Cell Biology and Organisation, revision notes

Overview

Every living thing is built from cells, the smallest structure able to carry out life's processes on its own. This chapter opens Form 4 Biology by examining the structure of a generalised animal cell and a generalised plant cell, explaining what each organelle does, and then tracing how individual cells cooperate as tissues, organs and systems inside a multicellular organism.

Structure and function are always tested together, so naming an organelle correctly is only half of what an examiner wants; the other half is explaining what that structure actually does inside the cell.

It also compares how life processes such as nutrition, respiration, excretion and response are carried out. In a unicellular organism like Amoeba, one cell alone performs every process, absorbing food, releasing energy and removing waste through its own cell surface.

In a multicellular organism such as a human, the same processes are shared out among specialised cells, tissues and organs that depend on one another to keep the whole body functioning, a division of labour a single free-living cell can never achieve.

A further content standard builds the levels of organisation from the ground up: similar cells grouped together form a tissue, different tissues working as a unit form an organ, related organs cooperating form a system, and all the systems together make up the complete organism. Each level exists only because the level below it is correctly structured, which is why examiners often ask you to place named human examples, such as muscle tissue, the heart and the circulatory system, in the right order, from the smallest level to the largest.

Across the whole chapter, the real skill being tested is linking a structure's shape to its role. A root hair cell is long and thin because that increases the surface area available for absorbing water, and a red blood cell is biconcave and has no nucleus so that it can carry more oxygen.

Memorising organelle names without this structure-function reasoning is the most common reason marks are lost in this chapter, more than any single organelle name forgotten under exam pressure.

The generalised cell and the job of each organelle

Content Standard 2.1 begins with a generalised cell bounded by a plasma membrane that encloses the cytoplasm, a jelly-like medium in which most of the cell's chemical reactions take place. Suspended in that cytoplasm are the organelles, each carrying out one main task: the nucleus controls the cell's activities and holds the chromosomes made of DNA, the mitochondria release energy through respiration, and the ribosomes build proteins by joining amino acids together.

A plant cell contains everything an animal cell has and adds three structures an animal cell never has. A cellulose cell wall lies outside the plasma membrane and gives the cell a fixed shape and support; chloroplasts containing the green pigment chlorophyll trap light energy for photosynthesis; and one large permanent vacuole filled with cell sap presses outwards on the wall to keep the cell firm, or turgid.

An animal cell has at most small, temporary vacuoles and none of the other two structures.

The skill this content standard rewards is naming a structure and stating its function in the same breath. 'Mitochondrion' on its own earns little; 'mitochondrion, the site of respiration that releases energy from glucose' is what a marker credits, because structure and function are assessed together throughout this chapter.

Comparing an animal cell with a plant cell

A side-by-side comparison is one of the most common ways this chapter is examined, so the shared and unique structures are worth setting out in a single table. Every value that appears in the 'Function' column must match the structure named on the left, because a labelled diagram question expects the function to be stated exactly, not paraphrased loosely.

How the structure of a specialised cell suits its function

A red blood cell is biconcave and loses its nucleus as it matures; the biconcave shape gives a larger surface area to volume ratio for oxygen to diffuse across, and the missing nucleus leaves more room inside for haemoglobin, so the cell carries more oxygen. A root hair cell is drawn out into a long, thin projection that increases the surface area in contact with soil water, speeding up the absorption of water and dissolved mineral salts.

A nerve cell has a long fibre so that an electrical impulse can travel from one part of the body to another over a distance, while a sperm cell has a tail for swimming towards the egg and a dense cluster of mitochondria packed behind the head to supply the energy that swimming demands. A guard cell changes shape as it gains or loses water, opening or closing the stoma it surrounds to control gas exchange in a leaf.

Specialisation is not limited to animals. A palisade mesophyll cell is column-shaped and densely packed with chloroplasts near the upper surface of a leaf, where the light is brightest, so that photosynthesis is as efficient as possible.

In every one of these examples the argument an examiner wants is the same: name the feature, then explain how that feature makes one particular function work better.

Life processes in a unicellular organism

Content Standard 2.2 uses organisms such as Amoeba and Paramecium, whose whole body is a single cell that must carry out every life process by itself. An Amoeba feeds by flowing its cytoplasm around a food particle until the particle is enclosed in a food vacuole, where it is digested; it respires as oxygen diffuses in and carbon dioxide diffuses out across the same cell surface, because no separate breathing organ exists.

Excess water that enters the Amoeba by osmosis collects in a contractile vacuole, which swells and then empties its contents out of the cell, keeping the water content in balance; nitrogenous waste leaves by diffusion across the cell surface. The Amoeba moves and captures food using pseudopodia, temporary extensions of its cytoplasm, and reproduces by dividing into two through mitosis.

A single cell can supply all its own needs by diffusion because it is small and has a large surface area compared with its volume, so no substance has far to travel to reach any part of the cell. A Paramecium achieves the same self-sufficiency with cilia for movement and an oral groove lined with cilia that sweep food particles inside, again with no transport system, since diffusion alone is fast enough over such a short distance.

Division of labour in a multicellular organism

As an organism grows large, its volume increases faster than its outer surface, so the surface area to volume ratio falls and diffusion across the body surface can no longer supply the cells buried deep inside. Content Standard 2.3 explains that a multicellular organism solves this by sharing the life processes out among specialised cells, tissues, organs and systems, an arrangement called division of labour that a single free-living cell can never achieve.

