What is the Difference Between Cyclic and Noncyclic Photophosphorylation?

Cyclic and noncyclic photophosphorylation are two processes used for synthesizing energy-rich ATP molecules in the presence of light energy during photosynthesis. The main differences between them are:

1. Involvement of Photosystems: Cyclic photophosphorylation involves only Photosystem I (PSI), while noncyclic photophosphorylation involves both Photosystem I (PSI) and Photosystem II (PSII).
2. Electron Flow: In cyclic photophosphorylation, electrons from PSI are cycled back to PSI via an electron transport chain, generating a proton gradient used to produce ATP by ATP synthase. In noncyclic photophosphorylation, electrons are transferred from PSII to PSI via an electron transport chain, and electrons from PSI are used to reduce NADP+ to NADPH.
3. Product Formation: Cyclic photophosphorylation only produces ATP, while noncyclic photophosphorylation produces both ATP and NADPH.
4. Oxygen Production: Noncyclic photophosphorylation involves the photolysis of water, which leads to the release of oxygen. In contrast, cyclic photophosphorylation does not involve water photolysis and does not release oxygen.
5. Occurrence: Cyclic photophosphorylation is predominantly found in photosynthetic bacteria and isolated chloroplasts, while noncyclic photophosphorylation occurs in algae, cyanobacteria, plants, and other oxygenic phototrophs.

Comparative Table: Cyclic vs Noncyclic Photophosphorylation

Here is a table summarizing the differences between cyclic and noncyclic photophosphorylation:

Cyclic photophosphorylation occurs in photosystem I, does not require water, and only produces ATP. In contrast, noncyclic photophosphorylation involves both photosystems I and II, requires water, and produces both NADPH and ATP molecules. The electrons lost by P680 in noncyclic photophosphorylation do not return to the system, whereas in cyclic photophosphorylation, the electrons expelled by photosystem I return to the system.