What you'll be able to do by the end
- Compare eukaryotic and prokaryotic cells
- Describe cell specialisation and differentiation
- Use microscopes and calculate magnification
- Describe mitosis and the cell cycle
- Explain the uses and issues around stem cells
- Explain diffusion, osmosis and active transport
- Explain how surface area to volume ratio affects exchange
- Describe the required practical on osmosis
- Describe photosynthesis and the factors limiting it
- Explain aerobic and anaerobic respiration
Part 1 — Cell structure
Eukaryotic and prokaryotic
| Eukaryotic | Prokaryotic | |
|---|---|---|
| Examples | Animals, plants, fungi | Bacteria |
| Nucleus | Yes | No — DNA is a single loop in the cytoplasm |
| Size | 10–100 µm | 1–5 µm |
| Mitochondria | Yes | No |
| Cell wall | Plants and fungi only | Yes — but not made of cellulose |
| Plasmids | No | Often |
Animal cell
| Part | Function |
|---|---|
| Nucleus | Contains genetic material controlling the cell's activities |
| Cytoplasm | Where most chemical reactions occur |
| Cell membrane | Controls what enters and leaves |
| Mitochondria | Site of aerobic respiration — releases energy |
| Ribosomes | Site of protein synthesis |
Plant cell
Everything above, plus:
| Part | Function |
|---|---|
| Cell wall | Made of cellulose — strengthens the cell |
| Chloroplasts | Contain chlorophyll; site of photosynthesis |
| Permanent vacuole | Contains cell sap; keeps the cell turgid |
Bacterial cell
- Cytoplasm, cell membrane, cell wall
- Single circular strand of DNA
- Plasmids — small rings of DNA
- Ribosomes
- No nucleus, no mitochondria, no chloroplasts
"Bacteria have no DNA." They do. It just isn't in a nucleus.
"Bacteria have a cell wall like plants." They have a wall, but not made of cellulose.
"The cell wall controls what enters the cell." That's the membrane. The wall is structural.
Part 2 — Specialisation and differentiation
Differentiation is the process by which a cell becomes specialised for a particular function.
In animals, most cells differentiate early and lose the ability to change. In plants, many cells retain the ability to differentiate throughout life.
| Cell | Adaptation | Function |
|---|---|---|
| Sperm | Tail, many mitochondria, enzymes in the head | Swims to the egg, penetrates it |
| Nerve | Long axon, branched endings, myelin sheath | Carries impulses over distance |
| Muscle | Protein fibres that contract, many mitochondria | Movement |
| Root hair | Long projection, large surface area, no chloroplasts | Absorbs water and minerals |
| Xylem | Hollow dead tubes, no end walls, lignin | Transports water upwards |
| Phloem | Living cells, sieve plates, companion cells | Transports dissolved sugars |
Never just name the adaptation. Link it to the function.
❌ "Sperm cells have a tail."
✅ "Sperm cells have a tail so they can swim to the egg, and many mitochondria to release the energy needed for that."
Part 3 — Microscopy
Light and electron microscopes
| Light | Electron | |
|---|---|---|
| Magnification | Up to ×2000 | Up to ×2,000,000 |
| Resolution | ~200 nm | ~0.2 nm |
| Specimens | Can be living | Must be dead |
| Cost and size | Cheap, portable | Very expensive, large |
| Can see | Cells, nuclei, chloroplasts | Sub-cellular structures, ribosomes, viruses |
Resolution is the ability to distinguish two points as separate. Higher resolution means more detail.
Electron microscopes revolutionised biology because they revealed structures — ribosomes, membrane detail, viruses — that no light microscope could resolve.
Magnification calculations
Worked example. A cell has a real diameter of 0.05 mm. It appears 20 mm wide in a photograph.
magnification = 20 ÷ 0.05 = ×400
Worked example with units. A cell appears 45 mm wide at ×1500 magnification. Give its real size in micrometres.
real size = 45 ÷ 1500 = 0.03 mm 0.03 mm × 1000 = 30 µm
Units
| Unit | Symbol | Relative to a metre |
|---|---|---|
| Millimetre | mm | 10⁻³ |
| Micrometre | µm | 10⁻⁶ |
| Nanometre | nm | 10⁻⁹ |
mm → µm: × 1000 · µm → nm: × 1000
Mixing millimetres and micrometres in one calculation produces an answer out by a factor of a thousand. Get everything into the same unit first.
