Introductory Crop Physiology (1+1)

Non cyclic Photophosphorylation

Non cyclic Photophosphorylation

The electron released from photosystem II goes through a series of enzymes and co-enzymes to photosystem I.
This is called non cyclic electron transport and the synthesis of ATP in non-cyclic electron transport is called non-cyclic photophosphorylation.
The main function of non cyclic electron transport is to produce the assimilatory powers such as NADPH2 and ATP and the process occurs in photosystem I and II.

• This process of electron transport is initiated by the absorption of a photon (quantum) of light by P680 form of chlorophyll a molecule in the pigment system II, which gets excited and an electron is ejected from it so that an electron deficiency or a hole is left behind in the P680 molecule.
• The ejected electron is trapped by an unknown compound known as Q.
• From Q, the electron passes downhill along a series of compounds or intermediated electron carriers such as cytochrome b6, plastoquinone, cytochrome f and a copper containing plastocyanin and ultimately received by pigment system I.
• At one place during electron transport i.e. between plastoquinone and cytochrome f, one molecule of ATP is formed from ADP and inorganic phosphate.
• Now, when a photon of light is absorbed by P700 form of chlorophyll molecule in the pigment system I, this gets excited and an electron is ejected from it.
• This ejected electron is trapped by FRS (Ferredoxin Reducing Substance) and it is then transferred to a non-heme iron protein called ferredoxin.
• From ferredoxin, electron is transferred to NADP so that NADP is reduced to NADPH + H+.
• The hole in pigment system I has been filled by electron coming from pigment system II. But, the hole or an electron deficiency in pigment system II is filled up by the electron coming from photolysis of water where, water acts as electron donor.
• In this scheme of electron transport, the electron ejected from pigment system II did not return to its place of origin, instead it is taken up by pigment system I. Similarly, the electron ejected from pigment system I did not cycle back and was consumed in reducing NADP. Therefore, this electron transport has been called as non–cyclic electron transport and accompanying phosphorylation as non–cyclic photophosphorylation.
• The non cyclic electron transport (photophosphorylation) takes the shape of Z and hence it is called by the name Z–scheme.
• Non cyclic photophosphorylation and O2 evolution are inhibited by CMU (3-(4’-Chlorophyl) – 1-1dimethyl urea and 3-(3-4-dichlorophenyl)-1, 1-dimethyl urea (DCMU).

Significance of non cyclic electron transport

1. It involves PS I and PSII
2. The electron expelled from P680 of PSII is transferred to PS I and hence it is a non cyclic electron transport.
3. In non cyclic electron transport, photolysis of water (Hill’s reaction and evolution of O2 ) takes place.
4. Phosphorylation (synthesis of ATP molecules) takes place at only one place.
5. The electron released during photolysis of water is transferred to PS II.
6. The hydrogen ions (H+) released from water are accepted by NADP and it becomes NADPH2
7. At the end of non cyclic electron transport, energy rich ATP, assimilatory power NADPH2 and oxygen from photolysis of water are observed.
8. The ATP and NADPH2 are essential for the dark reaction wherein, reduction of CO2 to carbohydrate takes place.