Cyclic Electron Flow

∞ generated and posted on 2016.12.03 ∞

A means during photosynthesis of biasing light reaction outputs towards increasing photophosphorylation over NADPH production.

Cyclic Electron Flow in a sense (well, metaphorically) is a 'short circuiting' of the normal z scheme of the light reaction of photosynthesis where energy that otherwise would have gone into making NADPH instead is diverted towards making more ATP.

Cyclic electron flow avoids both photosystem II and the donation of electrons to NADP+. Electrons instead are passed from photosystem I to an electron transport chain, resulting in proton pumping, and then returned to reduce photosystem I, returning that photosystem to its ground state. Also can be described as a cyclic photophosphorylation.

This cyclical process is rather than those reducing electrons being originally sourced from photosystem II. The process is described as cyclical because rather than electrons flowing from one component of the light reaction to another (photosystem II to NADP+), they instead cycle back to the same component (photosystem I to photosystem I).

The effect of cyclic electron flow is simply the diversion of more captured photons to ATP production than otherwise would be the case with noncyclic electron flow alone. The reason for this diversion is simply that more ATP is required to complete photosynthesis than is generated relative to NADPH production by noncyclic electron flow alone.