Required practical — using a light microscope
- Prepare the slide — a thin specimen, a drop of stain, lower the coverslip at an angle to avoid air bubbles
- Clip the slide onto the stage
- Select the lowest power objective
- Use the coarse focus to bring the specimen into view
- Sharpen with the fine focus
- Switch to a higher power and refocus using the fine focus only
- Draw with a sharp pencil, using clean unbroken lines, with a magnification and a scale
Common stains: iodine for plant cells, methylene blue for animal cells. Stains make structures visible that are otherwise transparent.
Part 4 — Cell division
The cell cycle
| Stage | What happens |
|---|---|
| Growth and DNA replication | The cell grows, sub-cellular structures increase in number, DNA replicates to form two copies of each chromosome |
| Mitosis | One set of chromosomes is pulled to each end of the cell; the nucleus divides |
| Cytokinesis | The cytoplasm and cell membrane divide to form two identical cells |
Mitosis produces two genetically identical daughter cells.
Used for: growth, repair of damaged tissue, replacement of worn-out cells, and asexual reproduction.
Mitosis and meiosis
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Cells produced | 2 | 4 |
| Chromosome number | Same as parent | Halved |
| Genetically | Identical | All different |
| Purpose | Growth and repair | Making gametes |
Stem cells
A stem cell is an undifferentiated cell capable of dividing to produce more cells of the same type, or differentiating into other cell types.
| Type | Source | Can become |
|---|---|---|
| Embryonic | Early human embryos | Almost any cell type |
| Adult | Bone marrow and other tissues | A limited range — mostly blood cells |
| Plant meristem | Growing tips of roots and shoots | Any plant cell type, throughout life |
Therapeutic cloning
An embryo is produced with the same genes as the patient, so stem cells taken from it are not rejected by the patient's immune system.
Potential uses: treating diabetes, paralysis, Parkinson's disease, replacing damaged tissue.
The arguments
| For | Against |
|---|---|
| Could cure conditions currently untreatable | Embryos are destroyed, which some consider ethically unacceptable |
| Could relieve serious suffering | Risk of viral transmission from cultured cells |
| Unused IVF embryos would otherwise be discarded | Long-term effects are not yet known |
| Adult stem cells are already used successfully | Some object on religious grounds |
This is an evaluate question and it will be levels-marked. You must give both sides and reach a judgement. There is no correct opinion. The marks are for the quality of the argument.
Plant stem cells
Meristem tissue in plants can differentiate into any plant cell type, throughout the plant's life.
Uses:
- Producing clones of rare species quickly, protecting them from extinction
- Producing large numbers of identical crop plants with desirable characteristics — disease resistance, high yield
Part 5 — Transport in and out of cells
Three mechanisms. Knowing which is which is worth marks on every paper.
| Diffusion | Osmosis | Active transport | |
|---|---|---|---|
| What moves | Any dissolved substance or gas | Water only | Dissolved substances |
| Direction | High → low concentration | Dilute → concentrated solution | Low → high concentration |
| Down or against gradient | Down | Down (for water) | Against |
| Energy required | No | No | Yes — from respiration |
| Membrane needed | No | Partially permeable | Carrier proteins |
Diffusion
The spreading out of particles of any substance in solution or a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
Examples: oxygen and carbon dioxide in gas exchange · urea from cells into blood plasma · digested food from the small intestine into the blood
Rate of diffusion increases with:
- A steeper concentration gradient
- A higher temperature — particles have more kinetic energy
- A larger surface area of the membrane
Osmosis
The movement of water from a dilute solution to a concentrated solution through a partially permeable membrane.
Water — not "particles" or "substances"
From dilute to concentrated — or "down its concentration gradient"
Through a partially permeable membrane
Missing any one loses a mark. Learn it as a single sentence.
What happens to cells:
| Solution | Animal cell | Plant cell |
|---|---|---|
| Dilute (water enters) | Swells and may burst — lysis | Becomes turgid — the wall prevents bursting |
| Concentrated (water leaves) | Shrinks and crenates | Becomes flaccid, then plasmolysed — membrane pulls away from the wall |
Plants rely on turgor for support. A wilting plant has flaccid cells.
Active transport
The movement of substances from a more dilute solution to a more concentrated solution — against a concentration gradient — using energy from respiration.
Examples:
- Root hair cells absorbing mineral ions from very dilute soil solution
- The small intestine absorbing glucose when its concentration in the gut is lower than in the blood
Moving substances against a gradient requires energy, and that energy comes from respiration. This is why root hair cells and intestinal cells contain many mitochondria — a detail worth a mark.
