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Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)
Question
Write the balanced symbol equation for photosynthesis and show by counting atoms that it is balanced. (4 marks)
Solution
Recall the balanced equation.
Count carbons: 6 on the left (in CO₂), 6 on the right (in glucose). Carbon balances.
Count hydrogens: 12 on the left (6 × 2 from water), 12 on the right (in glucose). H balances.
Count oxygens: 12 (in CO₂) + 6 (in water) = 18 on the left; 6 (in glucose) + 12 (in O₂) = 18 on the right. O balances.
Answer
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. C: 6=6, H: 12=12, O: 18=18 — all atoms balance.
Question
Explain why photosynthesis is described as an endothermic reaction. (2 marks)
Solution
Energy is transferred IN (not out).
The energy comes from light and is stored in the chemical bonds of the glucose produced.
Answer
Photosynthesis is endothermic because energy is taken IN from the surroundings (1). Light energy is absorbed by chlorophyll and stored in the chemical bonds of glucose (1).
Question
A pondweed produces 20 bubbles per minute when a lamp is 10 cm away. Calculate the expected number of bubbles when the lamp is moved to 30 cm away (assuming all other factors stay constant). (3 marks)
Solution
Distance trebles: 10 cm → 30 cm. By inverse-square, intensity falls by factor 3² = 9.
Rate is proportional to intensity (assuming light is the limiting factor), so bubbles = 20/9.
20/9 ≈ 2.2 bubbles per minute.
Answer
≈ 2.2 bubbles per minute (intensity falls to 1/9 of original).
Question
A greenhouse is brightly lit but the air contains only 0.04% CO₂. Suggest the limiting factor and ONE thing the grower could do to increase the rate. (2 marks)
Solution
With plenty of light and average CO₂, CO₂ is likely the limiting factor.
Burn paraffin heaters (or pump in CO₂) to raise the indoor concentration.
Answer
Carbon dioxide is the limiting factor (1). The grower could burn paraffin heaters or pump in CO₂ to raise the concentration (1).
Question
State FIVE ways a plant uses the glucose it makes by photosynthesis. (5 marks)
Solution
Respiration — releases energy.
Stored as starch (insoluble storage).
Used to make cellulose for cell walls.
Used to make lipids (stored in seeds).
Combined with nitrate ions to make amino acids and proteins.
Answer
(1) Respiration; (2) converted to starch for storage; (3) used to make cellulose for cell walls; (4) used to make lipids in seeds; (5) combined with nitrate ions to make amino acids/proteins.
Question
Plants convert glucose to starch for long-term storage. Explain why storing the carbohydrate as starch is better than storing it as glucose. (3 marks)
Solution
Starch is insoluble in water.
So it does not affect the water potential of the cell — no osmotic effect.
It also doesn't dissolve and leak away, and packs into dense granules.
Answer
Starch is insoluble (1) so does not affect osmosis or water potential of the cell (1). It also stays where it is stored, packed into dense granules (1).
Examiner note
Always say 'insoluble' AND 'osmosis' for top marks.
The endothermic reaction in chloroplasts that uses light energy to convert carbon dioxide and water into glucose and oxygen.
The green pigment in chloroplasts that absorbs light energy for photosynthesis.
A reaction that takes in energy from the surroundings — temperature of the surroundings falls.
The factor (e.g. light, CO₂, temperature) in shortest supply, which limits the rate of a reaction. Increasing it raises the rate; increasing any other factor will not.
Light intensity from a point source is inversely proportional to the square of the distance: I ∝ 1/d².
Permanent change in shape of an enzyme's active site so that it can no longer bind its substrate. Usually caused by high temperatures (above ~45 °C) or extremes of pH.
An insoluble polymer of glucose used by plants for storage; tests blue-black with iodine solution.
An insoluble polymer of glucose forming strong fibres in plant cell walls; gives cells their rigid shape.
Mineral ions absorbed from soil by root hair cells via active transport. Provide nitrogen for amino acid (and protein) synthesis.
Mistake
Saying 'chloroplast absorbs light'.
Why it happens
Mixing the names.
How to avoid it
Chloroplast = ORGANELLE. Chlorophyll = PIGMENT inside the organelle. Chlorophyll absorbs light.
Mistake
Writing CO₂ + H₂O → C₆H₁₂O₆ + O₂ (no 6s).
Why it happens
Forgetting balancing.
How to avoid it
Add the 6's: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Mistake
Calling photosynthesis exothermic.
Why it happens
Confusing with respiration.
How to avoid it
Photosynthesis IN-takes energy = endothermic. Respiration RELEASES energy = exothermic.
Mistake
Saying 'doubled distance → halved intensity'.
Why it happens
Students forget to square.
How to avoid it
I ∝ 1/d² — double the distance gives 1/4 the intensity, not 1/2.
Mistake
Drawing a plateau for temperature like the other factors.
Why it happens
Habit from light and CO₂ graphs.
How to avoid it
Temperature rises to an OPTIMUM then falls sharply (enzymes denature). Not a plateau.
Mistake
Saying 'the rate slows because of less light' instead of 'light is the limiting factor'.
Why it happens
Students avoid the AQA keyword.
How to avoid it
Use the phrase 'limiting factor' explicitly — it picks up marks.
Mistake
Saying plants absorb amino acids or proteins from soil.
Why it happens
Confusion with how animals get protein.
How to avoid it
Plants absorb NITRATE IONS, then combine them with glucose inside the plant to MAKE amino acids.
Mistake
Saying starch is soluble.
Why it happens
Mixing up starch and glucose.
How to avoid it
Glucose = soluble (sweet sugar). Starch = INSOLUBLE polymer of glucose. That's the whole point of storing as starch.
Mistake
Listing only 'respiration' as the use of glucose.
Why it happens
Students remember the main one.
How to avoid it
Always list all FIVE on an open-ended question. Most exam mark schemes credit each correct answer.