit or root. Sugars are translocated through the phloem from source regions to sink regions.

From: Translocation

Loading sugar into the phloem at the source lowers the water potential there, drawing water in from the xylem and raising the pressure. Removing sugar at the sink raises the water potential there, so pressure is lower. This pressure difference pushes the sugar solution through the connected sieve tubes from the high-pressure source end to the low-pressure sink end.

From: Translocation

Yes. Some organs switch role during their lifetime: a storage root or tuber acts as a sink while it fills up with sugar, but later becomes a source when its stored sugar is broken down and exported to support new growth, such as sprouting buds. Whether an organ is a source or a sink at any moment depends on whether it is a net supplier or a net user of food.

From: Translocation

In a tropism, the direction of the plant's response depends on the direction from which the stimulus comes, and the response is a slow, usually permanent growth movement. In a nastic movement, the direction of response does not depend on the stimulus direction, and the movement is usually a rapid, often reversible change caused by turgor pressure changes rather than growth.

From: Types of Responses in Plants

No. Although touch triggers the response, the leaves always fold in the same way regardless of which direction the touch comes from, so the direction of movement does not depend on the stimulus direction. This makes it a nastic movement, not a tropism, and it is caused by a rapid loss of turgor pressure in special cells rather than by growth.

From: Types of Responses in Plants

Yes. The opening and closing of stomata is triggered by light and by the plant's water status, and it counts as a nastic movement rather than a tropism because the guard cells respond to the presence of the stimulus, not to the direction it comes from. This shows that the type of stimulus alone does not decide whether a response is a tropism or a nastic movement, the direction-dependence of the movement is what decides it.

From: Types of Responses in Plants

Auxin, produced mainly at the shoot tip, promotes the elongation of cells just behind the tip, causing the shoot to grow longer. Because it can accumulate unevenly, such as on the shaded side of a shoot exposed to light from one direction, it also causes bending responses like phototropism.

From: Phytohormones

Abscisic acid inhibits growth and maintains seed dormancy, keeping a seed inactive until conditions such as water and temperature are suitable for germination. Gibberellin promotes germination by triggering the production of enzymes that break down stored food in the seed. The balance between these two hormones helps determine whether a seed stays dormant or germinates.

From: Phytohormones

Because ethylene is a gas at normal temperatures, it can diffuse away from the fruit that produced it and spread through the surrounding air, affecting nearby fruit as well as the fruit that released it. This is why ripening fruit placed close together, or kept in an enclosed space, tends to ripen faster than fruit left in an open, well-ventilated area, since the gas accumulates rather than dispersing.

From: Phytohormones

A synthetic form of auxin is sprayed at a much higher concentration than a plant would naturally produce. Broad-leaved weeds absorb it readily and respond with excessive, uncontrolled growth that disrupts their normal development and kills them, while narrow-leaved crop plants such as cereals are far less affected, allowing the weedkiller to target weeds selectively.

From: Application of Phytohormones in Agriculture

Normally, fertilisation triggers the hormones that cause a fruit to develop around the seeds. Applying gibberellin directly to an unpollinated flower can trigger fruit development without fertilisation taking place, so the fruit grows without seeds forming inside it.

From: Application of Phytohormones in Agriculture

A synthetic auxin has a similar effect to the plant's own auxin but is not broken down as easily inside the plant, so it stays active for longer and can build up to a damaging level. Sprayed at a high concentration, it causes broad-leaved weeds to grow in an uncontrolled way that disrupts their normal development and kills them. Narrow-leaved crops such as cereals absorb much less of it and are far less affected, so the weedkiller can remove weeds selectively from a crop.

From: Application of Phytohormones in Agriculture

In tissue culture, a small piece of plant tissue is grown on a nutrient medium containing cytokinin, which promotes cell division and the formation of new shoots. Because it starts from a small, carefully selected and sterilised sample, the plantlets can be kept free of the diseases carried by the parent plant. The method also produces a very large number of identical plantlets quickly and in a small space, which is why growers use it to raise disease-free planting material on a commercial scale.

From: Application of Phytohormones in Agriculture

A stamen is the male reproductive part of a flower, consisting of an anther that produces pollen grains on a supporting filament. A carpel is the female reproductive part, made up of a stigma, style and ovary containing ovules, where fertilisation and seed development take place.

From: The Structure of a Flower

A wind-pollinated flower typically has small, dull or absent petals since it does not need to attract insects, no nectar, anthers that hang loosely outside the flower to release pollen freely, and a large, feathery stigma exposed outside the flower to catch airborne pollen grains.

From: The Structure of a Flower

Each fertilised ovule develops into a seed, which contains the embryo of the new plant along with a food store. The ovary wall surrounding the ovules develops into a fruit, which protects the developing seeds and, in many species, later helps disperse them away from the parent plant.

From: The Structure of a Flower

Meiosis halves the chromosome number, producing haploid cells from a diploid parent cell. This is essential because pollen grains and the embryo sac must ultimately provide haploid gametes; when a haploid male gamete fuses with a haploid egg cell at fertilisation, the resulting zygote has the correct diploid chromosome number.

From: Development of Pollen Grains and the Embryo Sac

A mature embryo sac typically contains one egg cell, flanked by two synergid cells that help guide the pollen tube, three antipodal cells at the opposite end, and one large central cell containing two polar nuclei, which later fuses with a male gamete to form the endosperm.

From: Development of Pollen Grains and the Embryo Sac

Double fertilisation is the fusion of one male gamete with the egg cell to form the zygote, and the simultaneous fusion of the second male gamete with the two polar nuclei to form the endosperm nucleus. It is important because it produces both the embryo and, at the same time, a ready food supply (the endosperm) to nourish that embryo as the seed develops.

From: Pollination and Fertilisation in Flowering Plants

In cross-pollination, the pollen comes from a genetically different plant of the same species, so the offspring inherit a new combination of alleles from two distinct parent plants, increasing genetic variation. In self-pollination, the pollen and egg cell come from the same plant (or from genetically identical flowers on it), so the offspring's genetic makeup stays closer to that single parent, producing more reliable but less varied offspring.

From: Pollination and Fertilisation in Flowering Plants

A xerophyte is a plant adapted to survive in a dry habitat with limited water supply, such as a desert. Its adaptations, including a thick waxy cuticle, sunken stomata, reduced leaf surface area, and extensive roots, all work to reduce water loss and maximise water uptake.

From: Adaptations of Plants in Different Habitats

The upper surface of a floating leaf is exposed to air, so it is the surface where gas exchange with the atmosphere can occur. The lower surface is in contact with water, so stomata there would be of no use and could even let water into the air spaces of the leaf.

From: Adaptations of Plants in Different Habitats

A mesophyte is a plant that grows where the water supply is moderate and fairly reliable, which includes most ordinary land plants and crops. Unlike a xerophyte, it does not need extreme water-saving features such as a very thick cuticle or sunken stomata, and unlike a hydrophyte it does not need air spaces for buoyancy or a reduced root system. Instead it shows a balance of features: a moderate cuticle, stomata mainly on the lower leaf surface, a normal leaf area, and roots adequate for its habitat.

From: Adaptations of Plants in Different Habitats

Binomial nomenclature is the scientific system of naming a species using two Latin-based words: the genus name followed by the species name. It is used so that every organism has one standard, internationally recognised name, avoiding the confusion caused by different common names in different languages.

From: System of Classification and Naming of Organisms

Species is the basic unit because it is defined by the ability of its members to interbreed naturally and produce fertile offspring. All the higher levels, from genus up to kingdom, are formed by grouping species together based on shared characteristics.

From: System of Classification and Naming of Organisms

Viruses are not placed into any of the five kingdoms because they are not considered fully living organisms: they have no cellular structure, cannot carry out life processes such as respiration or growth on their own, and can only reproduce by taking over the machinery of a host cell. For this reason, viruses are studied separately from the classification hierarchy used for cellular organisms.

From: System of Classification and Naming of Organisms

A habitat with many species has more feeding relationships and alternative food sources built into its food web. If one species is lost or declines, other species can often fill its role, so the ecosystem is less likely to collapse than one with very few species.

From: Biodiversity

Genetic diversity is the variation of alleles within a single species, which is why individuals of one species are not identical. Species diversity is the variety of different species living together in a habitat. Genetic diversity lets a species adapt to change, while species diversity helps a whole community stay stable.

From: Biodiversity

Viruses show some characteristics of living things, such as having genetic material and being able to reproduce, but only inside a host cell. Because they cannot carry out life processes like feeding, respiration or growth on their own, they are usually placed on the boundary between living and non-living things rather than classified as true organisms.

From: Microorganisms and Viruses

A bacterium is a complete cell with cytoplasm, a cell membrane and genetic material, and it can carry out life processes and reproduce on its own. A virus is not a cell at all, it is genetic material in a protein coat that can only reproduce by infecting and hijacking a host cell.

From: Microorganisms and Viruses

Decomposer bacteria and fungi break down the bodies of dead organisms and organic waste into simpler substances, releasing nutrients such as nitrogen and mineral ions back into the soil. Without decomposers, nutrients would remain locked up in dead matter and would not be available for producers such as plants to absorb again, so this role is essential for nutrient cycling in an ecosystem.

From: Microorganisms and Viruses

In mutualism, both organisms in the relationship benefit, as when bees collect nectar from flowers while pollinating them. In commensalism, only one organism benefits while the other is unaffected, as when a plant such as an orchid grows on a tree trunk purely for physical support without harming the tree.

