Guard cells are types of cells that surrounds stomates in plants. Plants lose water through stomates. Guard cells are able to open and close the stomates to save water. Guard cells are activated by sunlight. To open a guard cell, (H+) protons 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. At night, guard cells lose water and close again.