Cyclic Electron Flow

The process depicted in Figure 17.12 of transferrring electrons from photosystem II (PSII) to photosystem I (PSI) and from water to PSII is called noncyclic electron flow. The generation of ATP by this process is called noncyclic photophosphorylation. An alternative pathway for the light reactions, called cyclic electron flow, utilizes the components of photosystem I, plastocyanin, and the cytochrome b 6f complex. (Figure 17.17)

Cyclic electron flow of electrons occurs when NADP+ is present in only small amounts in the chloroplast stroma. Under these conditions, electrons from ferredoxin (Fd) are transferred to the b 6f complex, which returns them to the P700 center. When this happens, no oxygen is released and no NADP+ is reduced. About one ATP is produced for every two electrons that complete the cycle in a process called cyclic photophosphorylation.

Cyclic photophosphorylation thus can produce ATP when little NADP+ is available to accept electrons (i.e., NADPH levels are high). This may be important when the dark reactions require more ATP than can be produced by noncyclic flow.

See also: Calvin Cycle, Photosystem Summary, Chlorophyll