Biological processes explained

Aerobic respiration

How aerobic respiration works, glucose and oxygen release energy, carbon dioxide and water, and how it is examined in SPM Biology.

Anaerobic respiration

How anaerobic respiration works in humans and yeast, the products, why it releases less energy, and how it is examined.

Diffusion

How diffusion works, the movement of particles down a concentration gradient, with everyday and biological examples.

Osmosis

How osmosis works, the movement of water across a partially permeable membrane, and its effects on animal and plant cells.

Active transport

How active transport works, moving substances against the concentration gradient using energy and carrier proteins.

Mitosis

How mitosis works, the four stages that make two genetically identical cells for growth and repair.

Meiosis

How meiosis works, the division that halves the chromosome number to make four different gametes.

Facilitated diffusion

How facilitated diffusion moves glucose, amino acids and ions across the cell membrane through carrier and channel proteins, without using energy.

Endocytosis

How endocytosis brings large particles and droplets into a cell by engulfing them in a vesicle, using energy from respiration.

Exocytosis

How exocytosis exports substances out of a cell, a vesicle fuses with the plasma membrane and releases its contents, using energy.

Enzyme action

How enzymes work, the lock-and-key model, the enzyme-substrate complex, and why enzymes are specific to one substrate.

Glycolysis

How glycolysis breaks down glucose into pyruvate in the cytoplasm, the first stage shared by aerobic and anaerobic respiration.

Inhalation and exhalation

How breathing works, the diaphragm and intercostal muscles change the volume and pressure of the thorax to draw air in and push it out.

Chemical digestion of food

How enzymes break down carbohydrates, proteins and lipids into small soluble molecules as food passes along the alimentary canal.

Peristalsis

How peristalsis moves food along the alimentary canal, waves of contraction by circular and longitudinal muscles behind and ahead of the food.

Absorption in the ileum

How the ileum absorbs digested food into the blood and lymph, villi, microvilli and the adaptations that speed up absorption.

Cardiac cycle

How one heartbeat works, atrial systole, ventricular systole and diastole, and the valves that keep blood flowing one way.

Blood clotting

How blood clotting stops bleeding, platelets, the conversion of fibrinogen to fibrin, and why the clot forms.

Double circulation

How double circulation works, blood passes through the heart twice for one full circuit, through separate pulmonary and systemic circulations.

Phagocytosis

How phagocytes engulf and destroy pathogens as part of the body's non-specific defence, without recognising a particular pathogen.

Immune response

How the specific immune response works, antigen recognition, antibody production by lymphocytes, and how memory cells are formed.

Nerve impulse transmission

How an electrical impulse travels along a neurone's axon, and how the myelin sheath speeds transmission up.

Synaptic transmission

How a nerve impulse crosses a synapse using neurotransmitter, and why transmission at a synapse only goes in one direction.

Reflex arc

How a reflex arc works, the pathway from receptor to effector that produces a rapid, automatic response.

Ultrafiltration

How ultrafiltration works at the kidney's glomerulus, high-pressure filtration that forms the filtrate.

Osmoregulation

How osmoregulation controls the water and solute balance of the blood using ADH and the kidney.

Thermoregulation

How the body keeps a constant temperature, skin responses to being too hot or too cold.

Deamination

How the liver breaks down excess amino acids and converts the toxic ammonia produced into urea.

Photosynthesis

How photosynthesis works, step by step, light and dark reactions, inputs and outputs, and how it is examined in SPM Biology.

Water uptake in roots

How root hair cells absorb water by osmosis and mineral ions by active transport.

Transpiration pull

How water evaporating from leaves pulls a continuous column of water up the xylem, the cohesion-tension mechanism.

Phototropism

How plants grow towards light, the role of auxin in directional growth responses.

Geotropism

How roots and shoots respond to gravity, why roots grow down and shoots grow up.

Nastic movement

How non-directional plant responses like the Mimosa leaf-folding reaction work, and how they differ from tropisms.

Seed germination

How a seed grows into a seedling, the conditions needed and how stored food reserves are mobilised.

Nitrogen cycle

How nitrogen cycles through fixation, nitrification, assimilation, ammonification and denitrification.

Carbon cycle

How carbon cycles through photosynthesis, respiration, feeding, decomposition and combustion.

Energy flow in ecosystems

How energy passes along food chains, and why less energy reaches each higher trophic level.

Water cycle

How evaporation, transpiration, condensation and precipitation move water between the Earth and the atmosphere.

Decomposition

How decomposers break down dead matter and recycle nutrients back into the ecosystem.