How Do Guard Cells Function?
Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. To understand how they function, study the following figures. As you look at the figures, keep in mind that an increase in solute concentration lowers the water potential of the solution, and that water moves from a region with higher water potential to a region of lower water potential.
Notice that in figure A the guard cells are turgid, or swollen, and the stomatal opening is large. This turgidity is caused by the accumulation of K+ (potassium ions) in the guard cells. As K+ levels increase in the guard cells, the water potential of the guard cells drops, and water enters the guard cells.
In figure B, the guard cells have lost water, which causes the cells to become flaccid and the stomatal opening to close. This may occur when the plant has lost an excessive amount of water. In addition, it generally occurs daily as light levels drop and the use of CO2 in photosynthesis decreases.
Procedure
1. Examine the slides of guard cells under a microscope.
2. Locate guard cells that are both open and closed, and diagram them below.
Questions
1. Where are guard cells located in leaves?
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2. What type of cells are guard cells?
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3. What do guard cells control?
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4. Why are guard cells important to plants?
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5. What specifically controls the opening and the closing of the stomata within guard cells?
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6. In general, what conditions in the environment, would cause a stomata to be open, and what conditions would cause it to be closed?
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7. Are stomata more likely to be open or closed on a hot day? Explain your answer.
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