What is "guard cell"

n. A kind of specialized cell located in the lower leaf epidermis of plants. Pairs of guard cells surround tiny stomatal airway pores and regulate their opening and closing.

Guard cells are specialized cells in the epidermis of leaves, stems and other organs that are used to control gas exchange.

The guard cells are produced in pairs with a gap between them that forms a stomatal pore (Figure 1). The stomatal pores are largest when water is freely available and the guard cells are turgid and closed when water availability is critically low and the guard cells become flaccid (Figure 1). Photosynthesis depends on the diffusion of Carbon dioxide (CO) from the air through the stomata into the mesophyll tissues. Oxygen (O), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open, water is lost by evaporation and must be replaced via the transpiration stream with water taken up by the roots. Plants must balance the amount of CO absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor and stomatal pore size.