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AQA GCSE Biology

Photosynthesis — AQA GCSE Biology revision

Free revision notes, key terms, common exam traps and 5 practice questions with answers. About 4 minutes to read.

The reaction

  • Carbon dioxide + water → glucose + oxygen (needs light energy, happens in chloroplasts).
  • It is endothermic: energy is transferred from the environment.

Limiting factors

  • Light intensity, carbon dioxide concentration and temperature can each limit the rate.
  • Light intensity follows the inverse square law: doubling the distance quarters the intensity.
  • Glucose is used for respiration, stored as starch, converted to cellulose, or made into proteins with nitrate ions.

Key terms

Limiting factor
The factor currently stopping the rate increasing.
Endothermic
Energy is taken in from the surroundings.
Chlorophyll
Green pigment in chloroplasts that absorbs light.
Photosynthesis
The process where plants use light energy to convert CO2 and water into glucose and oxygen.
Compensation point
The point where the rate of photosynthesis equals the rate of respiration.
Stomata
Tiny pores, mostly on leaf undersides, that allow gas exchange in and out of the leaf.

Common exam traps

  • Writing 'sunlight' as a reactant in the equation — it's a condition, not a reactant.
  • Saying temperature always increases rate — beyond the optimum, enzymes denature.
  • Forgetting plants respire 24/7 as well as photosynthesising.

Practice questions with answers

  1. 1. Write the word equation for photosynthesis.

    Answer: Carbon dioxide + water → glucose + oxygen

    Light and chlorophyll go above the arrow, not in the reactants.

  2. 2. A graph of rate vs light intensity levels off. What does this suggest?

    • • Light is still limiting
    • • Another factor such as CO₂ or temperature is now limiting
    • • The plant has died
    • • Respiration has stopped

    Answer: Another factor such as CO₂ or temperature is now limiting

    A plateau means light is no longer the limiting factor.

  3. 3. Give two uses of the glucose made in photosynthesis.

    Answer: Respiration and storage as starch (also cellulose, or amino acids with nitrates).

    Any two correct uses.

  4. 4. Which gas is a product of photosynthesis?

    • • Carbon dioxide
    • • Oxygen
    • • Nitrogen
    • • Hydrogen

    Answer: Oxygen

    Photosynthesis releases oxygen as a by-product.

  5. 5. Write the word equation for photosynthesis.

    Answer: Carbon dioxide + water → glucose + oxygen (using light energy).

    Light energy is absorbed by chlorophyll to drive this reaction.

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