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During photosynthesis, carbon dioxide is converted into carbohydrates and this is called fixing of carbon dioxide. Many processes like respiration, combustion, volcanic activity, etc. release carbon dioxide into the atmosphere. This carbon dioxide of the atmosphere is used by the terrestrial plants while hydrophytes use the carbon dioxide dissolved in the water.
During photosynthesis, hydrogen of water is used to fix carbon dioxide and its oxygen is released. Water is obtained through the root hairs by absorption.
They are pigments capable of absorbing radiant energy of the sun. Pigments are chemically porphyrin molecules which have a metal ion at the centre. The metal ion in haemoglobin is iron and in chlorophyll, it is magnesium.
There are two types of photosynthetic pigments - chlorophylls and carotenoids. Chlorophylls are the main pigments as they are involved in the conversion of light energy into chemical energy. The carotenoids also absorb light energy but they pass it to the chlorophyll molecules. Chlorophylls are blue-green (chlorophyll-a) or green (chlorophyll-b) in colour whereas carotenoids are orange (carotenes) or yellow (xanthophyll).
The radiant energy from the sun is the source of both light and heat energy to photosynthesis. Light energy is harvested by the pigments in order to carry out the breaking down of water molecule into hydrogen and oxygen. This is an energy-intensive process. Sunlight can be split into seven different colours which is called the spectrum.
Each colour is associated with different quantity of energy. Different pigments absorb different colour lights. Chlorophyll pigments absorb red and violet portions of the light and reflect green portion. Hence, they appear green. Leaves look green because they absorb most of the violet and red region of the incident light. These pigments cannot absorb the green region of the spectrum. Therefore they reflect the green light and appear green in colour.
The biochemical reactions which convert carbon dioxide into carbohydrates are controlled by enzymes. These enzymes require an optimum temperature to be active. The temperature is maintained by the heat energy of the sun.
Minerals like magnesium are essential as they form the structure of the pigment molecules. In fact, deficiency of magnesium results in the yellowing of leaves. Minerals are obtained through water in the form of dissolved salts.
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