From: Community and Ecosystem

Energy is lost at every trophic level through respiration, movement, body heat and undigested waste, so only a small part of the energy in one level passes to the next. After four or five links there is too little energy left to support a further level of consumers, which is why food chains are short and top predators are few in number.

From: Community and Ecosystem

Ecologists use the mark-release-recapture method: catch and mark a sample of animals, release them, then take a second sample later and count how many are marked. The estimated total population size equals the number marked in the first catch multiplied by the total caught in the second catch, divided by the number of marked individuals found in the second catch.

From: Population Ecology

Carrying capacity is the maximum population size that a habitat can sustainably support, based on the available resources such as food, water, and space. As a population approaches its carrying capacity, its growth rate slows because these resources become limiting.

From: Population Ecology

The method assumes that marked individuals mix randomly with the rest of the population before the second sample is taken, that marking does not affect an animal's survival or behaviour, and that the population is effectively closed during the study, meaning births, deaths, immigration and emigration between the two catches are negligible. If any of these assumptions is broken, the estimate becomes unreliable.

From: Population Ecology

The greenhouse effect is caused by gases in the atmosphere, mainly carbon dioxide and methane, trapping heat that radiates from the Earth's surface. This is a natural process that keeps the Earth warm enough for life, but human activities like burning fossil fuels increase greenhouse gas levels, intensifying the effect and causing global warming.

From: Threats to the Environment

Deforestation destroys the habitat that many species of plants and animals depend on for food, shelter, and breeding sites, forcing them to migrate, decline in number, or become extinct if no suitable habitat remains. It also removes trees that absorb carbon dioxide, contributing further to the greenhouse effect.

From: Threats to the Environment

Pollution introduces harmful substances into the air, water, or land that poison organisms or disrupt ecosystem processes directly, whereas overexploitation removes a resource, such as fish stocks or timber, faster than natural processes can replace it. Both threats reduce biodiversity, but pollution acts mainly through toxicity and habitat disruption, while overexploitation acts mainly through resource depletion and scarcity.

From: Threats to the Environment

A reforestation programme that replants native tree species on land previously cleared by logging is an example of restoration. It actively rebuilds a damaged ecosystem back towards its original, functioning state, rather than simply protecting it as it currently is or managing its continued use.

From: Preservation, Conservation and Restoration of Ecosystems

Once excess nutrients have already triggered an algal bloom, depleted the water's dissolved oxygen and killed fish, the ecosystem has already been damaged, so using the water more carefully from that point on would only slow further harm, not undo it. Restoration actively repairs the damage already done, for example by cutting the nutrient input further, removing the excess algae and allowing oxygen levels and species to recover.

From: Preservation, Conservation and Restoration of Ecosystems

Sustainable development means developing to meet the needs of people today in a way that does not reduce the resources or environmental quality available to future generations. It requires balancing economic growth, social needs, and environmental protection together, rather than treating them as separate concerns.

From: Environmental Sustainability

An Environmental Impact Assessment is a formal study carried out before a major development project begins, to predict its likely effects on the surrounding environment and to identify ways to reduce or manage any harm. It is a key tool used to support sustainable development decisions.

From: Environmental Sustainability

The syllabus most often tests the 3R (reduce, reuse, recycle), the use of renewable energy such as solar, wind, and hydroelectric power, sustainable agriculture practices such as crop rotation and selective logging, and conservation approaches such as gazetting forest reserves. A good answer names one specific practice rather than listing three vague ones, and links it to the environmental benefit it produces.

From: Environmental Sustainability

A Tt x Tt cross produces offspring genotypes in the ratio 1 TT : 2 Tt : 1 tt. Because the dominant allele T is expressed whenever it is present, both TT and Tt individuals show the dominant phenotype, giving a combined 3 parts dominant to 1 part recessive, the classic 3:1 ratio.

From: Monohybrid Inheritance

A homozygous recessive individual, such as tt, can only produce gametes carrying the recessive allele, so it cannot mask the alleles coming from the other parent. This means the phenotypes of the offspring directly reveal the unknown parent's gametes: a single offspring phenotype indicates a homozygous dominant parent, while a mix of phenotypes indicates a heterozygous parent. Crossing against any other genotype could hide this information.

From: Monohybrid Inheritance

The 9:3:3:1 ratio results from the law of independent assortment, which states that if two genes are on different chromosomes, their alleles are distributed into gametes independently of each other. This means all four possible combinations of alleles occur in gametes with equal likelihood.

From: Dihybrid Inheritance

A double heterozygote, such as TtYy, produces four different gamete types in equal proportion, because each of the two heterozygous gene pairs can contribute either allele, and the two genes assort independently. These four types combine to give the sixteen boxes of a dihybrid Punnett square.

From: Dihybrid Inheritance

A gene is a segment of DNA that codes for a particular characteristic, occupying a fixed position on a chromosome. An allele is one specific version of that gene, different alleles of the same gene produce different forms of the characteristic, such as tall or short.

From: Genes and Alleles

Alleles are located at the same locus, or fixed position, on each of the two chromosomes in a homologous pair. One allele comes from the mother's chromosome and the other from the father's chromosome, which is why an organism normally carries two alleles for every gene.

From: Genes and Alleles

An individual organism normally carries only two alleles for a gene, one on each chromosome of a homologous pair. However, across a whole population, more than two different alleles of the same gene can exist, this is called multiple alleles. The ABO blood group gene in humans is a well-known example, with three different alleles (A, B and O) existing in the population, even though any one person carries only two of them. This concept of multiple alleles distinguishes blood group inheritance from simpler examples, such as height in pea plants, which involve only two alleles.

From: Genes and Alleles

No. Environmental factors, such as nutrition or sunlight exposure, can change how a characteristic is expressed in the phenotype, for example affecting a person's final height or body mass, but they do not alter the underlying genotype an individual inherited from their parents.

From: Types and Factors of Variation

Only to a very limited extent. Because a discontinuous characteristic such as blood group is controlled by one or a small number of genes with a fixed set of possible alleles, environmental factors have almost no effect on which category an individual falls into. This differs from a continuous characteristic, where environmental factors can shift an individual's value considerably within the range that their genes allow.

From: Types and Factors of Variation

Siblings differ because meiosis shuffles the parents' chromosomes and alleles differently each time a gamete is formed, through independent assortment and crossing over, and because fertilisation combines a random sperm with a random egg. Each sibling therefore inherits a different combination of alleles from the same two parents.

From: Variations in Humans

ABO blood group is a clear example of discontinuous variation in humans. A person's blood group falls into one of four distinct categories, A, B, AB, or O, with no intermediate forms, and it is controlled by a single gene with little environmental influence.

From: Variations in Humans

Yes. When measured by a test score, intelligence shows continuous variation, since scores spread smoothly from low to high rather than falling into a small number of fixed categories. Unlike a purely single-gene trait, a person's score is also shaped by non-inherited factors such as nutrition, upbringing and access to education, which further broadens the range of values seen across a population.

From: Variations in Humans

Yes. Two genetically identical individuals, such as identical twins, can still differ in traits such as body mass or skin tone if they are exposed to different diets, amounts of sunlight, or lifestyles. This kind of variation is not passed on to the next generation, because unlike variation caused by mutation, it does not involve any change to the DNA itself.

From: Variations in Humans

A gene mutation is a small change in the DNA base sequence within a single gene, caused by substitution, insertion, or deletion of bases. A chromosomal mutation is a larger-scale change affecting the structure or number of whole chromosomes, which typically affects many genes at once.

From: Mutation

Down syndrome is caused by a chromosomal mutation in which an individual has an extra copy of chromosome 21, a condition called trisomy 21. This usually results from an error during meiosis in which chromosome 21 fails to separate properly into gametes.

From: Mutation

Yes. Although many mutations are harmful or have no noticeable effect, a mutation occasionally produces a characteristic that happens to help an organism survive or reproduce in its environment, in which case it is beneficial. Because mutation is the only process that creates entirely new alleles, beneficial mutations are the ultimate raw material for the genetic variation that natural selection can act on within a population.

From: Mutation

Restriction enzymes are used to cut DNA at specific recognition sequences. In genetic engineering, the same restriction enzyme is used to cut out the desired gene from its source DNA and to open up the vector, so that the two pieces have matching cut ends and can be joined together.

From: Genetic Engineering

The human gene for insulin is isolated and inserted into a bacterial plasmid using restriction enzymes and DNA ligase. The recombinant plasmid is then inserted into bacteria, which use their normal cell machinery to read the human gene and produce human insulin, which is later extracted and purified for medical use.

From: Genetic Engineering

Genetic engineering transfers a specific gene from one organism into another, giving the recipient a new characteristic, such as a bacterium gaining the ability to produce human insulin. Cloning instead produces a genetically identical copy of an existing organism or DNA sequence, without necessarily introducing any new gene, so the two techniques serve different purposes even though both are tools of biotechnology.

From: Genetic Engineering

Traditional biotechnology uses an organism's existing natural processes, such as fermentation by yeast or bacteria, without changing its genetic material. Modern biotechnology directly manipulates genetic material using techniques such as genetic engineering or tissue culture, to produce specific desired outcomes more precisely.

From: Biotechnology

Tissue culture is used to grow large numbers of new plants from a small piece of tissue taken from a single parent plant, under sterile, controlled laboratory conditions. It produces genetically identical, disease-free plants quickly, which is especially useful for propagating high-value crops.

From: Biotechnology

A recombinant plasmid is a bacterial plasmid that has had a gene from another organism inserted into it and joined with ligase. It is called a vector because it carries the new gene into a host bacterium, where the gene can be expressed. Plasmids are used as vectors because they can pass easily into bacteria and copy themselves as the bacterium divides.

