Explain how the structure of a chloroplast is related to its role in photosynthesis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The double‑membrane envelope of the chloroplast creates an internal environment that is rich in CO₂, NADPH and ATP, and it regulates the passage of materials such as CO₂ and glucose.
Inside the envelope, the thylakoid membranes form stacks (grana) that provide a large surface area for the light‑dependent reactions. Photosystems and electron‑carrying proteins are embedded in these membranes.
The thylakoid lumen (interior) holds a proton gradient that drives ATP synthesis via ATP synthase during photophosphorylation.
The surrounding stroma is an aqueous medium that contains the enzymes of the Calvin cycle, such as RuBisCO, and provides the site for the light‑independent reactions.
Inside the envelope, the thylakoid membranes form stacks (grana) that provide a large surface area for the light‑dependent reactions. Photosystems and electron‑carrying proteins are embedded in these membranes.
The thylakoid lumen (interior) holds a proton gradient that drives ATP synthesis via ATP synthase during photophosphorylation.
The surrounding stroma is an aqueous medium that contains the enzymes of the Calvin cycle, such as RuBisCO, and provides the site for the light‑independent reactions.
Examiner tips
- Use the word "envelope" for the double membrane and "stroma" for the aqueous matrix.
- Mention the thylakoid lumen as the site of the proton gradient.
- Show the link between the large surface area of thylakoids and the light‑dependent reactions.
- Include the role of the stroma in the Calvin cycle.
Mark scheme (4 marks)
- The thylakoid membranes provide a large surface area for the light-dependent reactions / for embedding photosystems and electron carriers.
- The thylakoid lumen / interior of the thylakoid maintains a proton gradient used for ATP synthesis via ATP synthase during photophosphorylation.
- The stroma provides the aqueous environment / contains the enzymes (e.g. RuBisCO) required for the Calvin cycle / light-independent reactions.
- The double membrane / envelope of the chloroplast maintains an internal environment with high concentrations of CO₂, NADPH, and ATP needed for photosynthesis, or controls the passage of materials such as CO₂ and glucose.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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