Photosynthesis

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Example 22:

In photosynthesis, chlorophyll present in the chloroplasts acts as a photosensitizer. Light is absorbed by chlorophyll and then this energy is used to split the water molecule into molecular oxygen and hydrogen equivalent. In the process, nicotinamide adenine dinucelotide phosphate (NADP⁺) is reduced to NADPH. During the process, a part of the energy is stored as ATP.

NADPH and ATP are then used in a series of dark reactions to fix carbon dioxide CO₂. An outline of the process is given below.

Light Reaction

Dark Reaction

Apart from vision and photosynthesis, the two most important phenomenons, which involve photochemical reactions, there are other applications where photochemical reactions come into play. They are:

i) photography

ii) photopolymerization

iii) photo etching in electronics

iv) printing.

Fast Reactions

All reactions are fast. At least, the individual steps of a reaction occur on an atomic time scale and are complete in less than 10^-9 seconds. The net rate of a reaction is slow due to the slowness with which reactant molecules get activated or come together.

Here those reactions are considered fast which occur in less than one second.

To study fast reactions special techniques are required. The various techniques that are used in study of fast reactions are:

i) flash photolysis

ii) relaxation technique

iii) flow technique.

The details of these techniques will not be discussed here.