Explain how the structure of the 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 chloroplast’s double‑membrane envelope creates a distinct internal environment, allowing selective transport of CO₂ in and glucose out, which is essential for photosynthesis.
Inside, the thylakoid membranes form a large surface area for the light‑dependent reactions; they house photosystems, chlorophyll and other pigments that absorb light.
The thylakoids are stacked into grana, further increasing surface area for light absorption while keeping the structure compact.
The fluid stroma surrounds the grana and contains the enzymes of the Calvin cycle, such as RuBisCO, where the light‑independent reactions fix CO₂ into sugars.
Inside, the thylakoid membranes form a large surface area for the light‑dependent reactions; they house photosystems, chlorophyll and other pigments that absorb light.
The thylakoids are stacked into grana, further increasing surface area for light absorption while keeping the structure compact.
The fluid stroma surrounds the grana and contains the enzymes of the Calvin cycle, such as RuBisCO, where the light‑independent reactions fix CO₂ into sugars.
Examiner tips
- Use the word ‘envelope’ for the double membrane, ‘thylakoid membranes’ for light reactions, ‘grana’ for stacked thylakoids, and ‘stroma’ for Calvin cycle enzymes.
- Show the link between structure (surface area, compartmentalisation) and function (light capture, enzyme localisation).
Common mistakes
- Confusing the envelope with the thylakoid membrane, or omitting the role of the stroma.
- Failing to mention the stacking of thylakoids into grana or the selective transport function of the envelope.
Mark scheme (4 marks)
- The thylakoid membranes provide a large surface area for the light-dependent reactions (housing photosystems/chlorophyll/photosynthetic pigments).
- Thylakoids are stacked into grana, increasing the surface area available for light absorption within a compact space.
- The stroma is the fluid-filled matrix where the light-independent reactions (Calvin cycle / carbon fixation) occur, as it contains the necessary enzymes such as RuBisCO.
- The double membrane (envelope) of the chloroplast maintains an internal environment suitable for photosynthesis by controlling the entry and exit of molecules 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 →
More Cell structure questions
- Explain how the structure of the nuclear envelope is related to its function in …
- Explain how the ultrastructure of a mitochondrion is related to its role in aero…
- Explain how the structure of the Golgi apparatus is related to its role in the p…
- Explain how the structure of the rough endoplasmic reticulum (rER) is related to…
- Distinguish between prokaryotic and eukaryotic cells in terms of their internal …
- Explain how the structure of the cell surface membrane is related to its functio…