What Is Cell Structure? A Visual Guide for IGCSE Biology Students

The building block of every living thing

Every organism on Earth, from a single bacterium in a pond to a blue whale crossing the Pacific, is built from cells. A cell is the smallest unit of life capable of carrying out the basic processes that keep an organism alive: taking in nutrients, releasing energy, growing, and reproducing. Some organisms consist of just one cell (unicellular), while others are made of trillions working together (multicellular). Understanding cell structure is one of the first and most important steps in your IGCSE Biology course, and it appears in almost every exam session.

Key facts at a glance

• All living things are made of cells.
• Animal cells have a cell membrane, nucleus, cytoplasm, mitochondria and ribosomes.
• Plant cells have all of the above plus a cell wall, chloroplasts and a permanent vacuole.
• Bacterial cells have no nucleus. Their DNA is a single circular loop in the cytoplasm, often with small extra rings called plasmids.
• New cells are produced by the division of existing cells.
• Specialised cells have structures adapted for particular functions.

Animal cell structures and their functions

An animal cell is the standard reference point in IGCSE Biology. If you can draw and label one from memory, you are already well placed for several marks across Papers 3, 4, 5 and 6.

Plant cell structures

A plant cell contains everything an animal cell has, plus three additional structures. These extra features are what allow plants to photosynthesise, maintain their shape without a skeleton, and store water and dissolved substances.

Bacterial cell structure

Bacteria are fundamentally different from animal and plant cells. The most important difference, and the one examiners test most often, is that bacteria have no nucleus. Their genetic material floats freely in the cytoplasm.

• Cell wall: present, but not made of cellulose (a different composition from plant cell walls).
• Cell membrane: inside the cell wall, controls entry and exit of substances.
• Cytoplasm: where chemical reactions occur.
• Ribosomes: smaller than those in animal or plant cells, but still the site of protein synthesis.
• Circular DNA: a single loop of DNA, not enclosed in a nucleus.
• Plasmids: small, extra rings of DNA that can carry genes for things like antibiotic resistance.

Notice what is absent: no mitochondria, no chloroplasts, no nucleus. Bacteria are classified as prokaryotes precisely because they lack a membrane-bound nucleus. Animal and plant cells, by contrast, are eukaryotes.

Comparing the three cell types

This comparison table is one of the highest-value revision tools for this topic. If you can reproduce it from memory, you can answer almost any cell structure comparison question.

From cells to organ systems: levels of organisation

In multicellular organisms, cells do not work alone. They are organised into increasingly complex levels, each building on the one before. For your IGCSE exam, you need to know this hierarchy clearly:

• Cell: the basic unit of life.
• Tissue: a group of cells with a similar structure and function working together (for example, muscle tissue).
• Organ: a structure made of different tissues working together to perform a specific job (for example, the heart contains muscle tissue, connective tissue and nervous tissue).
• Organ system: a group of organs working together to carry out a major body function (for example, the circulatory system includes the heart, blood vessels and blood).
• Organism: a complete living thing made of multiple organ systems working in coordination.

This hierarchy applies to both animals and plants. In a plant, for instance, palisade mesophyll cells form photosynthetic tissue, which is part of the leaf (an organ), which belongs to the shoot system (an organ system). Examiners in the Cambridge IGCSE frequently ask students to arrange these levels in order or to give examples at each level, so practise until the sequence is automatic.

Specialised cells: when structure follows function

Not every cell looks like the textbook diagrams above. Once a cell differentiates (develops specialised structures for a particular job), its shape, size and internal features change to match its role. This principle, that structure is adapted to function, runs through the entire IGCSE Biology syllabus.

How to draw and label cells in exams

Biological drawing is assessed in Papers 5 and 6. The examiners are not looking for artistic talent; they want accuracy, clarity and correct labelling. European and international schools that follow Cambridge programmes tend to emphasise these practical skills from the start, and for good reason: marks for drawing are among the easiest to earn if you follow the rules.

1. Use a sharp pencil. Never use a pen for biological drawings. Lines must be clean, continuous and unbroken.
2. Draw large enough. Your drawing should fill at least half the available space. Small, cramped diagrams lose marks.
3. No shading, no colouring. Represent differences in tone with dots (stippling) if absolutely necessary, but the default is clean outlines only.
4. Label with straight, horizontal lines. Use a ruler. Each label line must touch the structure it identifies and must not cross another label line.
5. Include a title. State what the drawing shows and, if you used a microscope, the magnification.
6. Proportions matter. If the nucleus takes up roughly a quarter of the cell in the specimen, it should take up roughly a quarter in your drawing.

Common exam question patterns

Cell structure questions appear in predictable formats. Knowing the pattern in advance lets you plan your answer before you write.

• "Identify structure X": a labelled diagram is shown, and you name the part indicated by an arrow. One word is enough.
• "State the function of...": a brief, precise sentence. "The mitochondria are the site of aerobic respiration" is perfect; "they give the cell energy" is too vague.
• "Compare an animal cell and a plant cell": give similarities AND differences. Use the comparison table as your mental checklist.
• "Explain why cell X is adapted for its function": name the structural feature, then link it to the function with "because" or "which means that."
• "Calculate the magnification": use the formula magnification = image size / actual size. Convert units first (1 mm = 1000 um).

Magnification and size calculations

The formula you need is straightforward:

Magnification = Image size / Actual size

Rearranging: Actual size = Image size / Magnification

Unit conversions you should know by heart:

• 1 mm = 1000 um (micrometres)
• 1 um = 1000 nm (nanometres)
• 1 mm = 1,000,000 nm

Self-check questions

1. Name three structures found in plant cells but not in animal cells.
2. What is the function of the mitochondria?
3. Explain why bacterial cells are classified as prokaryotes.
4. A root hair cell has a long, thin extension. Explain how this adaptation helps the cell carry out its function.
5. A cell image measures 45 mm across. The actual cell is 0.03 mm wide. Calculate the magnification.
6. State two differences between a cell wall and a cell membrane.
7. Why do red blood cells have no nucleus?