From: Biotechnology

Many fermented foods rely on traditional biotechnology. Yeast makes bread rise and ferments sugars into alcohol, bacteria turn milk into yoghurt and cheese, and moulds together with bacteria are used to make tempeh from soybeans and soy sauce from soybeans and wheat. In each case a microorganism's natural process changes the food, without any change to the organism's genes.

From: Biotechnology

Carbon dioxide + water, in the presence of light energy and chlorophyll, produces glucose + oxygen. The light energy is absorbed by chlorophyll, and the oxygen is released as a by-product from the splitting of water.

From: Photosynthesis

In the light reaction, chlorophyll absorbs light energy and uses it to split water, releasing oxygen and capturing energy as ATP and hydrogen carriers. In the dark reaction, that energy is used to combine carbon dioxide with hydrogen to make glucose. The dark reaction does not need light directly but depends on the products of the light reaction.

From: Photosynthesis

A limiting factor is the single condition that is in shortest supply and therefore sets the rate of photosynthesis at that moment. If light is limiting, adding more light speeds the process up; if carbon dioxide is limiting, extra light makes no difference. Examiners test this idea with graphs that rise and then level off, the plateau shows that some other factor has taken over as the limit.

From: Photosynthesis

Glucose + oxygen produces carbon dioxide + water + energy. The energy released is transferred to ATP, which the cell uses for its life processes. Aerobic respiration takes place mainly in the mitochondria.

From: Aerobic respiration

Yes. Every living plant cell respires aerobically all the time to release the energy it needs. In daylight, photosynthesis in the green parts produces more oxygen and glucose than respiration uses, so the net exchange looks like the reverse of respiration, but the two processes run at the same time.

From: Aerobic respiration

In humans, anaerobic respiration in muscle cells breaks glucose down to lactic acid, releasing a small amount of energy. In yeast, anaerobic respiration (fermentation) breaks glucose down to ethanol and carbon dioxide, also releasing a small amount of energy. Both occur without oxygen.

From: Anaerobic respiration

During hard exercise, muscles respire anaerobically and build up lactic acid. Oxygen debt is the extra oxygen the body needs to take in afterwards to break down that lactic acid. This is why you keep breathing hard for a while after intense exercise, even though the exercise has stopped.

From: Anaerobic respiration

The oil layer stops oxygen from the air dissolving into the yeast and glucose solution. Without oxygen, the yeast is forced to respire anaerobically, so the carbon dioxide that turns the lime water cloudy can be attributed to fermentation and not to aerobic respiration.

From: Anaerobic respiration

Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated, down the concentration gradient. It happens because particles move randomly, and it requires no energy, so it is a form of passive transport. It continues until the particles are evenly spread.

From: Diffusion

Diffusion is faster when the concentration gradient is steeper, the surface area is larger, the distance to diffuse is shorter, and the temperature is higher. This is why gas-exchange surfaces such as the alveolus have a very large, thin surface and a good blood supply to keep the gradient steep.

From: Diffusion

No. Facilitated diffusion uses channel or carrier proteins to help large or charged particles such as glucose and ions cross the membrane, but the particles still move down their concentration gradient and no ATP is used, so it is passive. Active transport uses carrier proteins and energy from respiration to move particles against their gradient.

From: Diffusion

Amoeba is tiny, so its surface area to volume ratio is high and every part of the cytoplasm is close to the membrane. In a large animal the volume is far greater than the surface, and the centre of the body is tens of centimetres from the exchange surfaces. Diffusion over that distance would take far too long, so a circulatory system carries substances close to every cell and diffusion only covers the last short distance.

From: Diffusion

Osmosis is the net movement of water molecules across a partially permeable membrane, from a solution with a higher water concentration (more dilute) to one with a lower water concentration (more concentrated). It requires no energy, so it is passive, and it continues until the two solutions have the same concentration or the cell wall prevents further movement.

From: Osmosis

If the solution outside is more concentrated than the cell sap, water leaves the cell by osmosis. The cell loses turgor and becomes flaccid; if water loss continues, the cell membrane pulls away from the cell wall, which is called plasmolysis. The plant wilts because its cells are no longer firm.

From: Osmosis

Osmosis is the movement of water only, across a partially permeable membrane, down a water potential gradient, and it uses no energy. Diffusion is the movement of any substance from a region of higher to lower concentration, with or without a membrane, and it also uses no energy. Active transport moves substances against their concentration gradient through carrier proteins and needs energy from ATP, which is why it stops when a cell is deprived of oxygen.

From: Osmosis

Active transport requires energy, which is supplied as ATP by respiration in the mitochondria. Cells that do a great deal of active transport, such as root hair cells and the cells lining the small intestine, therefore contain abundant mitochondria to release the energy needed to keep moving substances against their concentration gradients.

From: Active transport

Cyanide blocks aerobic respiration, so the cell can no longer produce ATP. Without ATP the carrier proteins cannot change shape, and active transport stops. Diffusion and osmosis continue because they are passive and do not depend on the cell's energy supply.

From: Active transport

Mitosis has four stages after interphase: prophase, when chromosomes condense and become visible; metaphase, when they line up in the middle of the cell; anaphase, when the copies are pulled to opposite ends; and telophase, when two new nuclei form. Cytokinesis then splits the cytoplasm to give two genetically identical daughter cells.

From: Mitosis

Before mitosis, during interphase, each chromosome is copied exactly. During mitosis, one copy of each chromosome goes to each daughter cell. Because both daughter cells receive an identical set of chromosomes, they have the same genes as each other and as the parent cell, which is essential for growth and repair.

From: Mitosis

In an animal cell, the plasma membrane pinches inward at the equator to form a cleavage furrow, which deepens until the cell is cut into two. A plant cell has a rigid cell wall, so it cannot pinch; instead, vesicles from the Golgi apparatus gather at the equator to form a cell plate, which grows outward until it joins the existing wall and becomes a new cell wall between the two daughter cells.

From: Mitosis

Mitosis makes two genetically identical diploid cells for growth and repair, in one division. Meiosis makes four genetically different haploid cells (gametes) in two divisions, and it halves the chromosome number. Meiosis also produces variation by shuffling alleles, whereas mitosis produces identical copies.

From: Meiosis

Gametes must be haploid so that when a sperm fuses with an egg at fertilisation, the normal diploid number is restored rather than doubled. If gametes kept the full diploid number, the chromosome number would double every generation. Meiosis therefore halves the number in the gametes to keep it constant across generations.

From: Meiosis

Meiosis I separates homologous chromosomes, so it is the reduction division that halves the chromosome number; crossing over and independent assortment both happen here. Meiosis II separates the sister chromatids of each chromosome, much like mitosis, and does not change the chromosome number. The two cells from meiosis I each divide again to give four haploid cells.

From: Meiosis

Molecules such as glucose, amino acids and ions are too large or too polar to pass directly through the phospholipid bilayer. They rely on carrier or channel proteins in the membrane to cross, even though they are still moving down their concentration gradient.

From: Facilitated diffusion

A channel protein forms a pore through the membrane and lets a specific ion or small molecule pass through without binding to it; some channels are gated and open only in response to a signal. A carrier protein binds the molecule at a specific site and changes shape to move it across. Both are specific and both are used in facilitated diffusion, but only carrier proteins are also used in active transport.

From: Facilitated diffusion

Yes. Endocytosis needs energy from respiration because the plasma membrane must actively fold, extend and pinch off to form a vesicle around the material being taken in. This makes it different from diffusion and osmosis, which are passive.

From: Endocytosis

Both are types of endocytosis. Phagocytosis is the uptake of solid particles, such as bacteria, by engulfing them in a vesicle. Pinocytosis is the uptake of liquid droplets containing dissolved substances in the same way.

From: Endocytosis

Active transport moves individual molecules or ions through a specific carrier protein embedded in the membrane, one at a time, against their concentration gradient. Endocytosis moves bulk material that is far too large for any protein by wrapping a section of membrane around it and pulling it in as a vesicle. Both are driven by ATP, but only endocytosis changes the surface area of the membrane.

From: Endocytosis

Each vesicle takes a patch of the plasma membrane into the cell, so repeated endocytosis would shrink the cell surface. The cell balances this by exocytosis, in which vesicles fuse with the plasma membrane and add their membrane back to the surface. Over time the membrane is recycled between the surface and the inside of the cell.

From: Endocytosis

Exocytosis needs energy from respiration to move vesicles to the plasma membrane and fuse with it. Gland cells that secrete large amounts of enzymes or hormones by exocytosis need a constant supply of energy, so they contain abundant mitochondria to produce enough ATP.

From: Exocytosis

At a synapse, a nerve impulse arriving at the end of a neurone triggers vesicles containing neurotransmitter to fuse with the presynaptic membrane by exocytosis. This releases the neurotransmitter into the synaptic cleft, allowing the impulse to be passed to the next neurone.

From: Exocytosis

Both are active processes that use ATP, but they are not the same thing. Active transport moves individual molecules or ions through a carrier protein against a concentration gradient. Exocytosis moves material in bulk inside a vesicle, without any carrier protein, and the vesicle membrane fuses with the plasma membrane. In an exam, name the two processes separately rather than treating one as a type of the other.

From: Exocytosis

Denaturation happens when extreme heat or extreme pH changes the shape of an enzyme's active site permanently, so the substrate can no longer fit. A denatured enzyme can no longer catalyse its reaction, even if conditions return to normal.

