Question

How does cyclic electron flow differ from non cyclic electron flow?

Answer 1

Light reaction in the photo system starts electron flow. In oxygenic photosynthetic organisms, flow of electrons is of two types: non-cyclic as well as cyclic. Non-cyclic electron flow begins with the absorption of a photon by PSII. Cyclic electron flow begins with the absorption of a photon by PSI.

Complete answer:
Noncyclic electron flow: It is a light induced electron transport from water to NADP+ and a concomitant evolution of oxygen. It involves a collaboration of two photosystems: PSII and PSI. Electrons move from water through PSII to PSI and then to NADP+. Electron transport leads to generation of a proton-motive force and synthesis of ATP. Formation of ATP due to light induced non-cyclic electron flow is called non-cyclic photophosphorylation.

Cyclic electron flow: In certain cases, photoexcited electrons flow cyclic path, which involves PSI but not PSII. In cyclic electron flow, photoexcited electrons from P700 of PSI move through the b6f complex and back to P700. This cyclic electron flow is coupled to protons pumping into the thylakoid lumen. When protons flow down their electrochemical gradient through ATP synthase complexes, ATP synthesis occurs. The formation of ATP due to light-induced cyclic electron flow is called cyclic photophosphorylation. There is no production of NADPH and no release of oxygen in cyclic electron flow.

Non - cyclic electron flow	Cyclic electron flow
Both PSI and PSII	Only PSI
Photolysis of water	No photolysis of water
Formation of oxygen	No oxygen formation
ATP synthesis occurs	ATP synthesis occurs
NADPH synthesis occurs	No NADPH synthesis

Note: In cyclic electron flow photoexcited electrons from PSI are returned from ferredoxin to P700 via the cytochrome complex and plastocyanin. This electron flow generates electrochemical proton gradient and thus ATP but produces no NADPH.