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A-Level Biology October/November 2024 Q7(b): Carotene and xanthophyll are chloroplast pigments. Describe the role played by these pi…
A-Level Biology · Paper 9700/42 · October/November 2024 · Question 7(b) · [3 marks]
Carotene and xanthophyll are chloroplast pigments. Describe the role played by these pigments in photosynthesis.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
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Carotene and xanthophyll are accessory pigments found within photosystems in the chloroplasts. Their primary role is to absorb light energy at wavelengths that are not effectively absorbed by the primary pigment, chlorophyll a. This serves to extend the absorption spectrum, broadening the range of light wavelengths that can be utilised for photosynthesis. The energy harvested by these pigments is then funnelled and passed on to the primary pigment, chlorophyll a, in the reaction centre of the photosystem.
How the marks are awarded
- B1 — Correctly identifying carotene and xanthophyll as 'accessory pigments', as stated in the first sentence.
- B1 — Stating that these pigments absorb light wavelengths that are not absorbed by the primary pigment, chlorophyll a. This also covers the idea of extending the range of wavelengths.
- B1 — Describing how the absorbed energy is transferred ('passed on') to the primary pigment/chlorophyll a in the reaction centre.
Common mistakes
- Stating that accessory pigments carry out photosynthesis themselves, rather than passing energy to the reaction centre.
- Vaguely writing 'they absorb light' without specifying that they absorb different wavelengths to chlorophyll a.
- Incorrectly stating that they pass electrons to the reaction centre; they pass excited energy via resonance transfer, not electrons.
- Confusing absorption with reflection, e.g., 'they reflect yellow and orange light so they can't absorb it'.
Examiner tip: For biological processes, learn the specific role of each component and how it links to the next step in the sequence, rather than just memorising a list of names.
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