Cyclic Photophosphorylation

 Significance of Cyclic Photophosphorylation

1. When cyclic photophosphorylation alone operates, the CO2 assimilation drops down because of the shortage of reduced NADP (NADPH2).

2. It generates only ATP molecules and as such can not drive dark reactions of photosynthesis.

3. It is an important system in providing ATP for synthetic processes (other than photosynthesis)like a synthesis of protein, lipids, nucleic acids, and pigments within chloroplasts.

Limitations of Cyclic Photophosphorylation

1. This system operates if the activity of the PSII is blocked.

2. Under these conditions :

a. only PS I remains active

b. Photolysis of water does not take place

c. blockage of non-cyclic ATP formation causes a drop in CO2 assimilation in dark reaction and, therefore,

d. there is a shortage of reduced NADP. (i.e., NADPH2)

2. Non-cyclic Photophosphorylation

It occurs in green plants and involves both PS I and PSII

* During this process, the electron is excited by the absorption of a photon (quantum) of light by P700 form of chlorophyll-a molecules in PS I.

* FRS traps this excited electron.

* The electron is now transferred to a non-heme iron protein called ferredoxin (Fd).

* From Fd, the electron is transferred to NADP via the intermediate protein electron carrier, ferredoxin-NADP-reductase so that NADP is reduced to NADPH2 in the presence of H+ released from the reactions of photolysis of water.

Schematic representation of light-induced electron transport in photosynthesis showing non-cyclic photophosphorylation. Two pigment systems, viz., PS PS II, Photolysis of water molecule, and generation of assimilatory powers and I are indicated. (PQ: Plastoquinone; PC: Plastocyanin: FRS: Ferredoxin Reducing Substances and Fd; Ferredoxin).

* Now, when a photon (quantum) of light is absorbed by P 680 form of chlorophyll ‘a” molecule in PSII, it gets excited and an electron is ejected from it so that an electron deficiency or a ‘hole” is left behind in the P680 molecule.

* A compound of unknown identity traps this ejected electron as PQ (sometimes called Q because it causes quenching of the characteristic fluorescence of chlorophyll a in PS II)

* From PQ the electron passes downhill along with a series of compounds or intermediate electron carriers and is ultimately received by PS I where it fills the “hole”.

* The series of compounds (or electron carriers) consists of :

i. Plastoquinone (PQ)

ii Cytochrome-b6

iii Cytochrome-f, a copper-containing protein, and

iv. Plastocyanin (PC)

*` During this electron transport, at one place, between cyt-b and cyt-f, phosphorylation of one molecule of ADP to form ATP molecule takes place(photophosphorylation).

* In the above scheme of Non-cyclic photophosphorylation, the electron ejected from PS II did not return to its place of origin; instead, it was taken by PSI. Similarly, the electron ejected from PS I did not cycle back and was consumed in reducing NADP. Therefore, this electron transport is called Non-cyclic Photophosphorylation (Fig.17). However, Non-cyclic Photophosphorylation is inhibited by chemicals like CMU (3-(4’-dichlorophenyl)-1, 1-dimethyl urea)). This entire process is called Hill Reaction or Light Reaction. With the production of ATP and NADPH2, the plant is now ready to reduce CO2 to form carbohydrates. This is also called as ‘Z’ Scheme of Photophosphorylation.

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Differences between Cyclic and Non-cyclic photophosphorylation :

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