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.
(1) Non-cyclic photophosphorylation
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.