Guard cells are specialized plant cells in the epidermis of leaves, stems and other organs that are used to control gas exchange. They are produced in pairs with a gap between them that forms a stomatal pore. The stomatal pores are largest when water is freely available and the guard cells turgid, and closed when water availability is critically low and the guard cells become flaccid. Photosynthesis depends on the diffusion of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), 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 CO2 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 pressure and stomatal pore size. Guard cell function Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. Light is the main trigger for the opening or closing. Each guard cell has a relatively thick cuticle on the pore-side and a thin one opposite it. As water enters the cell, the thin side bulges outward like a balloon and draws the thick side along with it, forming a crescent; the combined crescents form the opening of the pore. Guard cells contain phototropin proteins which are serine and threonine kinases with blue-light photoreceptor activity. Phototrophins contain two light, oxygen, and voltage sensor (LOV) domains, and are part of the PAS domain superfamily. The phototropins trigger many responses such as phototropism, chloroplast movement and leaf expansion as well as stomatal opening. Not much was known about how these photoreceptors worked prior to around 1998. The mechanism by which phototropins work was elucidated through experiments with broad bean (Vicia faba). Immunodetection and far-western blotting showed blue light excites phototropin 1 and phototropin 2, causing protein phosphatase 1 to begin a phosphorylation cascade, which activates H+-ATPase, a pump responsible for pumping H+ ions out of the cell. The phosphorylated H+-ATPase allows the binding of a 14-3-3 protein to an autoinhibitory domain of the H+-ATPase at the C terminus. Serine and threonine are then phosphorylated within the protein, which induces H+-ATPase activity. The same experiment also found