Explain how the structure of a chloroplast is related to its function in photosynthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The thylakoid membranes (granum) give a large surface area for photosystems and ATP synthase, so the light‑dependent reactions can occur rapidly.
The thylakoid lumen is a proton reservoir; protons pumped into it during the light reactions create an electrochemical gradient that drives ATP synthesis by chemiosmosis.
The stroma contains the enzymes of the Calvin cycle (e.g. RuBisCO) and is where carbon fixation, reduction of G3P and regeneration of RuBP occur, using the ATP and NADPH produced in the thylakoids.
The double envelope membrane keeps the internal environment distinct, and the close proximity of thylakoids to the stroma allows efficient transfer of ATP and NADPH between the light‑dependent and light‑independent reactions.
The thylakoid lumen is a proton reservoir; protons pumped into it during the light reactions create an electrochemical gradient that drives ATP synthesis by chemiosmosis.
The stroma contains the enzymes of the Calvin cycle (e.g. RuBisCO) and is where carbon fixation, reduction of G3P and regeneration of RuBP occur, using the ATP and NADPH produced in the thylakoids.
The double envelope membrane keeps the internal environment distinct, and the close proximity of thylakoids to the stroma allows efficient transfer of ATP and NADPH between the light‑dependent and light‑independent reactions.
Examiner tips
- Use the exact terms: thylakoid, granum, lumen, stroma, envelope; link structure to function; keep each point concise and separate.
- Show the flow: light reactions → proton gradient → ATP + NADPH → Calvin cycle in stroma.
- Mention the large surface area and proton reservoir explicitly.
Common mistakes
- Confusing the thylakoid lumen with the stroma; not stating the role of the envelope membrane; omitting the link between ATP/NADPH production and the Calvin cycle.
Mark scheme (4 marks)
- The thylakoid membranes (granum/grana) provide a large surface area for the attachment of photosystems and ATP synthase, maximising the rate of the light-dependent reactions.
- The thylakoid lumen acts as a proton reservoir; protons are pumped into it during the light-dependent reactions, creating the electrochemical gradient that drives ATP synthesis via chemiosmosis.
- The stroma contains the enzymes of the Calvin cycle (e.g. RuBisCO) and is where carbon fixation / reduction of GP / regeneration of RuBP occurs, using the ATP and NADPH produced in the thylakoids.
- The double membrane of the chloroplast (envelope) maintains a distinct internal environment, and the proximity of the thylakoids to the stroma allows efficient transfer of ATP and NADPH between the light-dependent and light-independent reactions.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Photosynthesis questions
- Explain how the light-dependent reactions of photosynthesis produce the molecule…
- Explain how changes in carbon dioxide concentration affect the rate of the Calvi…
- Explain how cyclic photophosphorylation differs from non-cyclic photophosphoryla…
- Explain how non-cyclic photophosphorylation results in the production of ATP and…
- Explain how oxygen is produced as a by-product during the light-dependent reacti…