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Why stomata and g

Answer :

A stoma (plural stomata) is a pore, found in the leaf epidermis that is used for exchange of gasses between the internal air spaces and the ambient environment. The stoma together with its guard cell and subsidiary cell called stomatal complex or stomatal apparatus.

Stomata and guard cells are called turgor operated valves because each stomata consists of a minute pore called stoma surrounded by two guard cells. The stoma acts as a turgor operated valve which closes and opens according to the turgidity of guard cells. The guard cells are the only epidermal cells that contain chloroplasts.


The guard cell function as hydraulically operated valve. When carbon dioxide required for photosynthesis the guard cells take up water and swell to open the pore, and lose water to close the pore when CO2 is not required for photosynthesis or when water stress overrides the photosynthetic needs of the plant. More than 90% of the CO2 and water vapour exchange between plant and its environment passes through the stomata.

The driving force for stomatal opening is known to be osmotic uptake of water by the guard cell and the consequent increase in the hydrostatic pressure. The result is the deformation of the opposing cells that increase the size of the opening between them. In the case of the elliptic guard cells the thickened walls become concave while in the dumbbell-shaped cells the handle separated but remain parallel. Stomata closure follows a loss of water, and the consequent decrease in hydrostatic pressure and the relaxation of the guard cell walls.

Stomata opening and closure is controlled by number of internal and environmental factors such as light, CO2 levels, water status of the plant and temperature. Stomata will open in the night in order to admit CO2 for photosynthesis or partially close when CO2 levels are high in order to conserve water while allowing photosynthesis to continue. On the other hand, conditions of extreme water stress should override the plant’s immediate photosynthetic needs and lead to closure, protecting the leaf against the potentially more damaging effects of desiccation.

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