In order for there to be photosynthesis, plants must be able to exchange gases in their leaves. This picture shows the process of gas exchange within a cell of the leaf. During transpiration, diffusion occurs and the water vapor from the moist air spaces would transfer to the drier air outside by means of the stomata. The water vapor that is lost is replaced by evaporation that occurs from the water film that coats the mesophyll cells. The evaporate would cause the air-water interface to delve deeper into the cell wall and cause a greater curvature. This curve would increase surface tension as well as the rate of transpiration. Then, it would pull water from the xylem into surrounding cells and air spaces in order to compensate for the water loss from transpiration.
Guard cells play a crucial role in gas exchange; they are the regulators of the entire process. The stoma must open to allow gas exchange and they must close to regulate water loss. Guard cells are the structures that regulate the opening and closing of the stomata.
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