During the light reactions (the primary process or the photochemical phase), ATP formation takes place through two types of phosphorylation reactions. These are

(1) non-cyclic photophosphorylation and

(2) cyclic photophosphorylation

The non-cyclic process involves both PS I and PS II. The cyclic process involves only PS I.

In the light reaction of photosynthesis, the formation of ATP from ADP and H₃PO₄ in presence of light and chlorophyll-a during the non-cyclic transfer of electrons is called non-cyclic photophosphorylation.

(1) It is the major pathway of the light reaction of photosynthesis.

(2) It is a photochemical reaction.

(3) It takes place in the grana of chloroplasts.

(4) It requires participation of both PS I and PS II.

(5) The transfer of electrons through the ETS is unidirectional or non-cyclic.

(6) The non-cyclic process involves :
(a) Photophosphorylation (ATP formation)
(b) Photolysis of water
(c) Formation of assimilatory power (ATP and NADPH₂)
(d) Liberation of oxygen

The non-cyclic photophosphorylation takes place as follows.

(a) Photoexcitation of PS I : The chlorophyll-a (P 700) of PS I is activated on receiving photons of light and so it expels electrons. As a result of this loss, it becomes ionized chl-a⁺.

The electrons from PS I are first accepted by FRS (ferredoxin reducing substance) and are transferred to co-enzyme NADP via Fd (ferredoxin). NADP retains the electrons and is thus reduced.

Chapter 5