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Plant Guard Cells: Function & Definition

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  • 0:01 The Role of Guard Cells
  • 0:55 How Guard Cells Work
  • 1:50 How They Know to Do Their Job
  • 3:20 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Plants 'sweat' through openings in their leaves, but what controls these openings? Guard cells are small but important cells in leaves that help plants maintain optimal health.

The Role of Guard Cells

When you get hot, you sweat, and when you sweat, water comes out of your skin through pores called sweat glands. Plants also 'sweat' through a process called transpiration, and the plant's pores, which are found on the leaves, are called stomata.

The difference between you sweating and the plant 'sweating' is that your body is purposefully pushing water out of your sweat glands to cool off. Plants do not like losing water, but it is a necessary trade-off because the stomata allow gases to exchange during photosynthesis. Photosynthesis is the process of converting sunlight into usable chemical energy in plants, and this process is necessary for the exchange of oxygen and carbon dioxide to occur.

So, how do plants keep from losing all their water through stomata? Surrounding each stomata are two guard cells, which regulate the opening and closing of stomata to facilitate gas exchange and control transpiration in plants.

guard cell and stoma

How Guard Cells Work

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.

Conversely, when guard cells lose potassium ions, water diffuses out of the cells by osmosis. As water leaves the cells, they become flaccid and less bowed, which closes the stomata between them.

How Guard Cells Know to Do their Job

How do guard cells know how to do their job? There are several signals that indicate to guard cells whether to open or close stomata, helping them maximize gas exchange while minimizing water loss.

Light is one environmental signal for guard cells. Guard cells usually keep stomata open during the day and closed at night because during the night there is no sunlight for photosynthesis. If the stomata were open at night, very little gas exchange would occur, but water loss would still continue.

Temperature also signals guard cells. Warmer temperatures can increase evaporation rates, which means more water will be lost through transpiration. However, warmer temperatures are also characteristic of optimal growing season for plants, so this can be a tricky balance for the plant.

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