Question

What are the alternate forms of electron flow in photosynthesis? What are the final products?

Make and label a diagram

Answer 1

In certain specialized conditions, the electrons which are excited by photons take an alternate path of photosynthesis called cyclic electron flow. In cyclic electron flow, photoexcited electrons from P700 of PSI move through the b6f complex and back to P700. There is no production of NADPH and no release of oxygen in cyclic electron flow. In plants, the cyclic flow of electrons is utilized only when the concentration of NADPH is sufficient but still needs ATP to power other activities in the chloroplast.

q Under certain conditions, the photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I (P700) but not photosystem II (P680).  This process produces no NADPH and no O_2, but it does make ATP. This is called cyclic photophosphorylation.  The chloroplast shifts to this process when the ATP supply drops and the level of NADPH rises.  Often the amount of ATP needed to drive the Calvin cycle exceeds what is produced in non-cyclic photophosphorylation.   Without sufficient ATP, the Calvin cycle will slow or even stop.  The chloroplast will continue cyclic photophosphorylation until the ATP supply has been replenished.  ATP is produced through chemiosmosis in both cyclic and non-cyclic photophosphorylation.