Either of a pair of cells that control opening and closing of a leaf pore (stoma). Each is a sausage- or kidney-shaped cell whose wall varies in rigidity. The wall bordering the pore is thickened and rigid, whereas the outside wall is thin and extensible. As the paired cells absorb water they swell, and the increased turgor pressure causes the thin-walled region to bend outwards, pulling the nonextensible thicker wall with it and opening the pore. Loss of water has the reverse effect, resulting in shrinking of the guard cells and closure of the pore. The guard cells respond to various environmental cues, including sunlight, water availability, and levels of carbon dioxide in the leaf. Movement of water in and out of the cell is controlled by the transport of protons (H⁺) and movement of potassium ions (K⁺) and other ions into and out of the cell. Sunlight activates a proton pump in the guard cell membrane, which actively pumps H⁺ out of the guard cell into the surrounding apoplast. The resulting electrochemical gradient drives K⁺ and other ions into the guard cells, raising their internal ionic concentration (i.e. making the water potential more negative) and causing water to follow by osmosis through pores (aquaporins) in the membrane. Hence the guard cells swell and the stomatal pore opens. In the absence of sunlight, the proton pumps become much less active and K⁺ ions diffuse out of the guard cells, causing them to lose water osmotically and shrink. These changes can occur in just a few minutes. If the leaf is stressed due to lack of water, the mesophyll cells release abscisic acid (ABA), which triggers the guard cells to close the stomata, even in the presence of sunlight, to conserve moisture within the leaf.