Part 6 — Surface area to volume ratio
As an organism gets larger, its volume increases faster than its surface area.
Worked example
| Cube side | Surface area | Volume | SA:V ratio |
|---|---|---|---|
| 1 cm | 6 cm² | 1 cm³ | 6 : 1 |
| 2 cm | 24 cm² | 8 cm³ | 3 : 1 |
| 3 cm | 54 cm² | 27 cm³ | 2 : 1 |
The bigger the object, the smaller its surface area to volume ratio.
Why it matters
Single-celled organisms have a large SA:V ratio and can exchange everything they need by diffusion across their surface.
Multicellular organisms have a small SA:V ratio. Diffusion across the body surface is far too slow, so they need specialised exchange surfaces and a transport system.
Effective exchange surfaces share three features
Large surface area · Thin membrane, giving a short diffusion path · A good blood supply, or ventilation, to maintain the concentration gradient
| Surface | How it achieves this |
|---|---|
| Alveoli | Millions of them; walls one cell thick; surrounded by capillaries; ventilated by breathing |
| Small intestine | Villi and microvilli; wall one cell thick; rich blood supply |
| Gills | Filaments and lamellae; thin; water flows over them continuously |
| Root hair cells | Long projections; thin wall; water constantly transported away |
| Leaves | Flattened shape; air spaces; stomata |
Large surface area, thin wall, maintained gradient. Five surfaces. One principle. Learn it once and answer any exchange question on the paper.
Part 7 — Required practical: osmosis
Aim: to investigate the effect of sugar or salt solution concentration on plant tissue.
Method
- Use a cork borer to cut potato cylinders of equal diameter
- Cut them to equal length and blot dry
- Measure and record the mass of each
- Place each in a different concentration of sugar solution — for example 0.0, 0.2, 0.4, 0.6, 0.8, 1.0 mol/dm³
- Leave for a set time, ideally 24 hours
- Remove, blot dry and re-measure the mass
- Calculate the percentage change in mass
Why percentage change and not just change
Because the cylinders may not have started at exactly the same mass. Percentage change allows fair comparison.
% change = (change in mass ÷ starting mass) × 100
Results
| Solution | Water movement | Mass change |
|---|---|---|
| Dilute (low concentration) | Water enters the cells | Increases |
| Concentrated | Water leaves the cells | Decreases |
| Same concentration as the cell contents | No net movement | No change |
The point where the graph crosses zero gives the concentration inside the potato cells. That's usually the question.
Control variables
Temperature · time in solution · volume of solution · surface area and size of the cylinders · same potato · blotting technique
Cylinders must be blotted before and after, in the same way. Surface water adds mass and has nothing to do with osmosis. This is a favourite "improve the method" question.
Part 8 — Photosynthesis
Photosynthesis is endothermic — energy is transferred from the environment to the chloroplasts.
What the glucose is used for
| Use | Detail |
|---|---|
| Respiration | Releasing energy |
| Converted to starch | For storage — insoluble, so it doesn't affect osmosis |
| Converted to cellulose | To strengthen cell walls |
| Converted to lipids | For storage in seeds |
| Used with nitrate ions to make amino acids | For protein synthesis |
Limiting factors
A limiting factor is the factor in shortest supply, which limits the rate of photosynthesis.
| Factor | Effect |
|---|---|
| Light intensity | Rate increases with light until another factor becomes limiting |
| Carbon dioxide concentration | Rate increases with CO₂ until another factor becomes limiting |
| Temperature | Rate increases until enzymes begin to denature, then falls sharply |
| Amount of chlorophyll | Reduced by disease or lack of magnesium |
Reading the graph
A photosynthesis graph rises and then plateaus.
The rising part: the factor on the x-axis is limiting.
The plateau: something else has become limiting.
The temperature graph is different — it rises, peaks, then falls, because enzymes denature. Light and CO₂ graphs plateau; temperature graphs peak.
Inverse square law
Light intensity is inversely proportional to the square of the distance from the source.
Double the distance → one quarter the light intensity.
Triple the distance → one ninth.
Economics of greenhouses
Growers can control the limiting factors:
| Method | Effect | Cost |
|---|---|---|
| Artificial lighting | Extends the growing day | Electricity |
| Paraffin heaters | Raise temperature and release CO₂ | Fuel |
| Enclosed space | Retains heat and CO₂ | Structure |
The decision is economic: growers increase a factor only while the extra yield is worth more than the extra cost.
