Question

Why and How Guard cells open and close stomata?

Answer 1

Hint: Stomata are tiny pores present on the surface of plants' parts like leaves stem and help in gaseous and water exchange during photosynthesis and transpiration. Stomata contain a small pore called ‘stoma’ which is surrounded by a pair of kidney-shaped cells called Guard cells.

Complete answer:
Guard cells have thicker walls on the inner side and thinner on the outer side. This uneven thickening is responsible for the opening and closing of the stoma. Also, the walls are semi-permeable in nature. When there is less water concentration in the cell so the water from the nearby regions enters the cell (osmosis) and makes the cell turgid. The cells bend away due to thick inner walls thereby opening the pore.
When the water potential is less in the nearby region then the water from the cell moves out and makes the cell shrunken or flaccid thereby closing the stoma due to the thick inner walls.

Many theories explain the water movement in and out from the guard cells. There are pH theory, sugar starch theory, and Active K+ theory. According to the K+ theory, the guard cells use ATP driven pumps to move H+ ions and thereby hyperpolarize the membrane. This causes the inward movement of the K+ ions inside and decreases the water potential inside which in turn causes the water movement inside the cell. Opposite to this, anions such as chloride (Cl-) are released through channels and depolarize membranes which in turn activates the outward K+ channels. This causes more water potential inside the cell so the water moves outside the cell thereby closing the gates.

The opening and closing mechanism of the stomata is necessary to allow the exchange of gases for photosynthesis and respiration. It is required for the movement of water for transpiration and to maintain moisture balance.

Note: The stomata are present only on the epidermis and are present on the inner and outer leaves. They are generally present on the underside of the leaf because direct contact with excessive sun can cause more transpiration thereby withering the plant.