Final answer:
Transpiration is the release of water vapor from plant leaves through stomata, driven by the water potential gradient. This regulates the plant's water content and contributes to the water cycle. The process does not require ATP and is affected by environmental factors.
Step-by-step explanation:
When plants release water vapor into the atmosphere through their leaves, the process is called transpiration. This occurs through tiny pores on the leaves called stomata. Plants absorb more water through their roots than is needed solely for photosynthesis and the excess water is exhaled into the atmosphere as vapor. This passive process is driven by the water potential gradient between the soil and the atmospheric water and does not require metabolic energy in the form of ATP. Transpiration not only plays a key role in the water cycle but also helps in the movement of water within the plant, being the main driver of water flow through the xylem.
Factors such as soil moisture content, atmospheric humidity, and stomatal regulation affect the rate of transpiration. During the day, when the stomata are open, this process contributes to the water cycle, affecting weather and climate systems by returning water vapor to the atmosphere. At night, when stomata are closed, water is conserved within the plant. The cohesion-tension theory explains how water is held in the plant during these times due to the cohesive and adhesive properties of water molecules.