Part 9 — Respiration
Respiration is an exothermic reaction occurring continuously in all living cells, transferring energy from glucose.
Breathing is ventilation — moving air in and out of the lungs.
Respiration is a chemical reaction inside every cell.
Plants respire. Bacteria respire. They do not breathe.
Aerobic respiration
Occurs in the mitochondria. Releases much more energy per glucose molecule than anaerobic respiration.
Anaerobic respiration in muscle
- Incomplete oxidation of glucose
- Releases much less energy
- Lactic acid builds up, causing muscle fatigue
Anaerobic respiration in yeast and plants
This is fermentation, and it is economically important in brewing and baking.
Response to exercise
During exercise the body needs more energy, so:
- Heart rate increases — delivering blood faster
- Breathing rate and depth increase — supplying more oxygen and removing more CO₂
If exercise is intense, insufficient oxygen reaches the muscles and anaerobic respiration occurs.
Oxygen debt
The amount of extra oxygen the body needs after exercise to react with accumulated lactic acid and remove it from the cells.
Blood carries lactic acid to the liver, where it is converted back to glucose.
This is why you continue breathing heavily after stopping — you are repaying the oxygen debt.
Exam-style questions
Answer all questions. Marks are shown in brackets. Total: 81 marks.
Question 1
(Total 10 marks)
Question 2
(Total 7 marks)
Question 3
(Total 10 marks)
Question 4
(Total 12 marks)
Question 5
A student investigated osmosis using potato cylinders in different sugar concentrations.
(Total 9 marks)
Question 6
(Total 9 marks)
Question 7
(Total 14 marks)
Question 8
(Total 10 marks)
TOTAL FOR PAPER: 81 MARKS
Mark scheme
Question 1
(a) 3 — any three: prokaryotes have no nucleus, DNA is a free loop · prokaryotes have plasmids · prokaryotes have no mitochondria · prokaryotes are much smaller · prokaryotic cell walls are not made of cellulose.
(b) 4 — any two, 2 marks each:
- Cell wall (1) — made of cellulose, strengthens and supports the cell (1)
- Chloroplasts (1) — contain chlorophyll, site of photosynthesis (1)
- Permanent vacuole (1) — contains cell sap, keeps the cell turgid (1)
| (c) 3. Mitochondria are the site of aerobic respiration | 1 |
| Respiration releases energy | 1 |
| The sperm needs energy to swim to the egg | 1 |
Question 2
(a) 2 — 32 ÷ 0.08 (1) = ×400 (1).
| (b) 3. 60 ÷ 2000 = 0.03 mm | 1 |
| × 1000 | 1 |
| = 30 µm | 1 |
(c) 2 — any two: much higher magnification · much higher resolution · can see sub-cellular structures such as ribosomes and viruses.
Question 3
| (a) 4. The cell grows and sub-cellular structures increase in number | 1 |
| DNA replicates, forming two copies of each chromosome | 1 |
| In mitosis, one set of chromosomes is pulled to each end and the nucleus divides | 1 |
| The cytoplasm and membrane divide, producing two identical cells | 1 |
(b) 2 — any two: growth · repair of damaged tissue · replacement of worn-out cells.
(c) 2 — an undifferentiated cell (1) able to divide to produce more of the same type, or to differentiate into other cell types (1).
| (d) 2. The embryo produced has the same genes as the patient | 1 |
| So the cells are not rejected by the patient's immune system | 1 |
Question 4
(a) 3 — the movement of water (1) from a dilute to a concentrated solution (1) through a partially permeable membrane (1).
| (b) 3. Water moves out of the cell by osmosis | 1 |
| The cell becomes flaccid | 1 |
| With further water loss the membrane pulls away from the cell wall — plasmolysis | 1 |
| (c) 3. Water enters both cells by osmosis | 1 |
| The animal cell has only a membrane, which cannot withstand the pressure, so it bursts | 1 |
| The plant cell has a rigid cellulose cell wall which resists the pressure, so it becomes turgid instead | 1 |
| (d) 3. The concentration of mineral ions in the soil is lower than inside the root hair cell | 1 |
| So the ions must move against the concentration gradient | 1 |
| This requires energy from respiration, which diffusion cannot provide | 1 |
Question 5
(a) 3 — any three: temperature · time in solution · volume of solution · size and surface area of cylinders · same potato · same blotting method.
