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PinkMonkey Online Study Guide-Biology


R. Hill (1937) suspended isolated chloroplasts in water in a beaker. He added some known hydrogen accepting compound (e.g. benzoquinone or ferric salt) to the water and exposed the beaker to the light. Immediately the oxidation of water took place and bubbles of oxygen were evolved. When the hydrogen acceptor in the beaker was examined, it was found to be reduced.

(A = hydrogen acceptor)

This is photolysis or the photochemical oxidation of water and is commonly called the Hill reaction.


The formation of ATP molecules (chemical energy) from ADP and H3PO4 in presence of light and chlorophyll-a during the photochemical phase of photosynthesis is called photophosphorylation or photosynthetic phosphorylation. More commonly, it is described as the conversion of light energy into chemical energy (ATP).

(ADP = adenosine diphosphate)

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.

Important features

(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.

Table of Contents

4.0 Introduction
4.1 Chloroplasts
4. 2 Overall Equation of Photosynthesis
4.3 Primary Process of Photosynthesis
4.4 Secondary Processes of Photosynthesis (Biochemical Phase, Dark Reaction)
4.5 Diversity in Photosynthesis Pathway
4.6 Significance of Photosynthesis

Chapter 5


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