In living organisms, water acts as a temperature buffer and a solvent, is a metabolite, and creates a living environment. Water is an effective and necessary solvent in living organisms.
Many molecular compounds such as amino acids and sugars dissolve in water, and water acts as a solvent for these chemical reactions to take place. It also helps transport compounds into and out of the cells in the body.
Water helps enzymes remain at an optimal temperature. Enzymes are especially sensitive to temperature fluctuations, and too much heat can denature most enzymes, leaving them inactive. Because water has a high heat capacity, it buffers temperature changes and helps enzymes work more efficiently.
As a living environment, water is crucial. Many organisms, such as fish, must live in water to survive. Even small organisms, such as some insects, live on the skin of the water that forms in some lakes and ponds.
As a metabolite, water is invaluable. All chemical reactions take place in the cell, and all of the chemical reactions that occur in living organisms are called the organism's metabolism. Water acts as a metabolite for many of these reactions, and can either be the reactant or a product of a reaction.