Stomatal pores in plants regulate the amount of water and solutes within them by opening and closing their guard cells using osmotic pressure.

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In order for plants to produce energy and maintain cellular function, their cells undergo the highly intricate process of photosynthesis. Critical in this process is the stoma. Stomata (multiple stoma) are located on the outermost cellular layer of leaves, stems, and other plant parts. An open stoma facilitates the process of photosynthesis in three ways. First, it allows light to enter the intercellular matter and trigger the process. Second, it allows for the uptake of carbon dioxide, a key chemical in producing plant energy. Third, it allows for oxygen to be expelled into the outside environment, a byproduct of photosynthesis that is no longer needed by the cell.

While an open stoma is necessary for the plant to undergo photosynthesis, it comes with a negative side effect: water loss. Over 95% of a plant’s water loss occurs through the stoma via water vapor. Therefore, a delicate balance must be maintained that allows light and gases to pass between cells, and does not put the plant at risk for dehydration.

This problem is mitigated with guard cells. Guard cells are a pair of two cells that surround each stoma opening. To open, the cells are triggered by one of many possible environmental or chemical signals. These can include strong sunlight or higher than average levels of carbon dioxide inside the cell. In response to these signals, the guard cells take in sugars, potassium, and chloride ions (i.e., solutes) through their membranes. An increase in solutes induces an influx of water across the guard cell membrane. As the volume of the guard cells increase, they “inflate” into two kidney-bean-like shapes. As they expand, they reveal the stoma opening in the center of the two guard cells (similar to a hole in the center of a doughnut). Once fully expanded, the stoma is open and gases can move between the cell and external environment.

The stoma’s pore closes in the opposite manner. Excess loss of water through the stoma, such as during a drought, triggers chemical reactions that signal water and ions to leave the guard cells. As solutes exit the guard cells, the pair “deflates,” subsequently closing the stoma like two flat balloons.

This summary was contributed by Allison Miller.

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“When a guard-cell pair accumulates solutes, the resultant turgor and volume changes cause the guard cells to bow outward because of cell-wall architecture, enlarging the pore between them. This simple explanation belies the underlying complexity of guard-cell turgor regulation and whole-plant responses.” (Outlaw Jr. 2003:503)

“During stomatal opening, the flanking guard cells accumulate K+ salts (Outlaw, 1983; Zeiger, 1983) and sucrose (Talbott and Zeiger, 1998). Osmotic H2O influx causes increased guard-cell turgor, asymmetric guard-cell enlargement, and a consequent increase in stomatal aperture size. During stomatal closure, solutes are dissipated. The essence of stomatal regulation is therefore regulation of membrane transport” (Outlaw Jr. 2003:505)

Journal article
Integration of Cellular and Physiological Functions of Guard CellsCritical Reviews in Plant SciencesJune 25, 2007
William H. Outlaw, Jr.

Journal article
Pallas Jr. JE. 1966. Mechanisms of guard cell action. The Quarterly Review of Biology. 41(4):365-383.The Quarterly Review of BiologyPallas Jr. JE

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