Explain the role of chlorophyll and accessory pigments in capturing light energy for photosynthesis.

IB DP Biology Standard Level (2023 syllabus) — C1.3 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)

Chlorophyll and accessory pigments absorb light at different wavelengths, broadening the spectrum that can be used for photosynthesis.
1. Chlorophyll a and b mainly absorb red and blue light.
2. Accessory pigments such as carotenoids absorb light in the green/yellow/orange region.
3. The energy absorbed by accessory pigments is transferred to chlorophyll a in the reaction centre by resonance energy transfer.
4. Chlorophyll a then uses this energy to excite electrons to a higher energy level, which enter the electron transport chain and drive the light‑dependent reactions.

Examiner tips

Use the exact terms ‘chlorophyll a’, ‘chlorophyll b’, ‘carotenoids’, ‘reaction centre’, and ‘resonance energy transfer’.
Show the sequence: absorption → energy transfer → electron excitation → electron transport chain.
Keep the answer concise – 4 points, one sentence each.

Common mistakes

Mixing up the colours absorbed by chlorophyll a and b (e.g. saying both absorb green).
Failing to mention that accessory pigments transfer energy to chlorophyll a rather than directly to the electron transport chain.
Using vague language such as ‘helps capture light’ without specifying wavelengths or energy transfer.

Mark scheme (4 marks)

Chlorophyll and accessory pigments absorb light at different / specific wavelengths, broadening the range of the spectrum used for photosynthesis.
Chlorophyll a and b absorb red and blue light mainly, whereas accessory pigments such as carotenoids absorb wavelengths in between (green/yellow/orange regions).
Energy absorbed by accessory pigments is transferred to chlorophyll a (the reaction centre) by resonance energy transfer.
At the reaction centre, chlorophyll a uses the absorbed energy to excite electrons to a higher energy level, which then enter the electron transport chain, enabling the light-dependent reactions to proceed.

Key terms in this question

chlorophyll · accessory pigments