Light Reactions
When light falls on the leaves, it
is absorbed by chlorophyll. The solar energy is used to split water into oxygen
and hydrogen and this is called photolysis (photo means light and lysis means
to break). The oxygen is released into the atmosphere as by-product of
photosynthesis. As this process takes place only in the presence of light, it
is called light reaction.
During light reaction, two compounds are formed when the solar energy is
converted into chemical energy these are:
1. NADPH (Nicotinamide Adenine Dinucleotide Phosphate)
2. ATP (Adenosine Triphosphate)
NADP, already exists in the cells of the leaf. The hydrogen released on the
splitting of water molecule is accepted by this compound and it is reduced to
NADPH.
ADP (Adenosine Diphosphate) is already present in the cell; it combines with
the phosphate group using light energy to form a compound called ATP.
These compounds are energy rich compounds which are needed for the dark
reactions of the process. Light reaction is called high dependent reaction.
(Diagram)
Dark Reactions
Using the energy of ATP and the
NADPH, water combines with carbon dioxide to form carbohydrate. Thus the solar
energy is now converted into chemical energy to form glucose. Other organic compounds
are also synthesized from this glucose.
This stage is completed in a series of chemical reactions with the help of
enzymes. Neither light energy nor chlorophyll is needed for dark reactions.
Therefore dark reaction is also called light independent reaction.
Various steps of the dark reactions were studied by a scientist called Melvin
Calvin so the dark reaction is also called the Calvin’s Cycle.
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