Why do plants use cyclic and non cyclic electron flow?

Question

Why do plants use cyclic and non cyclic electron flow?

Answer 1

This is a very complex issue currently under study by many leading researchers in the photosynthetic research community.  Noncyclic electron flow is usually mostly coupled to reduction of NADP to NADPH which is used to fix CO2.  But a little of this flow can be diverted to reduce O2 and nitrate.  A big advantage of this "alternative" (i.e. not associated with CO2 fixation) electron flow is that little or no ATP is consumed by the downstream processes but large amounts can be synthesized during this electron flow.  Hence, temporary deficits in ATP availability for CO2 fixation and other biosynthetic processes can be corrected.  Cyclic electron flow is another potential way of using light energy to make "extra" ATP.  But, there is real controversy as to whether cyclic flow is actually coupled to synthesis of ATP.  We are not even sure we know how much cyclic flow normally exists.  Great question but there is no firm answer as yet.