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 free 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'.