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Stomates are located in the leaf and stem epidermis of plants. They are the sites of gas exchange--carbon dioxide enters and is used for photosynthesis, oxygen and water vapor then exit. And for respiration, oxygen enters and carbon dioxide and water vapor can exit the stomates. Guard cells are responsible for regulating the size of the stoma or pore. It seems that light intensity and high humidity trigger the electrical potential of the cells to become negative. This opens potassium channels and potassium uptake increases. This causes a higher solute concentration, and causes diffusion of water into the cell by osmosis. This leads to an increase in turgor pressure which in turn elongates the guard cells and this creates an opening for gases to move in or out of a leaf. This opening is called the stomate. In dry conditions, abscisic acid is released, which in turn causes chloride ions to exit the cells. This allows a gradient to exist and water to move out of the cells by osmosis, which causes the guard cells to shrink and the stomates to close. This is an adaptation to conserve water during dry periods.
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