From: Enzyme action

At low substrate concentration, active sites are often empty, so adding substrate increases collisions and the rate rises. At high concentration every active site is occupied as soon as it becomes free; the enzyme is saturated and works at its maximum rate. Adding more substrate cannot raise the rate further, only adding more enzyme, and therefore more active sites, can.

From: Enzyme action

No. Glycolysis takes place in the cytoplasm and does not require oxygen, which is why it is the first stage of both aerobic and anaerobic respiration. Oxygen is only needed for the stages of respiration that follow glycolysis inside the mitochondrion.

From: Glycolysis

If oxygen is available, pyruvate moves into the mitochondrion and is broken down further during aerobic respiration, releasing much more ATP. If oxygen is not available, pyruvate is converted into lactic acid in muscle cells, or into ethanol and carbon dioxide in yeast, during anaerobic respiration.

From: Glycolysis

Glucose is a stable molecule, and the two ATP used at the start add phosphate groups that make it reactive enough for the enzymes to split it. This investment is repaid when four ATP are produced later in the pathway, leaving a net gain of two. Think of it as spending a little energy to unlock a larger amount.

From: Glycolysis

Breathing, or ventilation, is the physical movement of air into and out of the lungs, using the diaphragm and intercostal muscles. Respiration is the chemical release of energy from glucose inside every cell, forming ATP. Breathing simply supplies the oxygen that cellular respiration needs and removes the carbon dioxide it produces.

From: Lung structure and function

During inhalation, the diaphragm contracts and flattens and the ribcage moves up and outward, increasing the volume of the thorax. This makes the pressure inside the lungs lower than the pressure outside the body, so air moves down the pressure gradient into the lungs.

From: Inhalation and exhalation

Bile does not catalyse a chemical reaction, so it is not an enzyme. Instead, it emulsifies large fat droplets into tiny droplets, increasing the surface area available for lipase to break the fat down into fatty acids and glycerol.

From: Chemical digestion of food

Carbohydrates such as starch are digested into glucose. Proteins are digested into amino acids. Lipids are digested into fatty acids and glycerol. These small, soluble molecules can then be absorbed through the wall of the ileum.

From: Chemical digestion of food

Each enzyme has an optimum pH, and each region of the canal maintains a different pH. Pepsin needs the acidic medium of the stomach, while pancreatic amylase, trypsin and lipase need the slightly alkaline medium of the duodenum created by bile and pancreatic juice. An enzyme carried into a region with the wrong pH is denatured and stops working, which is why salivary amylase stops acting once food enters the stomach.

From: Chemical digestion of food

Peristalsis moves food in one direction because circular muscles contract behind the food, squeezing it forward, while longitudinal muscles contract ahead of the food, widening that region to receive it. This coordinated wave of contraction and relaxation always moves along the gut in the same direction.

From: Peristalsis

Yes. Peristalsis is caused by muscle contraction in the gut wall, not by gravity, so food is pushed along the alimentary canal whether a person is standing, sitting or lying down.

From: Peristalsis

Fibre is not digested, so it stays in the gut and adds bulk to the contents. This bulk stretches the gut wall more, which stimulates stronger and more regular peristaltic waves, so material moves through the intestine steadily. A low-fibre diet produces a smaller, drier mass that stretches the wall less; peristalsis becomes weaker and slower, more water is absorbed from the faeces in the colon, and constipation results.

From: Peristalsis

The ileum has villi and microvilli that give it a very large surface area, a wall that is only one cell thick to keep the diffusion distance short, and a dense network of blood capillaries and a lacteal to carry absorbed substances away quickly and maintain a steep concentration gradient.

From: Absorption in the ileum

Fatty acids and glycerol are absorbed into the epithelium cells of the villus and reassembled before passing into the lacteal, a lymph vessel at the centre of the villus, rather than directly into the blood capillary. They eventually reach the bloodstream through the lymphatic system.

From: Absorption in the ileum

Glucose and amino acids enter the capillaries of the villus and travel in the hepatic portal vein to the liver. The liver stores excess glucose as glycogen and releases the rest into the general circulation, so the concentration of glucose in the blood leaving the liver is regulated before it reaches the rest of the body.

From: Absorption in the ileum

The atrioventricular valves close when the ventricles contract, to stop blood flowing backward into the atria. The semilunar valves close when the ventricles relax, to stop blood flowing backward from the pulmonary artery and aorta into the ventricles. This keeps blood flowing in one direction through the heart.

From: Cardiac cycle

Both sounds are made by valves closing, not by the muscle contracting. The first, lower sound is the atrioventricular valves closing at the start of ventricular systole. The second, sharper sound is the semilunar valves closing at the start of ventricular diastole. Reading a pressure graph, the first sound sits where the ventricle line crosses above the atrium line and the second where it falls below the aorta line.

From: Cardiac cycle

Working muscles respire faster and need more oxygen and glucose while producing more carbon dioxide. The rise in carbon dioxide is detected by the brain, which sends impulses that make the pacemaker fire more often, and adrenaline released from the adrenal glands has the same effect. Each cardiac cycle is completed in less time, so more blood is pumped each minute. When exercise stops, the rate returns to its resting value.

From: Cardiac cycle

Fibrinogen is soluble and dissolved in the blood plasma, so on its own it cannot trap blood cells. The enzyme thrombin converts it into insoluble fibrin threads, which form a solid mesh across the wound that traps blood cells and creates a clot.

From: Blood clotting

As a clot dries, it forms a scab that seals the wound and acts as a physical barrier. This barrier helps stop pathogens on the skin or in the environment from entering the body through the damaged tissue while it heals.

From: Blood clotting

The clotting proteins circulate in inactive, soluble forms: prothrombin rather than thrombin, and fibrinogen rather than fibrin. The reaction only starts when platelets touch a damaged, rough surface and release their triggering substances. The smooth lining of an intact vessel gives platelets nothing to stick to, so the sequence is never started there.

From: Blood clotting

Both are needed for the conversion of prothrombin into thrombin. Without enough of either, thrombin forms slowly, fibrinogen is converted to fibrin slowly, and a wound takes longer to stop bleeding. This is the standard explanation expected when a question describes a person with a vitamin K deficiency.

From: Blood clotting

In double circulation, blood returns to the heart after the lungs and is pumped again at high pressure before travelling to the rest of the body. This keeps blood pressure and flow rate high throughout the body, delivering oxygen more efficiently than a single circulation, where blood pressure would already be low after passing through the lungs.

From: Double circulation

Blood first passes through the right side of the heart to the lungs and back in the pulmonary circulation, then through the left side of the heart to the body and back in the systemic circulation. This two-part path keeps oxygenated and deoxygenated blood separate and allows blood to be re-pressurised between the two circuits.

From: Double circulation

No. A fish has a single circulation: its two-chambered heart pumps deoxygenated blood to the gills, where it picks up oxygen, and the same blood then flows on at reduced pressure to the rest of the body before returning to the heart. Blood passes through the heart once per circuit, which is why fish generally have a lower metabolic rate than mammals and birds.

From: Double circulation

The capillaries surrounding the alveoli are extremely thin so that gases can diffuse across quickly. High pressure would damage them and force fluid into the alveolar space. The right ventricle therefore has a thinner wall and produces only enough pressure to move blood the short distance to the lungs and back, while the left ventricle produces the high pressure needed for the long systemic route.

From: Double circulation

Phagocytosis is non-specific because a phagocyte engulfs and digests any pathogen it encounters in the same way, without first recognising which particular pathogen it is. This is different from the specific immune response, where lymphocytes recognise a particular antigen before responding.

From: Phagocytosis

Once a pathogen is enclosed inside a vacuole within the phagocyte, digestive enzymes are released into the vacuole to break the pathogen down. The debris left over from this digestion is then removed from the phagocyte.

From: Phagocytosis

The lysosome is a small membrane-bound sac in the phagocyte's cytoplasm that stores digestive enzymes. After the pathogen is enclosed in a vacuole, lysosomes move to the vacuole and fuse with it, releasing their enzymes inside. This keeps the enzymes contained so they digest the pathogen without damaging the phagocyte's own cytoplasm.

From: Phagocytosis

After the first exposure to a pathogen, some activated lymphocytes remain in the body as memory cells. If the same antigen is encountered again, these memory cells allow antibodies to be produced faster and in larger quantities than during the first exposure.

From: Immune response

Each pathogen carries its own antigens, and the antibodies and memory cells made against one pathogen only bind to antigens of that shape. Memory cells for measles recognise nothing on a cold virus. This is also why a pathogen that changes its surface antigens, such as the influenza virus, can infect the same person again and why some vaccines have to be updated.

From: Immune response

The myelin sheath is a fatty layer that insulates most of the axon, leaving only small gaps called nodes of Ranvier exposed. The impulse jumps from node to node instead of travelling continuously along the whole membrane, which makes transmission much faster than in an unmyelinated axon.

From: Nerve impulse transmission

A stimulus causes sodium ions to rush into the axon at one point, reversing the charge across the membrane. This change triggers the same reversal in the next part of the membrane, so the impulse moves along the axon as a travelling wave of depolarisation, followed by a return to the resting charge behind it.

From: Nerve impulse transmission

A nerve impulse is only produced if the stimulus reaches a minimum strength called the threshold. Below the threshold there is no impulse at all; at or above it, an impulse of the same fixed size is produced every time, however strong the stimulus. The nervous system signals a stronger stimulus by sending impulses more frequently and along more neurones, not by sending bigger impulses.

