Light reaction is the first step in photosynthesis occurring in grana of chloroplast and needs the utilization of light energy. It consists of following three phases:
(a) Photolysis of water:
The light energy trapped by chlorophyll molecule decomposes water into its constituent elements, called photolysis of water.
H2O →(←) 4H+ + OH–
4OH– → 2H2 + 4e– +2O2
The electrons produced during the photolysis of water pass via 2 photosystems (PS –I and II). Each photo system has its own trap center and a primary pigment molecule.
It is the process of synthesis of ATP from ADP using light energy.
ADP+ ip → (light) ATP
It is of further two types:
1) Non-cyclic photo-phosphorylation:
High energy electrons released from P680 of PS-II are accepted by primary electron acceptor. The electrons pass via a series of electron acceptor i.e. PQ- cytochorome complex- PC and finally to P700 of PSI.
Again, the electrons given out by P700 of PS-I are taken up by primary pigment molecule and are ultimately passed to NADP through Fd. The electrons combine with ions and reduce NADP to NADP H2.
The net result of non-cyclic photo phosphorylation is the formation of 1oxygen (as a waste), 2 NADP H2 and 1 ATP molecule.
2) Cyclic photo-phosphorylation:
High energy electrons expelled from P700 of PS-I are taken up by primary pigment molecule, when the pass through series of electron acceptors i.e. Fd-PQ-Cytochorome complex-PC and finally to the same pigment molecule from which they have been originated.
There is formation of 2ATP molecules at the end.
(c) Photo reduction:
Chloroplast contains naturally occurring electron acceptor NADP. With addition of H+ from photolysis, it is reduced to NADP H2.
NADP + 2 H+ + 2e– →(light) NADPH2
Note: Light reaction is linked with dark reaction in a sense that the products i.e. ATP and of light reaction are used for carbon fixation in the dark reaction.