Explain why plants need light in order to carry out photosynthesis.

AQA GCSE Biology (8461) — 4.4.1 Photosynthesis · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Plants are found in a variety of environments, from dense forests to open fields. In all these environments, plants rely on photosynthesis to produce food.

Model answer (4 marks)

Light supplies the energy required for photosynthesis.

The overall reaction is endothermic, so energy must be taken in from the surroundings.

Without light, CO₂ and H₂O cannot be converted into glucose and O₂.

The energy is absorbed by chlorophyll in the chloroplasts of leaf cells.

Examiner tips

Use the word ‘energy’ and ‘endothermic’ to show understanding of the reaction type.
Mention chlorophyll and chloroplasts to link light absorption to the process.
Explain the consequence of no light – no glucose/O₂ production.

Common mistakes

Saying ‘light is necessary’ without explaining the energy transfer.
Confusing light with heat or not mentioning endothermic nature.
Omitting chlorophyll or chloroplasts from the explanation.

Mark scheme (4 marks)

Light provides the energy for photosynthesis
Photosynthesis is an endothermic reaction (so energy must be transferred in from the environment)
Without light, carbon dioxide and water cannot be converted into glucose (and oxygen)
Light is absorbed by chloroplasts / chlorophyll in the leaves

Key terms in this question

photosynthesis