From: Nerve impulse transmission

The presynaptic membrane is the only side with vesicles that release neurotransmitter, and the postsynaptic membrane is the only side with receptors that can bind it. Because of this arrangement, neurotransmitter can only move from the presynaptic to the postsynaptic side, so transmission is always one-way.

From: Synaptic transmission

Once enough neurotransmitter has bound to receptors on the postsynaptic membrane and triggered a new impulse, the neurotransmitter is broken down by an enzyme or reabsorbed by the presynaptic neurone. This clears the synaptic cleft and allows the synapse to respond to the next impulse.

From: Synaptic transmission

When an impulse reaches the synaptic knob, calcium ions diffuse in and trigger the vesicles to fuse with the presynaptic membrane, so without calcium no neurotransmitter is released. The mitochondria supply ATP, which is needed to synthesise neurotransmitter from recycled fragments and to reload the vesicles. A synapse starved of oxygen therefore fails after a short time, even though the axon can still carry impulses.

From: Synaptic transmission

The order is receptor, sensory neurone, relay neurone (in the spinal cord), motor neurone, then effector. A stimulus is detected by the receptor, and the impulse travels along this pathway until the effector, usually a muscle, produces the response.

From: Reflex arc

The reflex action is the response itself, the jerk of the hand or the blink of the eye. The reflex arc is the pathway of neurones along which the impulse travels to produce that response. In an answer, describe the arc when you are asked for the pathway, and name the action when you are asked what the body does.

From: Reflex arc

The afferent arteriole carrying blood into the glomerulus is wider than the efferent arteriole carrying blood out. This difference restricts outflow more than inflow, building up a high hydrostatic pressure inside the glomerular capillaries, which forces small substances out into the Bowman's capsule.

From: Ultrafiltration

Blood cells and plasma proteins stay in the blood because they are too large to pass through the capillary walls and the wall of the Bowman's capsule. Only water and small dissolved substances, such as glucose, amino acids, urea and mineral salts, pass into the filtrate.

From: Ultrafiltration

Glucose molecules are small enough to pass through the glomerular filter, so all of the glucose in plasma enters the filtrate. As the filtrate flows along the proximal convoluted tubule, the glucose is reabsorbed into the blood by active transport, so none remains in the urine of a healthy person.

From: Ultrafiltration

Drinking a large volume of water dilutes the blood, raising its water potential. The hypothalamus detects this, and the pituitary gland reduces ADH secretion. With less ADH, the distal tubule and collecting duct become less permeable to water, so less water is reabsorbed and a large volume of dilute urine is produced.

From: Osmoregulation

Any change in the blood's water potential, whether too high or too low, triggers a response, a change in ADH secretion, that brings the water potential back towards normal. Because the response opposes the original change, this control mechanism is called negative feedback.

From: Osmoregulation

ADH acts on the distal convoluted tubule and the collecting duct. It makes the walls of these tubules more permeable to water, so water leaves the filtrate by osmosis and returns to the blood in the surrounding capillaries. The glomerulus and proximal convoluted tubule are not affected by ADH; the proximal tubule reabsorbs most of the water regardless of hormone level.

From: Osmoregulation

Sweat is mostly water secreted onto the skin surface. As it evaporates, it absorbs heat energy from the skin, and this heat loss cools the body. This is why sweating increases when the hypothalamus detects that blood temperature is too high.

From: Thermoregulation

Shivering is the rapid, involuntary contraction of skeletal muscles. Muscle contraction requires respiration, which releases heat as a by-product, so shivering generates extra heat that helps raise body temperature back towards normal.

From: Thermoregulation

The hypothalamus contains thermoreceptors that monitor the temperature of the blood flowing through it, and it also receives impulses from thermoreceptors in the skin. It compares this information with the set point of about 37°C and sends nerve impulses to the effectors in the skin and muscles. It acts as both the detector and the control centre of the feedback loop.

From: Thermoregulation

Ammonia is very toxic to cells even in small amounts. The liver converts it into urea, which is far less toxic, so it can be safely carried by the blood to the kidneys and stored briefly before being excreted in urine without harming body cells.

From: Deamination

Excess amino acids are transported to the liver, where deamination removes the amino group to form ammonia, which is converted to urea and later excreted by the kidneys. The remaining part of each amino acid can be respired to release energy or converted into glycogen or fat for storage.

From: Deamination

Glucose can be converted into glycogen and stored in the liver and muscles, and fat can be stored in adipose tissue, because both are storage forms the body can later break down for energy. There is no equivalent storage molecule for amino acids: any amino acids not used for building protein remain in the blood, and the liver removes them by deamination so that their nitrogen can be excreted as urea and their carbon can be used as fuel.

From: Deamination

The liver makes urea by deamination and releases it into the blood; the kidney removes it. In the nephron, urea passes into the Bowman's capsule by ultrafiltration, is not fully reabsorbed along the tubule, and leaves in urine. An answer that stops at the liver gains only part of the marks, so always finish by naming the kidney, ultrafiltration and excretion in urine.

From: Deamination

Water moves into root hair cells by osmosis because there is a water potential gradient from the dilute soil water to the more concentrated cell sap. Mineral ions such as nitrate are often already less concentrated in the soil than inside the root hair cell, so the plant must use active transport, which needs energy, to absorb them against their concentration gradient.

From: Water uptake in roots

Root hair cells have a long, thin extension that greatly increases the surface area in contact with soil water, a thin cell wall that shortens the diffusion distance, and a large vacuole that maintains a strong water potential gradient. These adaptations make water uptake efficient.

From: Water uptake in roots

Excess fertiliser raises the solute concentration of the soil water, lowering its water potential. If the soil water potential drops below that of the root hair cell sap, water moves out of the root by osmosis instead of into it. The plant loses water and wilts even though the soil is moist, and the root hair cells may become plasmolysed.

From: Water uptake in roots

It explains how water rises up the xylem: water evaporating from leaves creates tension that pulls the water column upward, while hydrogen bonding between water molecules (cohesion) keeps this column continuous and unbroken from the roots to the leaves, so the whole column moves together.

From: Transpiration pull

The pull is created by evaporation, which is driven by heat energy from the environment, not by the plant's own respiration. The water column then moves upward passively because of the water potential gradient and the cohesive forces between water molecules, so no active transport is involved.

From: Transpiration pull

Anything that increases the rate of evaporation from the leaf increases the pull. High temperature gives water molecules more kinetic energy; low humidity and wind steepen the water vapour gradient between the leaf and the air; light opens the stomata. High humidity, still air and darkness have the opposite effect and slow the transpiration stream.

From: Transpiration pull

Since auxin is produced at the shoot tip, removing the tip removes the main source of the hormone. Without enough auxin being made and redistributed, the shoot below would show little or no phototropic bending towards a one-sided light source.

From: Phototropism

No. Shoots show positive phototropism and bend towards the light. Roots show negative phototropism, bending away from a one-sided light source, because root cells respond to a higher auxin concentration by elongating more slowly rather than faster. The hormone still collects on the shaded side; it is the sensitivity of the tissue that differs.

From: Phototropism

In both organs, auxin accumulates on the lower side due to gravity. Shoot cells are promoted to elongate by this higher auxin concentration, so the lower side of the shoot grows faster and it curves upward. Root cells are much more sensitive to auxin, so the same concentration inhibits elongation on the lower side, making the root curve downward instead.

From: Geotropism

Positive geotropism is growth towards gravity, shown by roots, which grow downward into the soil. Negative geotropism is growth away from gravity, shown by shoots, which grow upward towards light and air.

From: Geotropism

Both are tropisms controlled by auxin, but the stimulus differs. Phototropism responds to light: auxin moves to the shaded side of a shoot, so the shoot bends towards the light. Geotropism responds to gravity: auxin settles on the lower side. In shoots, both responses work through faster elongation of the side with more auxin. In roots, the higher auxin concentration inhibits elongation, so the root bends towards gravity and, weakly, away from light.

From: Geotropism

Touch triggers potassium ions to move out of cells in the pulvinus, and water follows by osmosis, so the cells lose turgor pressure and become flaccid. Because this loss of turgor happens regardless of where the touch came from, the folding response is the same strength in every case, this is what makes it a nastic movement rather than a tropism.

From: Nastic movement

Reopening is the reverse of folding and takes a few minutes. Active transport, powered by ATP from respiration, pumps potassium ions back into the extensor cells of the pulvinus. Water then re-enters the cells by osmosis, turgor pressure is restored, and the cells push the leaflets back into their open position.

From: Nastic movement

The radicle grows out first so that it can anchor the developing seedling in the soil and begin absorbing water, which the seedling needs for its cells to grow and expand. Only after this initial root growth does the plumule push upward, breaking through the soil toward the light.

From: Seed structure and function

Fresh mass rises sharply as soon as the seed imbibes water, which tells you nothing about new tissue. Dry mass removes the water and measures the organic matter that is actually present. That is why it falls during early germination, when reserves are respired, and rises only after the seedling begins photosynthesis.

From: Seed germination

Legumes have root nodules containing Rhizobium, nitrogen-fixing bacteria that convert atmospheric nitrogen gas into ammonium compounds the plant can use. When the legume plant dies or its roots decay, this fixed nitrogen becomes available in the soil, improving its fertility for other crops.

From: Nitrogen cycle

Nitrogen fixation converts nitrogen gas from the atmosphere into ammonium or nitrate compounds, carried out by nitrogen-fixing bacteria or lightning. Nitrification is a separate, later step in which nitrifying bacteria in the soil convert ammonium ions into nitrite and then nitrate ions.

