As the opposite charge attract one another, hydrogen atoms of one water molecule are weakly bonded to the oxygen atoms in adjacent water molecules. These weak bonds are called hydrogen bonds and this is the reason behind the high melting and boiling points of water when compared to similar compounds.
The polar nature of the water molecule is the reason that water is such a good solvent as it can easily ionize substances.
Often in living organisms substances must be solutions and water is the solvent which makes this a possibility. Eg. Plants can only obtain mineral salts in solution and human digestion will only dissolve soluble foods, so large starch molecules must be broken down in soluble sugars as glucose.
Gas exchange is another important factor and it requires a moist surface as it takes place in solution. In mammals the alveoli in the lungs are moist and in plants their leaves have moist surfaces for the gas exchange to occur.
Many organisms living in water spend much or all of their time under the surface so they need oxygen gas to respire and as water is such a good solvent the required oxygen gas is dissolved in the water.
As water has a high specific heat capacity, this means that to raise the temperature of a certain amount of water by as little as 1oc a large amount of energy is needed therefore the temperature of water is not easily changed. This then shows that bodies composed largely of water are very thermo stable and less prone to heat damage by changes in environmental temperatures.
This leads on to why mammals sweat. Mammals sweat when their body temperature is too high and the large amount of energy required to evaporate the sweat from the surface of the organisms is taken from the organism, this giving a large cooling effect. Plants also loose water through the process of transpiration, and this helps water uptake by the roots. It has an effect on the temperature of the plant and also helps to keep this from rising too high.
Water is also used in the process of transport. Many substances produced in organisms are often needed to be transported to other parts of the organism. Eg. In humans blood is used to transport food, hormones, oxygen and waste products and in plants sap is used to transport food. As both of these are made up of water, the water acts as the solvent which allows them to be dissolved so they can be transported.
Lubricants which are made up of water are very important aspect, as the lubricant makes sure it gives protection. Eg the lubricants of joints and bones make sure they don’t scrape against each other causing damage and enables free easy movement by reducing friction. There are many other fluids which act as lubricants-
Synovial fluid- lubricates many vertebrate joints
Pleural fluid- minimises friction between lungs and the rib cage during breathing.
Mucus- allows food to pass more easily through the gut.
As water cools towards its freezing point the individual molecules slows down sufficiently for each one to form its maximum number of hydrogen bonds. For this to happen the water molecule are widely separated, while in liquid form they are closer together although less tightly bonded. Therefore the water expands, and as it freezes it becomes less dense than liquid water and is able to float upon its surface.
As the ice forms on the surface of ponds and lakes it insulates organisms in the water and below it. The floating ice also gives penguins and polar bears somewhere to stand.
From this we can see that water is the most useful substance on earth, without it, there would be no life. Water in biological terms has great importance both inside cells and externally.
