Photosystem I at Higher Level: An AHL statement: the second photosystem, and the product that carries hydrogen to the Calvin cycle.
Practise this as you read
- Outline the role of photosystem I.
- State the products of the light-dependent reactions.
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The big idea: Electrons from photosystem II reach photosystem I, where light re-excites them. Photosystem I emits them, and in the stroma they reduce NADP, together with a proton. Reduced NADP carries the hydrogen to the Calvin cycle.
The points to remember
- Electrons arrive at photosystem I from photosystem II, through the carriers.
- Light absorbed by photosystem I re-excites them to a higher energy level.
- Photosystem I emits them; they reduce NADP on the stroma side.
- NADP + 2e⁻ + H⁺ → reduced NADP (the proton comes from the stroma).
- Reduced NADP and ATP are the two products used by the Calvin cycle.
Remember it as: Second light, second boost, NADP reduced.
Real example: Emerson's enhancement effect. In the 1950s, Robert Emerson found that algae lit with far-red light photosynthesised slowly, but adding shorter-wavelength red light raised the rate more than either light alone: two photosystems work together.
Trap: NADP is reduced (gains electrons); it is not NAD, the carrier used in respiration.
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How it is asked: Expect a statement about what happens in the light-dependent reactions, or an Outline of the role of photosystem I.
IB-style questionOutline[3 marks]
Outline the role of photosystem I in the light-dependent reactions.
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See the answer broken into clear points, with guidance on how each point earns marks.