In a human, the digestive system breaks down and absorbs food, the respiratory system exchanges oxygen and carbon dioxide at the alveoli, the circulatory system transports these substances between organs, and the excretory system removes waste; no one system works alone, because the nutrients absorbed by the gut only reach the body's cells once the blood has carried them there. Each system depends on the others to keep the whole organism alive.

Comparing one process across the two kinds of organism makes the difference clear. An Amoeba exchanges gases over its entire cell surface, whereas a human relies on millions of alveoli for a large gas-exchange surface and on the blood to carry the gases to and from every cell, the same job, but done by a chain of specialised structures rather than by one cell acting alone.

Levels of organisation from cell to organism

Content Standard 2.4 builds a hierarchy in which each level is made from the level below it: similar cells working together form a tissue, different tissues combined into one working unit form an organ, related organs cooperating form a system, and all the systems together form the complete organism. Because each level depends on the correct structure of the level beneath it, damage low in the hierarchy, faulty cells in a tissue, can disrupt the organ and system built from it.

The clearest way to fix the sequence is to trace one named example all the way up. In an animal: a muscle cell forms muscle tissue, muscle tissue together with nervous and connective tissue forms the heart, the heart with the blood vessels forms the circulatory system, and all the systems together form the human.

In a plant: a palisade cell forms palisade mesophyll tissue, different tissues together form the leaf, leaves and stem form the shoot system, and the shoot and root systems together form the whole plant.

Examiners test this standard by giving a jumbled list of levels or named examples and asking for the correct order from smallest to largest, or by asking you to place one named structure, the blood, the heart, a muscle cell, at its correct level. The blood is a tissue, the heart is an organ built from more than one tissue, and the circulatory system links the heart to every blood vessel, so keeping the definitions of tissue, organ and system separate is what earns the marks.

Key concepts to master

• Cell structure, A generalised cell has a nucleus that controls its activities, cytoplasm where chemical reactions take place, mitochondria that release energy through respiration, and ribosomes that build proteins. A plant cell adds three structures an animal cell lacks: a rigid cellulose cell wall for support, chloroplasts that trap light energy, and one large permanent vacuole that keeps the cell turgid.
• Animal vs plant cells, Plant cells have a cellulose cell wall outside the plasma membrane, chloroplasts for photosynthesis, and a large permanent vacuole for water storage and turgor; animal cells have none of these three structures. Both cell types still share a nucleus, cytoplasm, mitochondria, ribosomes and a plasma membrane that controls what enters and leaves.
• Living processes in unicellular organisms, Organisms such as Amoeba and Paramecium consist of a single cell that must carry out nutrition, respiration, excretion, growth, movement and response entirely on its own, without any other cell to share the work. Food, oxygen and waste all cross the same cell surface, and the cell has no separate organs to help, unlike the many cells of a multicellular animal.
• Living processes in multicellular organisms, In a multicellular organism, no single cell performs every life process alone. Specialised cells, tissues, organs and systems divide the work between them, nutrients absorbed by the digestive system are transported by the circulatory system to cells all over the body, which could not happen inside one isolated cell.
• Levels of organisation, Multicellular organisms are built as a hierarchy: similar cells form a tissue, different tissues working together form an organ, related organs form a system, and every system together forms the organism. A fault at one level, such as damaged cells in a tissue, can disrupt the organ and system built from it. Examiners often test this hierarchy by asking you to place a named human example, such as a muscle cell, muscle tissue, the heart and the circulatory system, at the correct level.
• Specialised cells, Cell structure matches function: a red blood cell is biconcave and lacks a nucleus so it can carry more oxygen; a root hair cell is long and thin to increase the surface area for absorbing water; a nerve cell has a long fibre to carry electrical impulses quickly over a distance. A sperm cell has a tail for swimming towards an egg, and a guard cell changes shape to open or close a leaf's stomata, showing that specialisation applies equally to plant and animal cells.
• Tissues in animals and plants, A tissue is a group of similar cells working together for one function, such as muscle tissue that contracts or xylem tissue that transports water in a plant. Recognising a named tissue is the link between the cell level and the organ level in the hierarchy of organisation. Other examples include epithelial tissue that lines surfaces such as the gut, and phloem tissue that transports dissolved food in a plant.
• Cell as the basic unit of life, Every reaction that keeps an organism alive, including respiration, protein synthesis and waste removal, happens inside cells. Because a cell is the smallest unit that can carry out these processes on its own, it is called the basic structural and functional unit of life, and damage at the cell level ultimately affects the whole organism.
• Selective permeability of the plasma membrane, The plasma membrane surrounds every cell and controls which substances enter and leave, letting in useful molecules such as oxygen and glucose while allowing waste products to pass out. This control keeps the cell's internal conditions suitable for the reactions it carries out, and the same basic membrane structure appears in every type of cell, from bacteria to human neurons.
• The nucleus and genetic control, The nucleus contains chromosomes made of DNA, carrying the genetic instructions that direct every activity inside the cell, including which proteins it builds. A cell without a working nucleus, such as a mature red blood cell, cannot divide or repair itself, and eventually breaks down and is replaced. Chromosome number is also characteristic of a species; human body cells, for instance, normally contain 23 pairs.

Quick recall checklist

1. Can you define and explain Cell structure?
2. Can you define and explain Animal vs plant cells?
3. Can you define and explain Living processes in unicellular organisms?
4. Can you define and explain Living processes in multicellular organisms?
5. Can you define and explain Levels of organisation?
6. Can you define and explain Specialised cells?
7. Can you define and explain Tissues in animals and plants?
8. Can you define and explain Cell as the basic unit of life?
9. Can you define and explain Selective permeability of the plasma membrane?
10. Can you define and explain The nucleus and genetic control?

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