Stomata are openings in the epidermis through which gas exchange occurs. They are found mainly on leaves, but also on stems. Each stomata is surrounded by two closure cells, which, unlike ordinary epidermal cells, contain chloroplasts. Trailing cells control the size of the stomatal opening by changing their turgidity. In the light, with sufficient moisture, the stomata are open, in the dark or with a lack of water, they are closed. The mechanism of work of stomata is determined by the following structural features of the trailing cells: they contain chloroplasts, while the rest of the epidermal cells do not contain them; trailing cells have a thickened wall on the side of the stomatal fissure. In the light there is a process of photosynthesis only in the closing cells; the resulting sugars increase the concentration of cell juice, which, due to the laws of osmosis, causes water to enter these cells. Turgor pressure rises, and the cells begin to swell, increasing in volume. But this is prevented by the cell wall, especially its side facing the stomatal fissure – thickened. As a result, the closure cells stretch toward the main epidermis, where the walls are thinner and the thick ones follow the entire cell – the stomata opens. At night, when photosynthesis does not occur, the trailing cells return to their place and close – the stomata closes. It is noted that when opening stomata, potassium ions move into the closure cells, which also determine an increase in turgor pressure and cell volume.
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