The chemistry of water.
Water is the medium in which all the reactions take place in living cells.
Water is a polar molecule because the electrons are held closer to the oxygen atom than the hydrogen atoms. This also makes it a polar solvent, this means that many ionic substances like sodium chloride will dissolve in it because the slightly positive hydrogen atoms attract the negative chloride ions. Water is also an excellent transport medium because so many different substances will dissolve in it.
Water is also a unique molecule because ice is less dense than water and it floats. When water reaches 4 degrees celsius it reaches its maximum density and as it cools further the molecules become more widely spaced, as a result ice floats and forms and insulating layer which prevents the water beneath it freezing and allows organisms to live in water even in countries where its cold enough to freeze in winter.
The hydrogen bonds between the molecules also require a lot of energy to break this means water has a high specific heat capacity and is slow to absorb and release heat. Water's specific heat capacity is 4181 J / kg °C. This means that the temperature of large bodies of water such as lakes and seas does not change much throughout the year, therefore water is a good habitat for living organisms.
Water is also useful in hydraulic mechanisms in living organisms because it doesn't compress, meaning it helps keep the turgidity of plants and other organisms.
Water is also useful in plants due to the molecules cohesiveness and adhesiveness, which helps the movement of water from the roots to the leaves of plants (cohesive) and in surface tension and plant transport systems (adhesive).
Waters surface tension is a useful property in helping plants transport water around the plant. it is also useful to some life at the surface of ponds and lakes because the attraction between the water molecules (hydrogen bonds) is greater than the attraction between the water molecules and the air forming a thin skin of surface tension and allowing insects to move across the waters surface.