| (b) 2. Surface water would add to the measured mass | 1 |
| This is not due to osmosis, so it would make the results inaccurate | 1 |
| (c) 2. The cylinders may not all have started with the same mass | 1 |
| Percentage change allows a fair comparison between them | 1 |
| (d) 2. At this concentration there is no net movement of water | 1 |
| So the concentration inside the potato cells is 0.35 mol/dm³ | 1 |
Question 6
| (a) 3. Surface area = 6 × (2 × 2) = 24 cm² | 1 |
| Volume = 2 × 2 × 2 = 8 cm³ | 1 |
| Ratio = 3 : 1 | 1 |
| (b) 3. Large organisms have a small surface area to volume ratio | 1 |
| Diffusion across the body surface would be far too slow to supply all the cells | 1 |
| So specialised exchange surfaces and a transport system are needed | 1 |
(c) 3 — large surface area (1) · thin membrane giving a short diffusion path (1) · good blood supply or ventilation to maintain the concentration gradient (1).
Question 7
(a) 2 — carbon dioxide + water → glucose + oxygen (1 for reactants, 1 for products).
(b) 2 — endothermic (1) — energy is transferred from the environment to the chloroplasts (1).
(c) 3 — any three: respiration · converted to starch for storage · converted to cellulose for cell walls · converted to lipids for storage · used with nitrate ions to make amino acids.
| (d) 4. At low light intensity, light is the limiting factor | 1 |
| As light increases, the rate of photosynthesis increases | 1 |
| At the plateau, light is no longer limiting | 1 |
| Another factor — carbon dioxide concentration or temperature — has become limiting | 1 |
| (e) 3. Increasing temperature increases the rate because enzymes and substrates have more kinetic energy | 1 |
| Above the optimum, enzymes begin to denature | 1 |
| The active site changes shape so the reaction slows and the rate falls | 1 |
Question 8
(a) 2 — glucose + oxygen → carbon dioxide + water (1 for reactants, 1 for products).
(b) 2 — any two: aerobic uses oxygen, anaerobic does not · aerobic releases much more energy · aerobic produces CO₂ and water, anaerobic in muscle produces lactic acid · aerobic completely oxidises glucose, anaerobic does not.
| (c) 4. Oxygen debt is the extra oxygen required after exercise | 1 |
| To react with the lactic acid that has accumulated | 1 |
| Blood transports lactic acid to the liver | 1 |
| Where it is converted back into glucose | 1 |
(d) 2 — breathing is the movement of air in and out of the lungs (1); respiration is a chemical reaction in cells releasing energy from glucose (1).
Common mistakes in this topic
"Bacteria have no DNA." They have DNA; it isn't in a nucleus.
Incomplete osmosis definition. Water, dilute to concentrated, partially permeable membrane. All three.
Saying osmosis moves "particles." It moves water, and only water.
"Active transport moves things down the gradient." It moves them against it, using energy.
Confusing breathing and respiration. One is air movement; one is a chemical reaction in every cell.
Saying plants only photosynthesise and don't respire. Plants respire continuously, day and night.
Mixing units in magnification calculations. Convert first.
Naming a limiting factor without explaining the plateau. The plateau means something else has become limiting.
Teaching notes
The three transport mechanisms should be taught as a single comparison table, not three separate lessons. Students confuse them because they meet them separately. Side by side, the distinctions are obvious: what moves, which direction, does it need energy.
The osmosis definition should be memorised word for word. It is three marks, it appears constantly, and partial definitions score partially.
Surface area to volume ratio is the idea that explains five topics. Alveoli, villi, gills, root hairs, leaves — all the same principle. Do the cube calculation properly so students see why size forces the change, then apply it everywhere.
The required practical rewards method detail, not results. Blotting, controlling temperature, using percentage change — these are what the questions ask about. Teach the reasoning behind each control variable rather than just listing them.
The photosynthesis graph shapes matter. Light and CO₂ plateau; temperature peaks and falls. Students who know why temperature is different — enzyme denaturation — can explain any graph on the paper.
"Respiration is not breathing" needs saying every lesson for a fortnight. It is one of the most persistent errors in the whole subject, and students who confuse the two lose marks in three separate topics.
Written for GCSE Biology, all major UK boards. Check your specification — stem cell detail, the inverse square law and some respiration content vary by board and tier. Reviewed 2026.