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48 Chapter Two

Sunlight

Enzymes
Chlorophyll
RuDP
ADP
Water ATP
NADPH Glucose CO 2
O 2
NADP
Light reaction Dark reaction
Figure 2.2 General process of photosynthesis.

independent (or dark reactions). The rate of the light reaction can be
increased by increasing light intensity, and the rate of the dark reaction
can be increased by increasing temperature to a certain extent (see Fig. 2.2).

2.3.1 Hill reaction (light reaction)
The process of formation of CO and O during photosynthesis is called
2
2
the Hill reaction or photolysis of water. This primary photochemical
reaction takes place in the presence of sunlight. The reaction is associ-
ated with chlorophyll, and after receiving light energy, the chlorophyll
becomes activated. The steps in the Hill reaction can be summed up in
the following manner:

1. Absorption of light and activation of chlorophyll Radiant
light contains very tiny energized particles called photons or quanta,
which are absorbed by the chlorophyll and it becomes activated.
2. Photolysis Is the dissociation of water molecules by light energy
that have been absorbed by the chlorophyll. The reaction can be rep-
resented as

Light energy
4H O ⎯⎯⎯⎯→ 4H 4OH
2
Chlorophyll
4H ⎯⎯⎯⎯→ 2H 2
H NADP → NADPH (Hydrogen acceptor)
2
2
Recombination
4OH ⎯⎯⎯⎯⎯→ 2H O O 2
2