Fundamentals of Molecular Biology. Part 1: Molecular Biology of the Cell - A. N. Ogurtsov 2011

Molecular mechanisms of photosynthesis
Cyclic electron transport chain

The electron-carrier protein ferredoxin (which transfers an electron to NADP+ in the Linear electron transport chain) can also donate electrons to the membrane quinone carrier associated with PSI on the stromal side of the thylakoid membrane.

In this case, such a quinone takes up two protons from the stroma to form the QH2 species, diffuses within the membrane, and binds near the luminal surface to the Qo center of the cytochrome $bf$ complex. Here, it releases two electrons to the cytochrome $bf$ complex and two protons into the lumen, thereby generating a proton-motive force.

Much like in linear electron transport, these two electrons are returned to PSI by luminal plastocyanin Proteins. This closed-loop cyclic electron flow, which bypasses PSII, is analogous to the cyclic processes occurring in purple bacterial Photosystems (Figure 183).

The Q-cycle pumps an additional two protons per pair of electrons transferred through the cyclic pathway, thereby increasing the proton-motive force. The proton-motive force generated by this cyclic electron transport in METABOLISM/14.html">Chloroplasts drives ATP synthesis via F0F1 complexes in the thylakoids, though without The formation of NADPH and O2.



Last update: 12/08/2026

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