ZIMSEC A Level · Biology · Form 6
The chloroplast
Thylakoid membranes (stacked into grana) contain photosystems; the stroma contains enzymes of the Calvin cycle.
Light-dependent reactions (thylakoids)
- Light excites electrons in chlorophyll in photosystems I and II.
- Photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂.
- Electrons pass along a transport chain; ATP is made by photophosphorylation (chemiosmosis); NADP is reduced.
- Non-cyclic photophosphorylation produces ATP, reduced NADP and oxygen; cyclic produces only ATP.
Light-independent reactions: the Calvin cycle (stroma)
- CO₂ combines with ribulose bisphosphate (RuBP, 5C) catalysed by rubisco, forming two molecules of glycerate 3-phosphate (GP).
- GP is reduced to triose phosphate using ATP and reduced NADP.
- Some triose phosphate forms glucose and other compounds; most regenerates RuBP using ATP.
Limiting factors
Light intensity, CO₂ concentration and temperature. Growers raise CO₂ and use lighting in greenhouses.
C4 plants
Maize and sorghum are C4 plants: they fix CO₂ first into a 4C compound in mesophyll cells and deliver it to bundle sheath cells, reducing photorespiration in hot, sunny conditions.
Exam tip: Link the two stages: the light-dependent reactions supply ATP and reduced NADP to the Calvin cycle.
Check yourself
- What is produced by the photolysis of water?
- Name the enzyme that fixes carbon dioxide in the Calvin cycle.
- Why are C4 plants such as maize productive in hot climates?
Show answers
- Protons, electrons and oxygen.
- Rubisco.
- They concentrate CO₂ around rubisco, reducing photorespiration at high temperature and light intensity.
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