The hydrogen bonds mean that water is liquid at room temperature, unlike many similar compounds. Hydrogen sulphide (H2S) is a gas, which means that oxygen, being a lighter element then sulphur should be in gaseous form at room temperature when bonded with hydrogen as water.
However, this means little in biological terms, until it is realised that the dipolar nature of water means that it a very good solvent, and therefore can be used to transport almost any substances (except lipids). This allows water to be used as a transport medium in blood, lymphatic and excretory systems.
When substances are in solution, the molecules and ions can move about freely, thus making the substance more reactive then if solid. Therefore, the majority of cell reactions occur in aqueous solutions. Even lipids, which are immiscible with water, are useful, because they can be used as membranes, to separate aqueous solutions.
Water is also the source of hydrogen and oxygen for many forms of life, most notably in autotrophic plants, where water along with carbon dioxide is synthesised into glucose.
Life is also based on water due to its high specific heat capacity. For warm- blooded animals, this means that a stable internal environment is possible; to change the internal temperature, a lot a heat must be acquired or lost. In oceans and seas, the water temperature varies little; around Britain, the sea temperature varies between 7ºC and 16ºC annually, which is very stable, relative to the air, which can vary from -10ºC to 30ºC. This means that for cold- blooded aquatic life, their external environment is stable, and therefore there is less chance of death due to extremes of temperature.
The high specific heat capacity of water also means that it has a high latent heat of vaporisation; it has to gain a lot of heat energy to evaporate. This means water is used for thermoregulation in animals and plants. For example, humans sweat; the water absorbs energy from the skin, which allows the water to vaporise, and the skin cools down. The same effect is achieved when animals pant; even cold- blooded alligators open their mouths on hot days to stop overheating. Also, the evaporation of water from leaf cells in plants achieves the same effect.
The density of liquid water is very important. Firstly, it is just right for aquatic animals. If water were more viscous, it would be difficult to move (and thus to swim) in (i.e. it would be difficult to move in fuel oil, or liquid tar). However, if water was more volatile, then aquatic organisms would find it hard to keep afloat (i.e. it would be hard to keep afloat in olive oil!).
The density of water is unusual when between 0ºC and 4ºC. The density of most liquids decreases as temperature increases, but when the temperature of water rises from 0ºC to 4ºC, its density actually increases. This means that ponds and lakes do not freeze from the bottom up, because the coldest water is also the least dense, so it stays at the top. This means that a layer of ice forms at the top. This layer of ice actually acts as an insulator, so the remaining liquid water stays liquid for longer (i.e. in Britain, ponds and lakes never freeze through; the thickest ice found is about six inches). This property means that aquatic life in freshwater lakes can survive air temperatures of -10ºC. The diagram below explains this, which is one of the many reasons why living organisms consist of at least 70% water.
Sources
The research sources used were Biological Science 1, by N.P.O. Green, G.W. Stout and D.J. Taylor, Advanced Biology by Mary and Geoff Jones. The websites and were used, as were sections from the New Scientist and Biological Sciences magazines
