Form 4 · Cell Biology and Organisation

Cell Structure and Function

The cell is the basic structural and functional unit of all living organisms, and each organelle inside it, nucleus, mitochondrion, ribosome and others, carries out a specific function that keeps the cell alive.

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The cell as the basic unit of life

Content standard 2.1 establishes the cell theory: all living organisms are made up of one or more cells, the cell is the basic unit of structure and function in an organism, and all cells arise from pre-existing cells. This standard expects you to identify the main organelles in animal and plant cells using a light micrograph or diagram, and to relate each organelle's structure to its function.

Key organelles and their functions

Organelles commonly compared in animal and plant cell diagrams at SPM level.
Organelle	Main function
Nucleus	Contains chromosomes (DNA) and controls the cell's activities
Mitochondrion	Site of aerobic respiration; releases energy as ATP
Ribosome	Site of protein synthesis
Endoplasmic reticulum	Transports proteins and lipids within the cell
Golgi apparatus	Modifies, packages and transports proteins for secretion
Cell (plasma) membrane	Controls movement of substances into and out of the cell
Cell wall (plant only)	Made of cellulose; provides shape and support
Chloroplast (plant only)	Site of photosynthesis; contains chlorophyll
Vacuole	Large and permanent in plant cells; stores cell sap and maintains turgor

Animal cells versus plant cells

Both cell types share a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells additionally have a cell wall, chloroplasts and a large permanent vacuole, which animal cells lack or have only in a much smaller, temporary form.

These differences relate directly to plant-specific functions such as photosynthesis and maintaining rigid shape without a skeleton.

Prokaryotic and eukaryotic cells

A eukaryotic cell (found in animals, plants, fungi and protists) has a nucleus enclosed by a nuclear membrane and membrane-bound organelles. A prokaryotic cell (found in bacteria) has no nuclear membrane, its genetic material lies free in the cytoplasm as a single circular chromosome, and lacks most membrane-bound organelles.

How it is examined

Common questions give a labelled or unlabelled diagram of a cell and ask you to name an organelle and state its function, or ask you to list structural differences between an animal cell and a plant cell. You may also be asked to distinguish a prokaryotic cell from a eukaryotic cell based on a diagram or description.

Levels of cellular organisation

In a multicellular organism, cells are organised into larger working units. A cell becomes specialised for a particular job through cell differentiation, and cells of the same type are grouped together to carry out a shared function.

Cell, the basic unit, specialised for a particular function, such as a red blood cell or a root hair cell.
Tissue, a group of similar cells working together, such as muscle tissue or epidermal tissue.
Organ, several tissues working together as one structure, such as the heart or a leaf.
System, a group of organs working together, such as the digestive system or the transport system in a plant.
Organism, all the systems working together make up the whole living thing.

Worked exam-style question

Question. A diagram shows a cell that has a cell wall, a large central vacuole, chloroplasts, a nucleus, cytoplasm and a cell membrane. (a) State whether this is an animal cell or a plant cell, and give two reasons from the diagram.

(b) Name the organelle where photosynthesis occurs and the organelle that controls the cell's activities. (c) Name the organelle that releases energy through aerobic respiration.

(d) State one structure present in this cell that is absent in a bacterial (prokaryotic) cell.

Model answer. (a) A plant cell, because it has a cell wall and chloroplasts (and a large permanent vacuole), which are not found in an animal cell. (b) Photosynthesis occurs in the chloroplast, and the nucleus controls the cell's activities.

(c) The mitochondrion. (d) A nucleus bound by a nuclear membrane (or any membrane-bound organelle, such as a chloroplast or mitochondrion), which a prokaryotic cell lacks.

Practice question

Try this. (a) List three structures found in both an animal cell and a plant cell. (b) A cell has no nuclear membrane and its DNA lies free in the cytoplasm as a single circular chromosome.

State whether it is prokaryotic or eukaryotic. (c) Name the organelle that modifies and packages proteins for secretion.

Exam tip

Key terms

Cell, the basic structural and functional unit of living organisms.
Organelle, a structure inside a cell with a specific function.
Nucleus, the organelle that contains DNA and controls the cell.
Tissue, a group of similar cells performing a shared function.
Cell differentiation, the process by which a cell becomes specialised for a role.

Source:SRC-DSKP-EN

Frequently asked questions

What organelles are found in a plant cell but not an animal cell?

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.

What is the main difference between a prokaryotic and a eukaryotic cell?

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.

How does a red blood cell's structure suit its 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.