Biology PHOTOSYNTHESIS IN HIGHER PLANTS

Photophosphorylation

Light phase includes the interaction of two pigment systems. PS I and PS II constitute various type of pigments. Arnon showed that during light reaction not only reduced NADP is formed and oxygen is evolved but ATP is also formed. This formation of high energy phosphates (ATP) is dependent on light hence called photophosphorylation.

When the light quantum is absorbed by various types of pigments (Like chlorophylls, phycobilins, carotenoids etc.), it is transferred to reaction centre i.e. P700 in PS I and P680 in PS II. Electrons excite from reaction centres due to funneling of energy. P700 gets photo excited and comes under first excited singlet state. As a result electron is lost, which is accepted by an electron, acceptor in the way. After absorbing light, excited electron liberated from reaction centre interacts with water.

Another important aspect of light reactions is the formation of ATP and NADPH2 (Assimilatory power). H+ from water and electron from chlorophyll are made available to NADP to form NADPH2. The electrons are accepted by NADP after passing through electron carriers. The carriers in the way undergo oxidation and reduction and are arranged in accordance with their redox potential value.

Photophosphorylation is of two types :
(a) Cyclic photophosphorylation
(b) Non cyclic photophosphorylation

(a) Cyclic photophosphorylation

It involves only PS I. Flow of electron is cyclic. When NADP is not available then this process will occurs. When the photons activate PS I, a pair of electrons are raised to a higher energy level. They are captured by primary acceptor which passes them on to ferredoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to reaction centre of PS I i.e. P700. At each step of electron transfer, the electrons lose potential energy. Their trip down hill is caused by the transport chain to pump H+ across the thylakoid membrane. The proton gradient, thus established is responsible for forming (2 molecules) ATP. No reduction of NADP to NADPH+ H+. ATP is synthesized at two steps.

(b) Non cyclic photophosphorylation

It involves both PS-I and PS-II. Flow of electron is unidirectional. Here electrons are not cycled back and are used in the reduction of NADP to NADPH2. Here H2O is utilized and O2 evolution occurs. In this chain high energy electrons released from 'P-680' do not return to 'P-680' but pass through pheophytin, plastoquinone, cytochrome b6-f complex, plastocyanin and then enter P-700. In this transfer of electrons from plastoquinone (PQ) to cytochrome b6-f complex, ATP is synthesized. Because in this process high energy electrons released from 'P-680' do not return to 'P-680' and ATP (1 molecules) is formed, this is called Noncyclic photophosphorylation. ATP is synthesized at only one step.

Pseudocyclic photophosphorylation

Arnon and his coworker (1954) demonstrated yet another kind of photophosphorylation. They observed that even in absence of CO2 and NADP, if chlorophyll molecules are illuminated, it can produce ATP from ADP and Pi (Inorganic phosphate) in presence of FMN or vit. K and oxygen. The process is thus very simple and requires no net chemical change but for the formation of ATP and water. Arnon called this oxygen dependent FMN catalysed photophosphorylation or pseudocyclic photophosphorylation which involves the reduction of FMN with the production of oxygen. FMN is an auto-oxidisable hydrogen acceptor with the effect that the reduced FMN is reoxidised by oxygen. Thus the process can continue repeatedly to produce ATP.
Since this process can be continuously self repeated, it appears that a single molecule of water should be sufficient to operate pseudocyclic photophosphorylation to meet the requirement of ATP.