While the cells of living organisms are made primarily of water, the biosphere is not a large reservoir. However, the water stored in living organisms is essential for cells to operate. Thousands and thousands of different organic chemicals, proteins, amino acids, carbohydrates, and so forth are dissolved within the water in the cell.

The processes of photosynthesis and cellular respiration both use water. Remember, photosynthesis is the process by which plants produce energy - in the form of glucose - for an ecosystem. And respiration is the chemical process by which organisms consume the glucose and release energy needed by their cells to stay alive and perform their functions. In photosynthesis, water in the atmosphere is combined with sunlight and carbon dioxide in the plants' chlorophyll, and converted into energy. In respiration, water is given off as a byproduct and stored in the cells.