Biological importance of water
Water is essential to every living organisms. Water is a major component of cells, it takes up around 70% to 95% of mass of the cell. Water possesses unique properties, for instance, high specific heat capacity, solvent properties, high incidence of hydrogen bonding and low viscosity. It is also present in most body fluids. These characteristics cause water to become biologically important to all living organisms. The biological importance of water is categorized into the following six major functions and is explained with corresponding examples.

Transport
Water is a medium for transporting substances around a body or plant. For example, in human bodies, useful substances like glucose and oxygen first dissolve in fluid plasma, they are then transported by plasma, a body fluid with water and protein, to different body cells for respiration, so energy will be produced. Carbon dioxide, released from cells after respiration, dissolves in plasma and is transported out the body by diffusion. In plants, water helps transport nutrients into their bodies. Water in the soil has some solutes dissolved in it, such as mineral ions (nitrate, phosphate, magnesium). However, the concentration of ions in soil water is much lower than that inside root hair cell, so soil water has a higher water potential. This allows water to enter root hair cell by osmosis, so minerals can be taken by plants for growth.

Metabolism
Water acts as a reactant in important chemical reactions. For instance, in photosynthesis of plants, light energy from sun is absorbed by chlorophyll. Together with carbon dioxide and water, plants carry out photosynthesis. The process produces glucose as energy supply and also oxygen as a by-product. Taking another example in animals, water is needed as reactant for hydrolytic reactions. In the digestion of proteins, carbohydrates and fats, hydrolysis is required for their break down during the chemical digestion process. Therefore, water is indispensable in metabolic reactions.
Besides acting as a reactant, water also provides an important environment for chemical reactions to take place. Take respiration process as an example, in which glucose is metabolized, it has to first dissolve in water because all enzyme reactions have to be carried out in solution.

Temperature control
Due to the hydrogen bonding between molecules, water has a high latent heat of vaporization. It helps body to cool down efficiently. When surrounding temperature is higher than body temperature, the body begins to sweat. The evaporation of sweat from skin surface absorbs a large amount of latent heat from our body, so cooling the body very efficiently.
Besides the high latent heat of vaporization, hydrogen bonding also gives a special characteristic to water that is a high specific heat capacity. This means a lot of energy is needed to heat up water. This feature is especially useful in both endothermic and aquatic organisms. As water does not heat up or cool down easily, the temperature of water does not fluctuate much. Thus, body temperature of species can be kept constant.
In addition, Water reaches its maximum density at about 4℃, ice is less dense than water and thus floats on top of it. In winter, water freezes from the top down, so this gives an insulating layer for the water below, keeping a higher temperature underneath. This is an advantage for aquatic organisms. Since water will not freeze solid. The aquatic animals can still maintain their activities and survive.
A steady temperature is also important for enzymes to function properly, because enzymes work best in a body at a constant optimum temperature. For human beings, water is also important for regulating body temperature. Our body contains a high proportion of water (around 65%). Therefore, our body temperature can be kept rather constant even when the temperature of the environment changes drastically.