From: Nitrogen cycle

Ammonification is carried out by decomposers and releases ammonium ions from proteins and urea into the soil, keeping nitrogen in a form that nitrifying bacteria can convert into nitrate. Denitrification is carried out by denitrifying bacteria in oxygen-poor soil and converts nitrate into nitrogen gas, removing nitrogen from the soil and returning it to the atmosphere. The first adds usable nitrogen to the soil; the second takes it away.

From: Nitrogen cycle

Producers such as green plants and algae remove carbon dioxide from the atmosphere during photosynthesis and fix the carbon into glucose and other organic compounds, which then form the basis of their tissues and of the food chains that depend on them.

From: Carbon cycle

Fossil fuels formed from the remains of organisms that lived millions of years ago, storing carbon that had been removed from the atmosphere long before. Burning these fuels releases this long-stored carbon back into the atmosphere as carbon dioxide, adding to the amount already being released by respiration and decomposition.

From: Carbon cycle

Yes. Every plant cell respires day and night, releasing carbon dioxide, while photosynthesis removes it only in light. During the day photosynthesis is faster than respiration, so the plant is a net absorber; at night only respiration runs, so the plant is a net releaser. Over a full day a healthy plant removes more than it releases, which is why forests act as carbon stores.

From: Carbon cycle

The syllabus treats carbon dioxide as a greenhouse gas that traps heat radiated from the Earth's surface. When combustion of fossil fuels and deforestation release more carbon dioxide than photosynthesis removes, the gas accumulates, more heat is trapped and global temperatures rise. Answers that link a named process to its direction of change in carbon dioxide, then to the heating effect, score fully.

From: Carbon cycle

At each trophic level, a large amount of energy is used up in respiration and lost as heat, while more energy is lost in materials the consumer cannot digest, in faeces, and in movement. Only the remaining small proportion, often estimated at around 10%, is available to be passed on when the organism is eaten by the next trophic level.

From: Energy flow in ecosystems

Because energy decreases sharply at each trophic level, there is eventually too little energy left to support another level of consumers. Beyond four or five levels, the amount of available energy would generally be too small to sustain a viable population at the next level.

From: Energy flow in ecosystems

Nutrients such as carbon and nitrogen move in a cycle: they pass from the environment into producers, along the food chain, and back to the environment through decomposers, and the same atoms are used again and again. Energy moves in one direction only: it enters as sunlight, passes along the chain, and leaves as heat that no organism can recapture. An ecosystem therefore needs a continuous input of light but not a continuous input of matter.

From: Energy flow in ecosystems

A pyramid of numbers counts organisms regardless of size, so one oak tree feeding thousands of caterpillars gives a narrow base and an inverted shape. A pyramid of energy records the energy stored at each level over a year, and because every transfer loses energy as heat and waste, each level must hold less than the one below it. The shape cannot be inverted.

From: Energy flow in ecosystems

Plants absorb water through their roots and lose much of it as water vapour through their leaves, in a process called transpiration. This adds a significant amount of water vapour to the atmosphere, alongside evaporation from oceans, rivers and lakes, contributing to cloud formation and rainfall.

From: Water cycle

Removing trees reduces transpiration, so less water vapour enters the atmosphere from that area and local rainfall can fall over time. It also removes the canopy, roots and leaf litter that slow rainwater, so more rain runs off the surface instead of soaking into the soil. The result is more flooding after storms and less groundwater in dry periods.

From: Water cycle

Decomposer enzymes work faster at higher temperatures, up to an optimum, increasing the rate at which dead matter is broken down. Decomposers also need water for growth and for their enzymes to function, so moist conditions support faster decomposition than dry ones.

From: Decomposition

As decomposers break down dead organisms and waste, they release nutrients such as nitrogen compounds and minerals back into the soil, and carbon dioxide back into the atmosphere. These recycled nutrients become available again for producers to absorb and use for growth.

From: Decomposition

A decomposer is a bacterium or fungus that digests dead organic matter outside its body by secreting enzymes and then absorbs the soluble products. A detritivore is an animal such as an earthworm, millipede or woodlouse that ingests fragments of dead matter and digests them internally. Detritivores speed up decomposition by breaking material into smaller pieces, but only bacteria and fungi complete the breakdown to mineral nutrients.

From: Decomposition

Each method removes a condition decomposers need. Refrigeration and freezing lower the temperature so enzyme activity and growth slow or stop. Drying removes the water that enzymes and absorption require. Salting and sugaring create a concentrated solution that draws water out of microbial cells by osmosis. Pickling in vinegar lowers the pH below the range in which most decomposer enzymes can act. Vacuum packing removes the oxygen that aerobic decomposers need for respiration.

From: Decomposition

An animal cell contains a nucleus that controls the cell, cytoplasm where reactions occur, a plasma membrane that controls what enters and leaves, mitochondria for aerobic respiration, and ribosomes for making proteins. It has no cell wall, no chloroplasts and no large permanent vacuole, which distinguishes it from a plant cell.

From: Animal cell structure

Centrioles are present in animal cells and absent in plant cells; they organise the spindle fibres during mitosis. Lysosomes are also more common in animal cells. In the other direction, the cell wall, chloroplasts and the large central vacuole are found only in plant cells.

From: Animal cell structure

Distilled water is hypotonic to the cytoplasm, so water enters the cell by osmosis. A plant cell has a cell wall that resists the pressure and stops it swelling further, so it becomes turgid. An animal cell has only a plasma membrane, which cannot withstand the pressure, so it swells and bursts.

From: Animal cell structure

A plant cell has three structures that an animal cell lacks: a cellulose cell wall outside the plasma membrane that gives support and a fixed shape, chloroplasts that contain chlorophyll for photosynthesis, and one large permanent vacuole filled with cell sap that keeps the cell turgid. Both cell types share a nucleus, cytoplasm, plasma membrane, mitochondria and ribosomes.

From: Plant cell structure

When a plant cell is placed in water, water enters by osmosis and the vacuole swells, pushing the cytoplasm against the cell wall. The cellulose cell wall is strong and resists this pressure, so the cell becomes firm (turgid) and stops swelling rather than bursting. An animal cell, which has no cell wall, would swell and burst in the same conditions.

From: Plant cell structure

The large permanent vacuole is filled with cell sap and is surrounded by a membrane called the tonoplast. It stores dissolved sugars, salts and pigments, and by pressing the cytoplasm against the cell wall it keeps the cell turgid, which helps support soft parts of the plant such as leaves and young stems.

From: Plant cell structure

The left ventricle pumps oxygenated blood out to the whole body, which needs high pressure to reach every organ, so it has a thick, muscular wall to generate that force. The right ventricle only pumps deoxygenated blood the short distance to the lungs at lower pressure, so its wall is thinner.

From: The human heart

In a double circulation blood passes through the heart twice for each complete circuit of the body: once on the way to the lungs (pulmonary circulation) and once on the way to the rest of the body (systemic circulation). This lets the heart raise the blood pressure again after the lungs, so oxygenated blood is delivered to the body organs quickly and efficiently.

From: The human heart

The valves make sure blood flows in one direction only. The atrioventricular valves between the atria and ventricles close when the ventricles contract, stopping blood from flowing back into the atria, while the semilunar valves at the base of the aorta and pulmonary artery stop blood flowing back into the ventricles. Without them, blood would flow backwards and circulation would be inefficient.

From: The human heart

First the glomerulus, a knot of capillaries under high pressure, forces water, glucose, salts and urea out of the blood into the Bowman's capsule by ultrafiltration, while large proteins and blood cells stay behind. Then, as this filtrate flows along the tubule, useful substances such as all the glucose, some salts and much of the water are reabsorbed into the blood. What remains, mainly urea and excess water, becomes urine.

From: The nephron

Ultrafiltration is filtration under pressure. The afferent arteriole is wider than the efferent arteriole, which raises the blood pressure in the glomerulus and forces small molecules such as water, glucose, salts and urea through the thin walls into the Bowman's capsule. Larger molecules such as proteins and blood cells cannot pass, so they stay in the blood.

From: The nephron

Glucose is small enough to be filtered out of the blood into the Bowman's capsule, so it does enter the filtrate. However, as the filtrate flows along the proximal convoluted tubule, all the glucose is reabsorbed back into the blood by selective reabsorption using active transport, so healthy urine contains no glucose.

From: The nephron

The alveolus has four main adaptations for efficient diffusion: a wall only one cell thick, which gives a very short distance for gases to diffuse; a very large total surface area from the millions of alveoli; a moist lining so that oxygen and carbon dioxide can dissolve; and a dense network of capillaries carrying blood close by, which keeps the concentration gradient steep. Together these make the exchange of oxygen and carbon dioxide fast.

From: The alveolus

Gases can only diffuse across a cell membrane once they are dissolved. The film of water on the inside of the alveolus dissolves oxygen from the air so it can pass through the alveolar wall and capillary wall into the plasma. Without moisture the rate of gas exchange would fall sharply.

From: The alveolus

Two things work together. Ventilation keeps bringing fresh air in, so the oxygen concentration in the alveolus stays high. At the same time the blood flow through the capillaries keeps carrying oxygenated blood away and bringing deoxygenated blood in, so the oxygen concentration in the blood beside the alveolus stays low. The difference between the two drives diffusion continuously.

From: The alveolus

Each villus has a wall only one cell thick, giving a short distance for absorbed food to reach the blood; a very large surface area, further increased by microvilli; a dense network of capillaries to carry away absorbed glucose and amino acids and keep the concentration gradient steep; and a lacteal to absorb fatty acids and glycerol. With millions of villi lining the small intestine, absorption is very efficient.

