Cyclic and Non cyclic photophosphorylation

Cyclic photophosphorylation :

The process of cyclic photophosphorylation involves only photosystem I.

A photon of light excites an electron of chlorophyll a molecule (of PS I) and the excitation energy is eventually passed on to the reaction centre P700.

This excited electron is expelled from the chlorophyll molecule and is accepted by an electron acceptor Fe-S.

By initiating this electron transfer chain, P700 molecule becomes oxidised.

Figure: Cyclic Photophosphorylation

Fe-S passes this electron to other acceptors in the following sequences : Ferredoxin, Plastoquinone, Cytochrome b6-f complex and plastocyanin.

The oxidised P700 serves as a natural electron acceptor from plastocyanin. All the above electron acceptors are in oxidised state before accepting electrons (therefore these act as electron acceptors) and get reduced after accepting the electron.

As the electron is transferred from one acceptor to another, certain amount of energy is released. This energy is used to generate A TP (while passing between Fd and PQ and/or PQ and cyt. b6-f complex).

When only PSI is functional, the electron is circulated within the photosystem and the phosphorylation occurs due to cyclic flow of electrons.

A possible location on where it occurs is in the stroma lamellae.

The stroma lamellae membrane lacks PSII as well as NADP reductase enzyme.

The excited electron does not pass on to NADP+ but is cycled back to the PSI complex through electron transport chain.

The cyclic flow hence, results only in the synthesis of ATP, but not of NADPH + H+.

Cyclic photo phosphorylation also occurs when only light of wavelengths beyond 680 nm are available for excitation.

(b) Non-cyclic photophosphorylation:

It involves photosystem I as well as photosystem II. Both these systems absorb light photons. For convenience, the non-cyclic photophosphorylation is described here in following steps:

(i) When P680 absorbs a photon of red light, it gives out an electron, and thus becomes positively charged. This electron from P680 passes to P700 through a series of electron carriers, like Phaeophytin (Primary acceptor), plastoquinone, cytochrome b6-f complex, plastocyanin.

This movement of electron is downhill in terms of redox potential scale.

In this series, enough energy is released when the electron is transferred from PQ to cytochrome b6-f complex. It is utilized to synthesize ATP.

(ii) Simultaneously, electron in the reaction centre of PS I also get excited when they receive red light of 700 nm and these electrons are transferred to another receptor having high redox potential. These electrons then move downhill again, this time to a molecule of NADP+. This addition of electron and a proton, reduce it to NADPH + H+.

Differences between Cyclic and Non-cyclic Photophosphorylation