Air enters the plant through these openings. The carbon dioxide is used in photosynthesis. Some of the oxygen produced is used in respiration. Surplus oxygen exits through these same openings. Also, water vapor goes into the atmosphere through these pores in a process called transpiration.
The pore is formed by a pair of cells known as guard cells. These adjust the size of the opening by opening or closing. Plants lose water through stomates. Guard cells are able to open and close the stomates to save water. Plants also open their guard cells to bring carbon dioxide into the leaf. To open a guard cell, protons (hydrogen ions, H+) are pumped into the guard cells and water follows, since the water potential inside the guard cell is lower. When water follows, the cells get hard and they push open.