Form 4 · Worked answers

Cell Biology and Organisation, worked answers

Fully worked answers for Cell Biology and Organisation, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Labelling a fully drawn animal or plant cell diagram and stating the precise function of each organelle shown.
Comparing animal and plant cells side by side in a table of shared and unique structures.
Arranging a mixed list of structures, cell, tissue, organ, system, organism, into the correct order of organisation.
Explaining how a named specialised cell's structure suits its one main function, using correct biological terms.
Comparing how the same life process, such as respiration or excretion, is carried out in a unicellular organism and in a human organ system.
Identifying an unfamiliar organelle correctly from a written description of its function alone, without seeing a labelled diagram.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

The diagram shows a generalised plant cell. (a) Name the structure that traps light energy and state the process it carries out. (b) Name the structure that keeps the cell turgid and state what it contains. (c) State one further structure present in this cell but absent from a human cheek cell.

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(a) The chloroplast traps light energy and carries out photosynthesis. (b) The large permanent vacuole keeps the cell turgid and contains cell sap. (c) The cellulose cell wall is present in the plant cell but absent from a human cheek cell, giving the plant cell support and a fixed shape. Each answer names the structure and states its role, which is what the mark scheme requires rather than the name alone.

chloroplastphotosynthesispermanent vacuolecell sapcell wall

State three structures found in a plant cell but not in an animal cell, and give the function of each.

[6]

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The cellulose cell wall lies outside the plasma membrane and gives the cell support and a fixed shape. The chloroplast contains chlorophyll and traps light energy for photosynthesis. The large permanent vacuole is filled with cell sap and presses outwards to keep the cell turgid. Each of the three structures must be paired with its own function to gain full credit, because naming the structure alone is only half the required answer.

cell wallchloroplastpermanent vacuoleturgid

A red blood cell is biconcave and has no nucleus. Explain how each of these two features suits its function of transporting oxygen.

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The biconcave shape gives the cell a larger surface area to volume ratio, so oxygen can diffuse into and out of the cell more quickly across a larger membrane area. The absence of a nucleus leaves more space inside the cell for haemoglobin, the pigment that binds oxygen, so each cell can carry more oxygen than it could if a nucleus took up part of that volume.

biconcavesurface area to volume rationucleushaemoglobin

Explain how an Amoeba carries out (a) feeding and (b) the removal of excess water, using only its single cell.

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(a) The Amoeba feeds by flowing its cytoplasm around a food particle until the particle is enclosed inside a food vacuole, where it is then digested; this is possible because the cell surface itself can change shape. (b) Excess water that enters the cell by osmosis collects in a contractile vacuole, which swells and then contracts to push the water back out of the cell, keeping the cell's water content balanced without any separate excretory organ.

food vacuolecytoplasmcontractile vacuoleosmosis

Arrange the following into the correct order of organisation, from smallest to largest: heart, muscle cell, circulatory system, muscle tissue, human. Then state which item in the list is an organ and which is a tissue.

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From smallest to largest: muscle cell, muscle tissue, heart, circulatory system, human. The heart is the organ, because it is built from more than one tissue working together, and muscle tissue is the tissue, because it is a group of similar cells with one function. Placing the levels in this order and separating the organ from the tissue is exactly what the content standard tests.

levels of organisationtissueorgansystem

The plasma membrane is described as selectively permeable. (a) Explain what 'selectively permeable' means. (b) Name one substance that enters and one that leaves a respiring cell across this membrane.

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(a) Selectively permeable means the membrane allows some substances to pass through while blocking or restricting others, so the cell can control what enters and leaves. (b) In a respiring cell, oxygen and glucose enter across the membrane, while carbon dioxide, a waste product of respiration, leaves across it. This control keeps the cell's internal conditions suitable for the reactions taking place inside it.

selectively permeableplasma membraneoxygencarbon dioxide

A mature red blood cell has no nucleus. State one advantage and one disadvantage of this to the cell.

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One advantage is that the space normally taken by the nucleus is free for extra haemoglobin, so the cell carries more oxygen. One disadvantage is that, without a nucleus, the cell cannot divide or repair itself, so it has a limited lifespan and must eventually be replaced by new cells made in the bone marrow.

nucleushaemoglobincannot dividelimited lifespan

Phrasing that earns marks

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.

Frequently asked questions

What is the difference between a plant cell and an animal cell?

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.

What are the levels of 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.

Why do cells become specialised?

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.