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Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata.
Guard cells are able to control how open or closed stomata are by changing shape. They are like an inflatable set of doors that make the opening between the two cells wider or narrower. The guard cells change shape depending on the amount of water and potassium ions present in the cells themselves.
When the guard cells take in potassium ions, water diffuses into the cells by osmosis. Osmosis is the movement of water from an area of low concentration of solute (dissolved substance) to an area of high concentration of solute. When the water enters the cells, they swell and become bowed. This causes the guard cells to bend away from each other, thereby opening the stomata.
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