Green plants can directly use sunlight to form food from simple inorganic substances. They manufacture food from carbon dioxide and water in the presence of chlorophyll and sunlight. This process of making food is called photosynthesis (photo =>light, synthesis => to make). Such a form of nutrition directly using sunlight as the source of energy, is called autotrophic mode of nutrition. All green plants are thus called autotrophic organisms or autotrophs.
Leaves are the sites of photosynthesis. All green leaves contain certain tiny cells organelles called chloroplasts which contain the green pigment- chlorophyll. Green plants use chlorophyll to trap energy from the sun during photosynthesis. Carbon dioxide required for the process of photosynthesis is obtained through stomata while water is absorbed by the plants through the roots and reaches the leaves through xylem vessels found in the veins.
Chlorophyll traps the solar energy and uses carbon dioxide and water to make glucose (or starch). Oxygen is produced as a by-product and is released into the atmosphere through stomata. Photosynthesis can be summarized in the following manner:
Thus, photosynthesis is the only process on earth by which sun’s energy is trapped by autotrophs and converted into food. In addition, it is the only natural process by which oxygen is produced in the atmosphere that sustains the entire animal kingdom.
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Glucose produced by the process of photosynthesis is converted into starch and stored I different parts of the plant body like leaves, stems and roots. Thus, when you eat potato you are nourished by the starch stored in the stem of the potato plant. Glucose and starch are two examples of a bigger class of organic compounds called carbohydrates. Thus, the easiest way to find out whether photosynthesis has taken place is to test for the presence of starch.