Water dissolves most substances in the body, exceptions being fats and other large polymers such as fibrous proteins.
As well as being the medium for many chemical reactions, water can also play a main part in important reactions. For example, water is a major raw material in photosynthesis, as well as being used in digestion, breaking down large food molecules by hydrolysis. .
Water is present in hydrated molecules. If the water is removed, the chemical and physical properties of the molecule are altered. Many organic molecules occur in a hydrated form.
Being used in the body to take part in chemical reactions is only a small part of what living organisms use water for. Water is a key element in homeostasis, especially temperature regulation within an organism. Water does not change state readily; it has a high latent heat of vaporisation. This means that it needs large amounts of heat to turn it from a liquid into a vapour. This property is very useful to many land animals, as they can use their body heat to evaporate the water from their surfaces. When the water evaporates, it therefore draws a lot of heat energy away from the body. This is the principle upon which sweating is based.
As well as being useful in temperature regulation within living organisms, water plays a key role in the temperature of the environment of an organism. Water has a very high specific heat capacity, which means that it takes a lot of heat energy to raise the temperature of water, but, once heated, the water will cool down only very slowly. This ability to retain heat better than most other substances is very important for organisms that live in water, but cannot control their own body temperature. Because of its high specific heat capacity, the temperature in bodies of water stays relatively constant; it does not fluctuate as rapidly as air temperature. This means that organisms living in water do not have to regulate their own temperature, enabling them to survive.
