Explain how the ultrastructure of the chloroplast is adapted for the two stages of photosynthesis.

IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

1. Thylakoid membranes in grana give a large surface area for light‑absorbing pigments and photosystems, enabling efficient capture of light energy in the light‑dependent reactions.
2. The thylakoid membrane hosts the electron transport chain and ATP synthase, creating a proton gradient that drives ATP synthesis by chemiosmosis during the light‑dependent reactions.
3. The stroma contains enzymes such as RuBisCO that carry out the Calvin cycle, fixing CO₂ and reducing it to triose phosphate in the light‑independent reactions.
4. The double‑membrane envelope of the chloroplast maintains the internal environment—concentrations of CO₂, ATP, NADPH—and controls the movement of substances like triose phosphate across the envelope.

Examiner tips

  • Use the word ‘adapted’ to link structure to function; mention both light‑dependent and light‑independent stages.
  • Show the flow: light absorption → electron transport → ATP/NADPH → Calvin cycle in stroma.
  • Include the envelope’s role in regulating internal conditions.
  • Keep each point concise and use specific terminology (thylakoid, grana, stroma, RuBisCO).

Common mistakes

  • Confusing the role of the envelope with that of the thylakoid membrane; the envelope regulates internal conditions, not the electron transport chain.
  • Lacking mention of the Calvin cycle enzymes (e.g., RuBisCO) in the stroma.
  • Using vague terms like ‘photosynthetic machinery’ instead of specific structures such as ‘thylakoid membranes’ and ‘stroma’.

Mark scheme (4 marks)

  1. Thylakoid membranes (in grana) provide a large surface area for photosynthetic pigments / light-harvesting complexes / photosystems to absorb light energy for the light-dependent reactions.
  2. The thylakoid membrane is the site of the electron transport chain and ATP synthase, allowing chemiosmosis / the proton gradient to drive ATP synthesis during the light-dependent reactions.
  3. The stroma contains enzymes (including RuBisCO) needed for the Calvin cycle / light-independent reactions, where carbon dioxide is fixed and reduced to produce organic molecules (triose phosphate / G3P).
  4. The double membrane / envelope of the chloroplast maintains the internal environment (concentrations of CO2, ATP, NADPH) needed for both stages, and the intermembrane space / envelope controls entry and exit of substances such as triose phosphate.

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