Biological significance of water

Some biological importance of water are;

Water is excellent solvent for polar substances. These includes ionic substances like salts, which contain charged particles and some non-ionic substances like sugar that contain polar groups (slightly negatively charged hydroxyl group)
When polar substance dissolve in water, ions and polar groups are surrounded by water molecules which separates the ions or molecules from each other.
Once the substance is in solution its molecules or ions can move freely thus making it more chemically reactive than if it is solid. Thus majority of biochemical reactions in cells take place in aqueous solution.
The non-polar molecules such as lipid are repelled by water and usually group together forming bulky molecules. Thus these non-polar molecules are known as hydrophobic.
Water is medium for diffusion

The solvent property of water make it as excellent transport medium in blood, lymphatic system, alimentary canal, xylem and phloem in plants.

Water helps in dispersal of seeds, gametes and larval stages of aquatic organism and also fertilization by swimming gametes
It also helps in germination of seeds by swelling and breaking open of testa and further development
Helps in osmosis and turgidity which results in cell growth
Helps in transpiration in plants
Helps is osmoregulation in animals such as cooling by evaporation such as sweating and panting
Water also helps in support mechanism, for example in annelida, water acts as hydrostatic skeleton
Water also helps in protection. For example lacrimal fluids, mucus

The heat capacity of water is the amount of heat required to raise the temperature of 1 kg of water by 1°C.
Water has a high heat capacity. This means that a large amount of heat is require to rise relatively small temperature. This is because much of the energy is used in breaking the hydrogen bond which restrict the movement of molecules.
Temperature change within water are minimized as a result of its high heat capacity. Biochemical processes therefore operate over a smaller temperature range, proceeding at more constant rates and less likely to be inhibited by extremes of temperature.
Water also provides a very constant external environment for many cells and organisms

Latent heat of vaporization is a measure of the heat energy required to vaporize a liquid that is to overcome the attractive forces between its molecules so that they can escape as gas.
A relatively large amount of heat energy is needed to vaporize water. This is due to the hydrogen bonding. As a result water has an unusually high boiling point for such a small molecule.
The energy transferred to water molecules to allow then to vaporize results in loss of energy from their surroundings, thus cooling takes places. Sweating and panting of mammals, opening of mouth of some reptiles such as crocodile and transpiration of leaves helps in cooling of body.
The high heat of vaporization means that a large amount of heat can be lost with minimal loss of water from the body.

Latent heat of fusion is a measure of the heat energy required to melt a solid.
Due to high heat capacity of water, ice requires relatively large amount of heat to thaw.
Conversely, liquid water must lose a relatively large amount of heat energy to freeze.
Contents of cells and their environments are therefore less likely to freeze. Ice crystals are particularly damaging if they develop inside the cells.

The density of water decrease below 4°C, therefore ice floats in water.
Since ice floats, it forms at the surface first and the bottom last. If ponds froze from the bottom upwards, freshwater organisms could not exist in arctic climates.
Ice insulates the water below it, thus increasing the chances of survival of the organism in water.
Also ice thaw more rapidly at surface.
Water below 4°C tends to rise upwards thus it helps in circulation of water resulting in nutrition cycling and colonization of water to greater depth.

Cohesion is the force whereby individual molecules stick together.
At the surface of a liquid, a force called surface tension exists between the molecules as a result of cohesive force between the molecules. These cause the surface of liquid to occupy the least possible surface area.
Water has a higher surface tension than any other liquid. The high cohesion of water molecule is important in cells and in translocation of water through xylem in plants.
Many small organism rely on surface tension of water to settle on water or to skate over its surface.

Water is biologically significant as an essential metabolite for biochemical reaction in cell.
Water is the source of hydrogen in photosynthesis and used in hydrolysis reaction.