Water is a polar chemical and it is a universal solvent in biological systems. All processes of catabolism are using water molecules. For example, the decomposition of polysaccharides, proteins and fats to their monomers ensured in the presence of water molecules. Water molecules are donors of electrons during photosynthesis. The amount of water in living organisms plays a crucial role in maintaining homeostasis. Water has a high heat capacity and a lower density at temperatures below 4 degrees Celsius, which play a huge role in functioning of ecosystems and climate of the planet.