From: The villus

The lacteal is a small lymph vessel that runs up the centre of each villus. It absorbs the products of fat digestion, fatty acids and glycerol, which enter the lymphatic system and are carried in the lymph before eventually joining the bloodstream.

From: The villus

When the concentration of glucose or amino acids in the gut is higher than in the blood, they diffuse into the villus. Once most of the food has been absorbed, their concentration in the gut becomes lower than in the blood, so diffusion alone would stop. Active transport then uses energy from respiration to absorb the remaining molecules against the concentration gradient, so nearly all the digested food is absorbed.

From: The villus

The myelin sheath is a fatty layer that insulates the axon and is interrupted by gaps called nodes of Ranvier. Instead of travelling smoothly along the whole membrane, the impulse jumps from one node to the next, a process called saltatory conduction. This makes the impulse travel faster than it would along a bare, unmyelinated axon.

From: The neuron

A motor neuron has its cell body at one end, in or near the central nervous system, and a long axon that carries impulses out to an effector such as a muscle. A sensory neuron has its cell body on a side branch partway along the fibre and carries impulses from a receptor towards the central nervous system. Relay neurones, which link the two, are short and lie inside the central nervous system.

From: The neuron

A leaf is broad and thin to give a large surface area for absorbing light and a short distance for gases to diffuse. Its palisade mesophyll near the upper surface is packed with chloroplasts to trap the most light, the upper epidermis and cuticle are transparent to let light through, the spongy mesophyll has air spaces for carbon dioxide to reach the cells, and veins bring water and carry away the glucose made.

From: Leaf structure

The lower surface is cooler and more shaded than the upper surface, so placing most stomata there lets the leaf take in carbon dioxide and release oxygen while losing less water by evaporation. The upper surface, which faces the sun directly, is left almost free of stomata to reduce water loss.

From: Leaf structure

Palisade mesophyll lies just below the upper epidermis; its cells are long, column-shaped and tightly packed with chloroplasts, so it is the main site of photosynthesis. Spongy mesophyll lies below it, has fewer chloroplasts and large air spaces between its rounded cells, so its main job is to allow gases to diffuse to and from the photosynthesising cells.

From: Leaf structure

A bacterium is a prokaryotic cell, so its DNA lies free in the cytoplasm as a nucleoid and it has no nucleus or other membrane-bound organelles such as mitochondria. An animal cell is eukaryotic, with DNA enclosed in a nucleus and organelles such as mitochondria and ribosomes bound by membranes.

From: Structure of a bacterium

A plasmid is a small circular piece of DNA separate from a bacterium's main chromosome. It often carries genes that are useful but not essential, such as antibiotic resistance, and can be copied and transferred between bacteria.

From: Structure of a bacterium

Yes. The enzymes for respiration are found in the cytoplasm and on the plasma membrane rather than in a mitochondrion. Some bacteria respire aerobically using oxygen, and others respire anaerobically; the yeast and lactic-acid bacteria used in food production are examples of the anaerobic type.

From: Structure of a bacterium

Muscle cells contract frequently and need a constant, large supply of ATP for this work. Having many mitochondria allows a muscle cell to carry out aerobic respiration at a high rate, releasing enough energy to keep the muscle contracting.

From: Mitochondrion structure and function

Folding the inner membrane into cristae increases its surface area within the same small space. This lets more respiratory enzymes attach to the membrane, so more ATP can be produced by aerobic respiration.

From: Mitochondrion structure and function

The mitochondrion contains a small loop of its own DNA and its own ribosomes, so it can make some of its own proteins and copy itself without relying entirely on the nucleus. This lets a cell increase the number of its mitochondria when its energy demand rises.

From: Mitochondrion structure and function

The chloroplast is the whole organelle in which photosynthesis takes place, including its membranes, grana and stroma. Chlorophyll is the green pigment found within the grana of the chloroplast, and it is chlorophyll that actually absorbs light energy.

From: Chloroplast structure and function

Cells near the upper surface of a leaf, such as palisade mesophyll cells, receive the most sunlight, so they are packed with closely spaced chloroplasts to absorb as much light as possible for photosynthesis. Cells deeper in the plant, such as root cells, receive little or no light and have no chloroplasts.

From: Chloroplast structure and function

Both organelles have a double membrane and their own enzymes, but they do opposite jobs. A chloroplast uses light energy to build glucose from carbon dioxide and water, releasing oxygen. A mitochondrion breaks glucose down with oxygen in aerobic respiration to release energy as ATP. Plant cells contain both; animal cells contain only mitochondria.

From: Chloroplast structure and function

Chromatin is the loosely coiled thread of DNA and protein found in the nucleus for most of a cell's life. When a cell is about to divide, the chromatin coils up tightly to form distinct, visible chromosomes, making it easier to separate the genetic material accurately into the two daughter cells.

From: Nucleus structure and function

The nucleolus is a dense region inside the nucleus that produces ribosomal RNA and assembles it with proteins to form ribosomes. These ribosomes are then sent out into the cytoplasm, where they carry out protein synthesis.

From: Nucleus structure and function

The nucleus contains the cell's DNA, which carries the coded instructions for making every protein, including the enzymes that control the cell's chemical reactions. By deciding which genes are copied into messenger RNA at any time, the nucleus decides which proteins are made, and so directs how the cell grows, works and divides.

From: Nucleus structure and function

The fluid mosaic model describes the plasma membrane as a flexible, fluid double layer of phospholipid molecules with proteins scattered throughout it, able to move sideways within the layer. The phospholipids give the membrane its basic structure, while the proteins act as channels, carriers and receptors, giving the membrane a mosaic-like, patchy pattern.

From: Plasma membrane structure (fluid mosaic model)

The plasma membrane allows small, uncharged molecules to diffuse across it directly through the phospholipid bilayer, but restricts larger or charged molecules unless they pass through a specific channel or carrier protein. This selective control over which substances can cross makes it partially permeable rather than freely open or completely sealed.

From: Plasma membrane structure (fluid mosaic model)

Small, uncharged molecules such as oxygen and carbon dioxide diffuse directly through the phospholipid bilayer, and water moves across by osmosis. Ions and larger molecules such as glucose pass through channel or carrier proteins by facilitated diffusion when moving down their gradient, or by active transport, which uses ATP, when moving against it. Large particles can be taken in whole when the flexible membrane folds around them to form a vesicle.

From: Plasma membrane structure (fluid mosaic model)

The vacuole of a root hair cell contains cell sap with dissolved solutes, giving the cell a lower water potential than the surrounding soil water. Water therefore moves into the cell by osmosis, passing through the partially permeable plasma membrane from the region of higher water potential to the region of lower water potential.

From: Root hair cell structure and function

Root hair cells are found underground, where there is no light, so they cannot carry out photosynthesis and do not need chloroplasts. Their internal space and energy are used instead for absorbing water and mineral ions, which is why they are packed with cytoplasm and mitochondria rather than chloroplasts.

From: Root hair cell structure and function

A xylem vessel loses its cytoplasm and nucleus as it matures, leaving an empty, hollow tube. This lack of living contents removes any obstruction, allowing water and dissolved minerals to flow through it with minimal resistance.

From: Xylem vessel structure and function

Lignin is deposited in the walls of xylem vessels, making them waterproof and mechanically strong. This allows the vessel to withstand the tension created as water is pulled upward during transpiration without collapsing, and it also gives the whole plant support so it can stand upright.

From: Xylem vessel structure and function

Water moves up the xylem mainly by the transpiration pull. As water evaporates from the leaves and escapes through the stomata, it lowers the water potential of the leaf cells, which draw water from the xylem. Because water molecules stick to one another and to the vessel walls, the whole column is pulled upward under tension, from the roots to the leaves.

From: Xylem vessel structure and function

A companion cell keeps its nucleus and is dense with mitochondria, which release the ATP needed to actively load sucrose into the neighbouring sieve tube element. Since the sieve tube element itself has no nucleus, it depends on its companion cell to control its activity and supply the energy for translocation.

From: Phloem tissue structure and function

Phloem carries substances from a source, a part of the plant making or releasing sugars such as a leaf, to a sink, a part using or storing them such as a root or fruit. Because different parts of a plant can act as a source or a sink at different times, phloem can translocate sugars either upward or downward as needed.

From: Phloem tissue structure and function

Phloem is made of living cells and carries dissolved food, sucrose and amino acids, in both directions by translocation. Xylem is made of dead, hollow cells and carries water and dissolved mineral salts one way, from the roots up to the leaves, pulled up by transpiration. Both tissues run together in the vascular bundles.

From: Phloem tissue structure and function

The lower epidermis is shaded from direct sunlight and stays cooler than the upper surface, so placing most stomata there reduces the rate of evaporation and water loss. This balances the plant's need for gas exchange with the need to conserve water.

From: Stoma structure and function

A stoma allows gas exchange, letting carbon dioxide in for photosynthesis and oxygen in for respiration while releasing the waste gases from each process. At the same time, it is the main route through which water vapour escapes from the leaf as transpiration.

From: Stoma structure and function

A stoma opens and closes through changes in the turgor of its two guard cells. In light the guard cells take in water by osmosis and become turgid; because their inner walls are thicker and less elastic, the cells curve apart and open the pore. When water is scarce or at night, the guard cells lose water and become flaccid, so they straighten and the pore closes.

