BIO203H5 Lecture Notes - Lecture 7: Vapor Pressure, Electrochemical Gradient, Guard Cell

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5 Nov 2020
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The area immediately inside stomata are usually large, air-filled interstitial spaces called substomatal cavities. These are surrounded by highly photosynthetic mesophyll cells. When stomata are open,transpiration is driven through a difference in vapour pressure between the substomatal space and the external environment. High vapour pressure has higher water pressure so water will flow out into the lower area -- water moves with gradient , water moves potential gradient from high potential to lower vapour pressure. As water evaporates from the substomatal space, the water potential of this area becomes lower than the in the surrounding cells. This causes water to move with this gradient, from the cells to the substomatal cavity, where it can be lost to evaporation. Now the mesophyll cells surrounding the substomatal cavity have a lower than surrounding cells. Water now flows down the gradient to replace the water lost to transpiration. Guard cells are opened and closed using turgor pressure.

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