Plants require food which can supply energy for their various metabolic activities. Plants can’t move from one place to another in search of food like animals. They stand still and make their own food. We know that green plants are autotrophic that is they synthesise their own food by the process of photosynthesis. In presence of chlorophyll plants use the energy in sunlight to prepare food from carbon dioxide and water. Plants are green due to chlorophyll present in the chloroplasts of the plants cells.
Therefore, the process by which green plants prepare their own food like glucose from carbon dioxide and water by using sunlight energy in the presence of chlorophyll is called photosynthesis. This process takes place in the leaves of the leaves of the plant.
Raw materials require for photosynthesis are:
1. Carbon dioxide
Now, we will understand how plants obtain Carbon Dioxide
On the surface of the leaves of the plants there are a large number of tiny pores known as stomata or stoma. For photosynthesis green plants take carbon dioxide from the air. The carbon dioxide enters the leaves of the plant through the stomata present on their surface. Each stomatal pore is surrounded by a pair of guard cells. The opening and closing of the pores of stomata is controlled by the guard cells only. When water flows into the guard cells, they swell, become curved and cause the pore to open. On the other hand, the guard cells lose water; they shrink, become straight and close the stomatal pore. A large amount of water is also lost from the cells of the plant leaves through open stomatal pores. So, when the plant does not need carbon dioxide and wants to conserve water, the stomatal pores are closed.
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During photosynthesis, the oxygen gas produces goes out through the leaves of the stomatal pores. Do you know that stomata are also present in the green stems or shoots of a plant? So, we can say that green stems or shoots also shows photosynthesis. In most broad-leaved plants, the stomata occur only in the lower surface of the leaf but in narrow-leaved plants, the stomata are equally distributed on both the sides of the leaf. In aquatic plants or plants that live in water use carbon dioxide gas dissolved in water for carrying out photosynthesis.
So, we can say that stomata pores allow the movement of gases in and out of plant cells. Therefore, the gaseous exchange in plants takes place through the stomata in leaves and other green parts.