From: Stoma structure and function

Because the outer wall of a guard cell is thinner and more elastic than its thicker inner wall, the outer wall stretches more when the cell becomes turgid with water. This uneven stretching makes the whole cell curve outward, which pulls the two guard cells apart and opens the pore between them.

From: Guard cell structure and function

In light, guard cells actively move solutes such as potassium ions into themselves, lowering their water potential compared with neighbouring cells. Water then moves into the guard cells by osmosis, making them turgid and causing the stoma to open.

From: Guard cell structure and function

Guard cells respond to light, opening the stomata in the day and closing them at night, so having chloroplasts links their activity to daylight and to photosynthesis in the leaf. Ordinary epidermal cells only form a protective layer and do not control the stomata, so they have no chloroplasts.

From: Guard cell structure and function

Alveoli have a very large total surface area because there are millions of them, and each has a wall only one cell thick, giving a short diffusion distance. Their moist inner lining lets gases dissolve before crossing the wall, and a dense network of surrounding capillaries maintains a steep concentration gradient by constantly replacing the blood.

From: Lung structure and function

The cartilage rings are rigid but flexible, and they hold the trachea open so it does not collapse when air pressure changes during breathing. This keeps the airway clear so that air can flow freely to and from the lungs.

From: Lung structure and function

Villi are finger-like folds of the ileum wall, and microvilli are even smaller projections on the surface of each villus epithelial cell. Together they greatly increase the internal surface area of the small intestine, allowing digested nutrients to be absorbed more quickly and efficiently.

From: Small intestine structure and function

A lacteal is a small lymph vessel found at the centre of each villus. It absorbs fatty acids and glycerol, the products of fat digestion, which then enter the lymphatic system before eventually reaching the bloodstream.

From: Small intestine structure and function

Small, soluble molecules such as glucose and amino acids are absorbed across the thin epithelium of the villi into the blood capillaries, partly by diffusion and partly by active transport, which uses energy to absorb them even when their concentration in the blood is higher than in the gut. Fatty acids and glycerol are absorbed into the lacteal and enter the lymphatic system before joining the blood.

From: Small intestine structure and function

The gastric juice in the stomach contains strong hydrochloric acid and the protein-digesting enzyme pepsin, both of which could damage the stomach's own cells. A continuous layer of mucus coats the lining and protects it from being digested by its own acid and enzymes.

From: Stomach structure and function

Hydrochloric acid kills most of the microorganisms present in swallowed food, helping to prevent infection. It also converts inactive pepsin into its active form and provides the strongly acidic environment, around pH 2, in which pepsin works best to digest proteins.

From: Stomach structure and function

Protein digestion begins in the stomach, where pepsin breaks proteins into shorter polypeptide chains in acidic conditions. It is completed in the small intestine, where proteases from the pancreas and intestinal wall break the polypeptides down into amino acids, which are then absorbed at the villi.

From: Stomach structure and function

Blood leaves the heart under high pressure, so an artery needs a thick, muscular and elastic wall to withstand this pressure without bursting. The elastic fibres also stretch and recoil with each heartbeat, helping to smooth the pulsing flow of blood as it travels away from the heart.

From: Blood vessels, arteries, veins and capillaries

Blood in veins is at low pressure and, in the limbs, often has to flow against gravity back to the heart, so it can easily flow backwards without help. Valves in veins prevent this backflow, keeping blood moving in one direction, whereas the high pressure in arteries already keeps blood flowing forward without needing valves.

From: Blood vessels, arteries, veins and capillaries

The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, and the pulmonary vein carries oxygenated blood from the lungs to the left atrium. These are the two exceptions to the rule that arteries carry oxygenated blood and veins carry deoxygenated blood, because the terms artery and vein describe direction relative to the heart, not oxygen content.

From: Blood vessels, arteries, veins and capillaries

Without a nucleus, a red blood cell has more internal space available to be filled with haemoglobin. This increases the amount of oxygen the cell can carry, since haemoglobin is the protein that binds to and transports oxygen. Losing the nucleus also makes the cell thinner and more flexible.

From: Red blood cell structure and function

A biconcave shape gives the cell a larger surface area relative to its volume compared with a simple sphere, allowing oxygen to diffuse into and out of the cell more quickly. The thin centre also shortens the distance oxygen must travel, and the shape makes the cell flexible so it can bend and squeeze through narrow capillaries.

From: Red blood cell structure and function

In the lungs, where oxygen is plentiful, haemoglobin binds oxygen to form oxyhaemoglobin. As the blood reaches respiring tissues where oxygen is low, the oxyhaemoglobin breaks down and releases oxygen, which diffuses out to the cells. Because the binding is reversible, the same haemoglobin can pick up oxygen and release it again on every circuit of the body.

From: Red blood cell structure and function

The cerebrum is the largest part of the brain and controls voluntary actions, thinking, memory and the interpretation of sensory information. The cerebellum is a smaller region that coordinates balance, posture and precise muscular movement, working alongside the cerebrum rather than in place of it.

From: Brain structure and function

The medulla oblongata controls automatic, involuntary actions that do not require conscious thought, such as heartbeat rate, breathing rate and reflexes like swallowing and coughing. It sits at the base of the brain, connecting it directly to the spinal cord.

From: Brain structure and function

Three layers work together. The cranium is a hard bony case around the brain, the meninges are three membranes that wrap around it, and a layer of cerebrospinal fluid cushions the soft nerve tissue against sudden shocks. The fluid also supports the brain and supplies it with nutrients.

From: Brain structure and function

This process is called accommodation. To focus on a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes rounder to bend light more strongly. To focus on a distant object, the ciliary muscles relax, the ligaments become taut, and the lens flattens to bend light less.

From: Eye structure and function

In bright light, the circular muscles of the iris contract, making the pupil smaller and reducing the amount of light entering the eye, which protects the sensitive retina from damage. In dim light, the radial muscles of the iris contract instead, widening the pupil to let in more light so the eye can still see clearly.

From: Eye structure and function

The fovea, or yellow spot, is the part of the retina directly behind the lens that is packed with cone cells, giving the sharpest and most detailed colour vision. The blind spot is where the optic nerve leaves the eye; it contains no receptor cells, so any light that falls on it forms no image.

From: Eye structure and function

A nephron is the microscopic functional unit of the kidney. It filters blood at the glomerulus and Bowman's capsule to remove small molecules such as water, glucose, salts and urea, then selectively reabsorbs useful substances back into the blood along its tubules, leaving urine to be formed from the remaining fluid.

From: Kidney structure and function

The loop of Henle in each nephron sets up a concentration gradient deep in the medulla, which allows the collecting duct to reabsorb a variable amount of water depending on the body's needs. This lets the kidney produce more dilute urine when the body has excess water, or more concentrated urine when water needs to be conserved.

From: Kidney structure and function

The kidney is supplied by the renal artery, which delivers a large, steady share of the blood the heart pumps out. This constant flow lets the kidney filter the entire blood volume repeatedly through the day, removing urea as fast as the liver produces it and keeping the water and salt content of the blood stable.

From: Kidney structure and function

A muscle can only actively pull by contracting and cannot push itself back to its original length. So to move a joint in two directions, two muscles must work as an antagonistic pair, with one contracting to produce movement in one direction while the other relaxes, and then the roles reversing to move the joint back.

From: Skeleton and joints, structure and movement

Cartilage is a smooth tissue that covers the ends of the bones where they meet, so their surfaces do not grind directly against each other, and it also absorbs shock. Synovial fluid fills the joint and acts as a lubricant, letting the bone surfaces slide over one another easily. Together they reduce friction and wear so the joint can move smoothly.

From: Skeleton and joints, structure and movement

Fertilisation normally takes place in the oviduct, where a sperm cell fuses with an egg cell after the egg has been released from the ovary. The resulting embryo is then carried along the oviduct into the uterus, where it implants and develops.

From: Human reproductive organs and their functions

Sperm production works best at a temperature slightly lower than normal body temperature. By holding the testis in the scrotum outside the main body cavity, the body keeps it a few degrees cooler, providing suitable conditions for healthy sperm to develop.

From: Human reproductive organs and their functions

The wall of the uterus is thick, muscular and highly elastic, so it can stretch as the foetus grows and then contract strongly during labour to push the baby out. Each cycle its lining also becomes thick and full of blood vessels, ready to receive an implanted embryo and supply it with nutrients and oxygen.

From: Human reproductive organs and their functions

The stamen is the male reproductive part of a flower, made up of the anther, which produces pollen, and the filament, which supports it. The carpel is the female reproductive part, made up of the stigma, which receives pollen, the style, through which the pollen tube grows, and the ovary, which contains the ovules.

From: Flower structure and function

Insect-pollinated flowers usually have large, colourful, scented petals and sticky or spiky pollen to attract and cling to insects, along with a sticky stigma. Wind-pollinated flowers tend to have small or absent petals, produce large amounts of light, smooth pollen carried by the wind, and have feathery, exposed stigmas to catch airborne pollen.

From: Flower structure and function

The cotyledon stores food reserves such as starch, protein and oil that were built up while the seed was forming. During germination, before the seedling has leaves capable of photosynthesis, these reserves are digested and used in respiration to supply the energy and nutrients needed for the radicle and plumule to grow.

From: Seed structure and function

A seed needs water, oxygen and a suitable temperature to germinate. Water is absorbed through the micropyle and activates the enzymes that digest the stored food; oxygen is needed for aerobic respiration to release energy; and a suitable temperature allows those enzymes to work at a good rate. Light is not needed to start germination, only later for photosynthesis.

From: Seed structure and function