Light Reaction (Hill’s Reaction)

Introduction:
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.
H_2­O       →(←)        4H⁺ + OH^-
4OH^-           →          2H₂ + 4e­­^- +2O₂

(b) Photo-phosphorylation:
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 H_2.

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 H_{2.
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.