Key Concepts: Photosynthetic Electron Transport Chain

A key concept for biochemistry and plant biology – the photosynthetic electron transport chain. Remember – the point of this is that the Calvin cycle needs energy in the form of ATP, and reducing power in the form of NADPH.


How it works:

1. Light hits the chlorophyll special pair at Photosystem II (PSII), resulting in charge separation and the loss of an electron.

2. The electrons at PSII are replaced by the photolysis of water, which releases electrons, oxygen and protons into the lumen.

3. In PSII the electron then passes to internal electron acceptors in PSII including a quinone (Q) and then to the mobile plastaquinone (PQ). PQ moves across the membrane and gives its electrons to the cytochrome b6f complex.

4. PQ is a hydrogen carrier so it takes H+ out of the stroma and releases them into the lumen.

5. Cytochrome b6f also moves H+ from the stroma to the lumen in its own right via a process known as the Q cycle.

6. Electrons are passed to plastacyanin (PC) and then to Photosystem I (PSI).

7. In PSI the electrons are passed to an iron-sulphur (FeS) cluster and then to Ferredoxin (Fd).

8. Ferredoxin NADP+ Reductase (FNR) catalyses the reduction of NADP+ to NADPH using the electrons from Fd.

9. The protons in the lumen pass through the F-type ATPase to provide energy for the production of ATP in the stroma.


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