The biological importance of water as a solvent and as a medium for living organisms, including change of state and specific heat capacity.
Water is a very useful biological substance. It has many properties that render it essential for the sustenance of life. It is a major constituent both of the tissues in the body, and of the diet. Typically, about 60% of the water we take in comes from drinks, 30% comes from food, and the remaining 10% is metabolic water - the product of respiration.
It is a fairly simple molecule, being made up of one oxygen atom and two hydrogen atoms. Each of the hydrogen atoms share two electrons with the oxygen atom, but due to oxygen's greater electronegativity, it has more pull on the electrons, creating a dipolar molecule, which has regions of slightly positive charge, and slightly negatively charged regions. These charged regions mean that the water molecules attract each other and form what are called hydrogen bonds. Hydrogen bonds give water many of its unique properties.
Water will dissolve many substances because of its ability to hydrogen bond. It is often called the universal solvent, because so many things dissolve in it. Polar and ionic substances also have an electrostatic charge, so they are readily attracted to the water molecules. They are called hydrophilic and dissolve easily in water. Non-polar substances are not attracted to the water molecules and therefore do not dissolve readily in water. They are called hydrophobic and include substances such as oil.
As water dissolves more substances than any other solvent, it is used inside organisms as a transport medium and as the medium for metabolic reactions; the compounds dissolved form ions in solution, which can move around, and are free to enter in to reactions with enzyme molecules. The great capacity of water to dissolve substances also allows it to be used to remove soluble waste products from the body.