Photosythesis, respiration, and transpiration are the three main functions that drive plant growth and development that are essential to a plant's survival.
Photosynthesis ia the process by which plant produce their own food for survival. Sunlight gives the energy in photosynthesis. H2O and CO2 are used to make sugar and O2. O2 is released through the stomata.
Respiration is when plants break down sugar and use the enrgy to do life functions. To get energy from the food it makes, plants break down the sugar through respiration. O2 from the air blend with sugar and makes CO2 and H2O. Energy is released so it can be used by the plant and gives off CO2 and H2O through the leaves.
Transpiration is when plants lose water through the leaves. Plants slow down transpiration by closing the stomata because plants must control the amount of water they lose or they would wither and die.