Explain why plants need both light and chlorophyll to carry out photosynthesis.

WJEC GCSE Biology (Wales) — 1.5.2 Photosynthesis · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Light supplies the energy required for photosynthesis.
Chlorophyll absorbs this light energy.
The absorbed energy drives the conversion of CO₂ and H₂O into glucose and O₂.
Without chlorophyll, light cannot be captured, so photosynthesis cannot occur.

Examiner tips

Use the word 'light' and 'chlorophyll' explicitly; link light to energy and chlorophyll to absorption.
Show the causal chain: light → energy → chlorophyll absorption → glucose/O₂ production.
Keep the answer concise, 4 points total, matching the scheme.

Common mistakes

Confusing chlorophyll with other pigments or omitting its role in light absorption.
Failing to mention that light is the energy source, not the reactant.
Using vague terms like 'sunlight' without linking to energy or chlorophyll.

Mark scheme (4 marks)

Light provides the energy needed for photosynthesis
This energy is used to convert carbon dioxide and water into glucose (and oxygen)
Chlorophyll absorbs the light energy
Without chlorophyll, light energy cannot be absorbed / photosynthesis cannot occur

Key terms in this question

photosynthesis · chlorophyll