Living Processes in Multicellular Organisms

Levels of organisation

• Cell, the basic structural and functional unit, for example a red blood cell or a root hair cell.
• Tissue, a group of similar cells working together to perform a specific function, for example muscle tissue or xylem tissue.
• Organ, different tissues working together to perform a more complex function, for example the heart or a leaf.
• Organ system, a group of organs working together for a major body function, for example the digestive system or the circulatory system.
• Organism, different organ systems working together to form a complete, functioning living thing.

Cell specialisation and division of labour

Content standard 2.3 expects you to relate the structure of specialised cells to their specific function, and to explain why multicellular organisms need division of labour. Because a large organism cannot rely on diffusion alone across every cell, groups of cells specialise: some absorb, some transport, some contract, and no single cell needs to do everything.

Examples of specialised cells

How it is examined

Common questions ask you to arrange the levels of organisation in the correct order, to name the tissue or organ formed by a described group of cells, or to explain how the structure of a named specialised cell (such as a root hair cell or red blood cell) suits its function. You may also be asked why a large multicellular organism needs specialised transport and exchange systems rather than relying on diffusion alone.

Worked exam-style question

Question. A sperm cell has a long tail (flagellum) packed with mitochondria, a streamlined head, and an acrosome containing digestive enzymes at its tip. (a) Name the specific structural feature that enables the sperm cell to move, and state the function of the mitochondria packed around it.

(b) State the function of the acrosome. (c) Explain, using the term division of labour, why a sperm cell can afford to be highly specialised for movement and fertilisation rather than carrying out every life process itself.

Model answer. (a) The flagellum enables movement by beating; the mitochondria packed around it release energy through aerobic respiration to power this movement. (b) The acrosome releases digestive enzymes that break down the outer layers of the egg cell, allowing the sperm to penetrate and fertilise it.

(c) Because the sperm cell belongs to a multicellular organism with division of labour, other cells and organ systems in the body handle nutrition, gas exchange and waste removal for it, so the sperm cell can devote almost all of its structure to the single specialised task of reaching and fertilising the egg.

Practice question

Try this. A xylem vessel is a hollow, tube-like structure with no cytoplasm and walls thickened with a waterproof substance called lignin. Explain how each of these two structural features suits the function of a xylem vessel.

Exam tip

Key terms

These Chapter 2 terms describe how a multicellular organism is built and organised:

• Cell differentiation, the process by which a cell becomes structurally specialised for one function.
• Tissue, a group of similar specialised cells working together.
• Organelle, a specialised structure inside a cell, such as the mitochondria packed into a sperm cell's tail.
• Nucleus, the organelle absent from a mature red blood cell, freeing space for more haemoglobin.

Source:SRC-DSKP-EN

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