<P> Guard cells are specialized 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 (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 . </P> <P> Opening and closure of the stomatal pore is mediated by changes in the turgor pressure of the two guard cells. The turgor pressure of guard cells is controlled by movements of large quantities of ions and sugars into and out of the guard cells . When guard cells take up these solutes, the water potential (Ψ) inside the cells decreases (creating a hypotonic solution), causing osmotic water flow into the guard cells . This leads to a turgor pressure increase causing swelling of the guard cells and the stomatal pores open . The ions that are taken up by guard cells are mainly potassium (K) ions and chloride (Cl) ions . In addition guard cells take up sugars that also contribute to opening of the stomatal pores . </P>

Place the events in the correct sequence to explain the function of guard cells in opening stomata