Photosystem Summary

1. Light energy causes electrons to be taken from water and end up in NADPH.

2. The summary of all reactions for photosystems I and II is given here. In addition, protons have been pumped from the stroma into the thylakoid lumen during the passage of each electron through the electron transport chain. The pH gradient arising from proton pumping can be quite large - as much as 3.5 pH units in brightly illuminated chloroplasts. The protons reenter the stroma only through ATP synthase complexes (Figure 17.15), as in oxidative phosphorylation.

3. ATP is generated in the thylakoid membranes of chloroplasts by a complex called CF 0-CF 1 and is very similar to the F 0 F 1 complex of mitochondria. As in mitochondria, passage of protons through the ATP synthase complex (CF 0-CF 1), is the driving force (and energy source) for ATP synthesis.

4. Photosystems I and II, the cytochrome b 6 f complex, and ATP synthase (CF 0-CF 1) are all individual entities in the thylakoid membrane, but are not necessarily contiguous. Components that link the photosystems (plastoquinone in the lipid phase of the membrane and plastocyanin in the thylakoid lumen) are mobile. Figure 17.16 shows the arrangement of components of the two photosystems on the thylakoid membrane. Note that the interior membrane layers of the grana are rich in PSII, whereas the stroma lamellae are rich in PSI.

See also: The F 0F 1 Complex (from Chapter 15), Photosystem II, Photosystem I, Cyclic Electron Flow, Thylakoid Lumen, Cytochrome b 6f, Plastoquinones,

INTERNET LINK: Photosynthesis Metabolic Scheme