Question

what are the functions of guard cells

Answer 1

Guard cells contain phototropins which are serine and threonine kinases mediated by light. They contain two light, oxygen, and voltage (LOV) domains, and are also part of the PAS domain superfamily[5]. These phototropins trigger many responses such as phototropism, chloroplast movement, leaf expansion, and stomatal opening[5]. Light is the main trigger for the opening or closing of stomata. Not much was previously known on how the photoreceptors work. To understand the mechanism to which phototropins work, an experiment on broad bean (Vicia faba) was done. 

Answer 2

guard. cells are surrounded by stoma.
FUNCTION- they help to regulate the rate of transpiration on plants.
they also open and close the stomatal pore
they protect stomata pore
they swell when water flows into them causing stomatal pore open. similarly the pore closes if the guard cell shrinks