Introductory Crop Physiology (1+1)

Cyclic electron transport and cyclic photophosphorylation

Cyclic elctron transport and cyclic photophosporylati

• The electrons released from photosystem I goes through a series of coenzymes and returns back to the same photosystem I.
• This electron transport is called cyclic electron transport. The synthesis of ATP occurring in cyclic electron transport is called cyclic photophosphorylation.
• The cyclic electron transport involves only pigment system I. This situation is created when the activity of pigment system II is blocked.
  
Under this condition,

1. Only pigment system I remain active
2. Photolysis of water does not take place
3. Blockage of noncyclic ATP formation and this causes a drop in CO2 assimilation in dark reaction
4. There is a consequent shortage of oxidized NADP

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• Thus, when P700 molecule is excited in pigment system I by absorbing a photon (quantum) of light, the ejected electron is captured by ferredoxin via FRS.
• From ferredoxin, the electrons are not used up for reducing NADP to NADPH + H+ but ultimately it falls back to the P700 molecule via number of other intermediate electron carriers.
• The electron carriers are probably cytochrome b6, cytochrome f and plastocyanin.


• During this electron transport, phosphorylation of ADP molecule to form ATP molecule take place at two places i.e., between ferredoxin and cytochrome b6 and between cytochrome b6 and cytochrome f.
• Thus, two ATP molecules are produced in this cycle. Since the electron ejected from P700 molecule is cycled back, the process has been called as cyclic electron transport and the accompanying phosphorylation as the cyclic photophosphorylation.
Significance of cyclic photophosphorylation

• During cyclic electron transport and phosphorylation, photolysis of water, O2 evolution and reduction of NADP do not take place.
• The electron returns or cycles back to original position in the P700 form of chlorophyll a. Here, chlorophyll molecule serves both as donor and acceptor of the electron.
• It generates energy rich ATP molecules at two sites and as such cannot drive dark reactions of photosynthesis.
• On the other hand, non- cyclic photophosphorylation does not produce sufficient ATP in relation to NADPH to operate the dark phase of photosynthesis.
  
• Therefore, the deficiency of ATP molecule in non–cyclic photophosphorylation is made up by the operations of cyclic photophosphorylation.
  
Secondly, the cyclic photophosphorylation may be an important process in providing ATP for photosynthesis and other processes such as synthesis of starch, proteins, lipids, nucleic acids and pigments within the chloroplast.

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