Cyclic electron flow

Cyclic electron flow (also known as cyclic photophosphorylation) is a process in photosynthesis that forms a part of the light-dependent reactions. It involves the use of photosystem I and is one of the two ways in which photosynthesis can take place in chloroplasts.

Overview

Cyclic electron flow occurs in the thylakoid membrane of the chloroplasts. It is initiated when light energy is absorbed by photosystem I, exciting an electron to a higher energy level. This electron is then passed to a series of electron carriers, creating a flow of electrons.

Unlike in non-cyclic electron flow, where the electron is eventually used to reduce NADP+ to NADPH, in cyclic electron flow the electron is returned to photosystem I. This allows the process to continue in a cyclic manner, hence the name.

Function

The primary function of cyclic electron flow is to generate a proton gradient across the thylakoid membrane. This is achieved through the pumping of protons from the stroma into the thylakoid lumen by the electron carriers. The resulting gradient is used to drive the synthesis of ATP via chemiosmosis.

Cyclic electron flow also plays a role in protecting the photosynthetic apparatus from damage. By diverting electrons away from the production of NADPH, it helps to prevent the over-reduction of the photosynthetic electron transport chain, which can lead to the production of harmful reactive oxygen species.

Comparison with Non-Cyclic Electron Flow

Cyclic electron flow differs from non-cyclic electron flow in several key ways. Firstly, it only involves photosystem I, whereas non-cyclic electron flow involves both photosystem I and photosystem II. Secondly, it does not produce NADPH, only ATP. Finally, it is thought to be an older process, having evolved before non-cyclic electron flow.

See Also

• Photosynthesis
• Light-dependent reactions
• Photosystem I
• Photosystem II
• Thylakoid
• Chloroplast