To understand the role of living organisms in the water cycle, you must first understand the general processes of the water cycle and its non-living components. The three main steps of the cycle are condensation, precipitation and evaporation.
Condensation is the change from a gas to a liquid; this occurs in the atmosphere under conditions of low temperature and pressure, resulting in the formation of clouds. Precipitation occurs when the water droplets formed among the clouds become large and heavy and fall to the ground, most commonly in the form of rain. Other forms of precipitation include snow, sleet and hail, and depend upon other atmospheric conditions.
The final step of the cycle is evaporation, during which water from the Earth's surface evaporates into gas, which rises back into the atmosphere. The greatest sources of evaporation are large bodies of water, such as lakes and the ocean. Living organisms, mainly plants and animals, also contribute to this last step of the cycle through processes, such as respiration, perspiration and transpiration.
Both plants and animals produce oxygen and water as a byproduct of respiration, the cellular processes responsible for producing the energy needed to carry out cell functions. During respiration, gas exchanges take place as part of the reaction; in the case of animals, gas exchange takes place in the lungs, where CO2 and water vapors are exhaled and oxygen is taken into the bloodstream. Animals contribute to the water cycle as they respire, breathing out water vapors that rise to the atmosphere.
Animals also contribute to the water cycle through perspiration. They perspire to prevent overheating; during this process, water is excreted through the pores and evaporates off the skin, cooling the animal by absorbing the external heat. Other forms of excretion, namely urination and defecation, carry high concentrations of water, which are absorbed into the ground and eventually merged into groundwater systems or other bodies of water.
Of all living organisms, plants contribute the most to the water cycle. To produce their energy, plants perform photosynthesis, in which energy from sunlight is harvested to produce sugars from CO2 and water. Plants draw water from the surface and/or from under ground with their roots, and it is then transported upward to the leaves where photosynthesis takes place.
While much of this water is used for the photosynthesis reactions, a great deal of evaporates from the surface of the leaves. This type of plant-water evaporation is called transpiration. Transpiration from the tree canopies in forests plays a huge part in the water cycle; in fact, about 10 percent of all water vapor cycled back into the atmosphere is from plant transpiration.