Explain how the structural features of the chloroplast enable it to carry out both the light-dependent and light-independent reactions of photosynthesis in separate compartments.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The thylakoid membranes in the grana give a large surface area for photosystems, electron carriers and ATP synthase, so the light‑dependent reactions take place there.
The pumping of protons into the thylakoid lumen during these reactions makes the lumen acidic, creating a proton gradient that drives ATP synthesis by ATP synthase (chemiosmosis).
The stroma is a separate aqueous compartment that contains the enzymes of the Calvin cycle (e.g. RuBisCO), so the light‑independent reactions use the ATP and NADPH produced in the thylakoids.
The double‑membrane envelope of the chloroplast isolates the organelle, maintaining the required ion concentrations, pH and enzyme complement for both stages of photosynthesis.
The pumping of protons into the thylakoid lumen during these reactions makes the lumen acidic, creating a proton gradient that drives ATP synthesis by ATP synthase (chemiosmosis).
The stroma is a separate aqueous compartment that contains the enzymes of the Calvin cycle (e.g. RuBisCO), so the light‑independent reactions use the ATP and NADPH produced in the thylakoids.
The double‑membrane envelope of the chloroplast isolates the organelle, maintaining the required ion concentrations, pH and enzyme complement for both stages of photosynthesis.
Examiner tips
- Mention the large surface area of thylakoids for light‑dependent reactions. Explain the proton gradient and ATP synthesis. State that the stroma hosts the Calvin cycle. Highlight the role of the envelope in isolating the environment.
Common mistakes
- Confusing the thylakoid lumen with the stroma. Omitting the role of the envelope. Using vague terms like "separate spaces" without naming stroma and thylakoids.
Mark scheme (4 marks)
- The thylakoid membranes (within the grana) provide a large surface area for the embedding of photosystems / electron carriers / ATP synthase, enabling the light-dependent reactions to occur there.
- The thylakoid lumen becomes acidic (low pH) as protons are pumped into it during the light-dependent reactions, generating a proton gradient that drives ATP synthesis via ATP synthase (chemiosmosis).
- The stroma is a separate aqueous compartment containing the enzymes of the Calvin cycle (e.g. RuBisCO), allowing the light-independent reactions to occur there using the ATP and NADPH produced in the thylakoids.
- The double membrane (envelope) of the chloroplast isolates the organelle's internal environment, maintaining the specific ion concentrations, pH, and enzyme complement needed for both stages of photosynthesis.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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