Cyclic Electron Transport


Cyclic electron transport is a light‐driven flow of electrons through a photosynthetic reaction centre with the electrons returning to the reaction centre via an electron transport pathway. Cyclic electron transport will normally generate an electrochemical potential (typically pH) gradient across a membrane but does not result in the net production of reductant. The pH gradient generated may drive the production of adenosine triphosphate (ATP) (cyclic photophosphorylation) or may regulate photosynthesis.

Keywords: ATP synthase; chloroplast; cyclic phosphorylation; nonphotochemical quenching; photosynthesis; plant stress physiology

Figure 1.

Pathways of higher plant photosynthetic electron transport. Higher plant photosynthetic electron transport takes place in the chloroplast in the thylakoid membrane. Linear electron transport starts with the light‐driven splitting of water by photosystem II (PSII), producing oxygen and electrons. These are transferred via plastoquinone (PQ) the cytochrome b/f complex (cyt b/f) and plastocyanin (PC) to photosystem I (PSI). PSI then reduces ferredoxin (Fd) in a second light‐driven reaction. Ferredoxin may reduce NADP to NADPH, catalysed by ferredoxin NADP oxidoreductase (FNR) or pass electrons back to PQ, probably via the cyt b/f complex. NADPH may be used to produce carbohydrate in the Benson–Calvin cycle or may reduce PQ, via an NADP–PQ oxidoreductase complex (ndh). Electron transport is coupled to the transfer of protons from the chloroplast stroma to the thylakoid lumen. These return to the stroma via ATP synthase, a process coupled to the synthesis of ATP. The pH gradient produced may also lead to downregulation of light harvesting by PSII – a process termed qE.



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Further Reading

Allen JF (2003) Cyclic, pseudocyclic and noncyclic photophosphorylation: new links in the chain. Trends in Plant Science 8: 15–19.

Bendall DS and Manasse RS (1995) Cyclic photophosphorylation and electron‐transport. Biochimica et Biophysica Acta 1229: 23–38.

Heber U (2002) Irrungen, Wirrungen? The Mehler reaction in relation to cyclic electron transport in C3 plants. Photosynthesis Research 73: 223–231.

Johnson GN (2005) Cyclic electron transport in C‐3 plants: fact or artefact? Journal of Experimental Botany 56: 407–416.

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Johnson, Giles N(Sep 2007) Cyclic Electron Transport. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001312.pub2]