What examiners actually want from cell structures
The classic question is 'state the function of structure X' for one mark, so you need a clean, function-linked phrase for every organelle. Write these as cause-and-effect, not vague labels.
- Nucleus: contains genetic material / DNA that controls cell activities and division.
- Cytoplasm: site where chemical reactions take place, controlled by enzymes.
- Cell membrane: controls what enters and leaves the cell; partially permeable.
- Mitochondria: site of aerobic respiration, releasing energy.
- Ribosomes: site of protein synthesis.
Notice the wording: respiration releases energy, it never 'makes' or 'produces' energy. Examiners cross that out. A common trap is writing that the membrane is 'fully permeable'; it is partially permeable, which is the exact phrase the mark scheme rewards. When asked to describe a structure, do not describe its function, and vice versa. Read the command word first.
Plant vs animal cells: the comparison that earns marks
Comparison questions want differences stated as matched pairs, not two separate lists. The features only in plant cells are the ones examiners reward:
| Feature | Plant cell | Animal cell |
|---|---|---|
| Cell wall (cellulose) | Present | Absent |
| Chloroplasts | Present (in some cells) | Absent |
| Large permanent vacuole | Present | Absent / small temporary only |
State the cell wall is made of cellulose and gives support. Both points score. Do not say 'plant cells have a vacuole and animal cells don't', because animal cells can have small temporary vacuoles; say 'large permanent vacuole'. The chloroplast contains chlorophyll and is the site of photosynthesis, linking neatly to photosynthesis.
Bacterial cells (and what's different)
A bacterial cell has a cell wall (not made of cellulose), cell membrane, cytoplasm and ribosomes, plus features that often appear in 'compare' questions: circular DNA / a single chromosome free in the cytoplasm (no nucleus), plasmids (small extra rings of DNA), and sometimes a flagellum or slime capsule. The mark-scheme distinction examiners look for is that bacteria have no nucleus and no mitochondria. A frequent slip is saying bacteria have 'no DNA'. They do, it is simply not enclosed in a nucleus. If a question asks why bacteria are used in biotechnology, plasmids are usually the answer they are steering you toward.
Specialised cells: structure linked to function
For specialised cells, every mark is a structure-to-function link. Learn these as 'feature → so that':
- Root hair cell: long extension increases surface area for water and mineral ion absorption.
- Red blood cell: biconcave shape and no nucleus give more room/surface area for haemoglobin to carry oxygen.
- Ciliated cell: cilia sweep mucus and trapped particles away.
- Nerve cell: long axon transmits impulses over distances.
- Sperm and egg cells: sperm has a tail/flagellum for swimming; the egg has food stores for the embryo.
- Palisade mesophyll cell: packed with chloroplasts for photosynthesis.
The hierarchy of organisation is also examined: cell → tissue → organ → organ system → organism. Define a tissue as a group of similar cells working together with a shared function. If you would like a tutor to drill these links with you, you can book a trial class.
Magnification and size calculations (where units cost marks)
Calculation questions are guaranteed marks if you control the triangle: Magnification = Image size ÷ Actual size (M = I ÷ A). Rearrange to find actual size: A = I ÷ M.
- Convert everything to the same unit first: usually millimetres or micrometres (1 mm = 1000 µm).
- Measure the image size with a ruler in mm.
- Divide by the magnification, then convert if the answer is asked for in µm.
The single biggest error is unit mismatch: dividing a millimetre image by a micrometre actual size gives an answer 1000 times wrong. (Extended only) Show your working. Even a wrong final answer can earn method marks. Magnification has no units because it is a ratio; never write '×500 µm'.
Key terms to connect with this topic
Use these definitions inside explanations and questions rather than memorising them as isolated sentences.
- Cell membrane: A thin, partially permeable barrier surrounding every living cell that controls which substances enter and leave. It is made of a phospholipid bilayer with embedded proteins and is present in both plant and animal cells.
- Cell wall: A rigid outer layer made of cellulose that surrounds the cell membrane of plant cells, bacterial cells and fungal cells. It provides structural support and prevents the cell from bursting when water enters by osmosis.
- Chloroplast: An organelle found in plant cells and some protoctist cells that contains the green pigment chlorophyll. It is the site of photosynthesis, where light energy is absorbed and used to make glucose from carbon dioxide and water.
- Cytoplasm: The jelly-like material that fills the cell between the cell membrane and the nucleus. Most chemical reactions of metabolism take place here, including many enzyme-controlled reactions.
- Magnification: The number of times larger an image is compared with the actual (real) size of the object.
- Mitochondrion: An organelle with a folded inner membrane where aerobic respiration takes place, releasing energy from glucose. Cells with high energy demands, such as muscle cells and sperm cells, contain large numbers of mitochondria.
- Nucleus: A large organelle enclosed by a nuclear envelope that contains the cell's chromosomes, made of DNA. It controls the activities of the cell by determining which proteins are made.
- Ribosome: A tiny organelle found in the cytoplasm where protein synthesis occurs. Ribosomes read the instructions from mRNA and assemble amino acids into polypeptide chains.
- Vacuole: A fluid-filled space enclosed by a membrane called the tonoplast. In plant cells, a large permanent vacuole contains cell sap (a solution of sugars, salts and pigments) and helps maintain turgor pressure. Animal cells may have small temporary vacuoles.
Browse the full IGCSE Biology glossary when a related term needs checking.
Apply the topic in practical work
These practical guides show how the Biology appears in methods, variables, measurements, graphs and evaluation.
- Biological Drawings and Magnification Calculations: Use clear single lines, no shading, large proportions and ruled label lines. For calculations, magnification = image size ÷ actual size, with both sizes converted to the same unit first.
- Using a Light Microscope: Set up the microscope with the lowest power objective first, focus using the coarse then fine adjustment, and always start viewing at low power before switching to higher magnification. Biological drawings must be accurate, with clear lines, correct proportions, and magnification stated.