Water is a polar molecule- the oxygen has a negative charge and the hydrogen has a positive charge. Water can therefore dissolve polar or ionic substances and keep them in solution. Substances that dissolve in water are known as hydrophilic substances. Ionic substances such as sodium chloride, NaCl, are made up of both positive and negative ions. Sodium chloride is held in it’s structure by the strong attraction between it’s positive sodium ions and negative chloride ions. Normally these ionic attractions require a large amount of energy to break, but when put into water, the negative oxygen side of the water molecules cluster around the positive sodium ions [Na+] and the positive hydrogen atoms cluster around the negative chloride ions [Cl-]. The attraction between the Na+ and the Cl- ions is weakened as the ions are separated.
Water can also separate covalently bonded molecules such as glucose. This is because glucose contains a polar hydroxyl group in its structure which forms hydrogen bonds with water molecules, so separating the glucose molecules from each other.
The ability of water to act as a solvent is vital to life as most biochemical reactions such as respiration occur in solution. Hence cytoplasm is made up of about 90% water. However, water cannot dissolve hydrophobic substances such as fats and oils. This is why these substances are used as cell membranes as they prevent water from entering the cells.
Water is also good at maintaining its temperature despite fluctuations in the temperature of the surrounding environment. Water has a high specific heat capacity, the result of this being that it takes 4.2 Joules of energy to raise the temperature of 1g of water by 1°C. This means that a lot of energy is required to raise the temperature of water significantly, but that once warm, it cools slowly. This is useful because the range of temperatures in which biological reactions can proceed is narrow, and most organisms cannot tolerate large variations in temperature. This is useful for humans, as it allows us to maintain a constant temperature in our bodies (homeostasis).
Also, because of the large number of bonds holding water together, it takes 2Kj per gram of water to separate the bonds and turn the liquid to vapour. Water is therefore described as having a high latent heat of evapouration. This fact is essential to life as animals use excess body heat to evapourate water from their surfaces, resulting in them transferring a lot of energy to the environment but only losing a little water. This is the rationale for sweating when hot.
Three hundred Joules of energy are required to turn 1g of solid ice into water. This means that it is not easy for water to freeze. This is vital in cells as cytoplasm (approximately 90% water) would be irreparably damaged if it was allowed to freeze. The solutes in the cytoplasm also disrupt the hydrogen bonds making it freeze at a much lower temperature, thereby protecting cells further.
Ice has a lower density that water which gives this liquid another unique property. A layer of ice can insulate the water below, insuring that it remains liquid and that aquatic life can continue to flourish.
Water is also a key medium in many reactions, most notable condensation reactions, where water is removed from molecules to bond them together, and in hydrolysis reactions, where water is added to molecules in order to separate them. Hydrolysis in particular is important in plants and animals as it allows them to break up large carbohydrate molecules into smaller ones which can then easily be stored. Photosynthesis also depends upon water as a source of hydrogen atoms which are used to produce glucose which can then be stored in plant cells as starch or used in respiration, which produces gaseous water molecules as a by-product.
The transparency of water is also important as it allows sunlight to pass through it so that aquatic plants can photosynthesise.
Another important property of water is that water molecules are extremely cohesive because of the hydrogen bonds between molecules. The cohesive properties of water allow plants to pull up water through xylem vessels from the roots to the leaves, in a process known as the transpiration stream. This also means that at the boundary where water meets air, water molecules will be held tightly together and the water molecules will form an ‘elastic film’ known as water tension. Water tension is the force that causes the surface of a liquid to contract so that it occupies the least possible area. Water has the highest surface tension of any liquid other than mercury. Small creatures can get stuck in the surface water as they cannot break the water surface tension. Also, creatures such as water skaters are able to ‘walk’ on the surface of water without sinking as they have hydrophobic feet which stop them from breaking the surface tension.
Finally, due to water’s strong hydrostatic forces, it is incompressible. This provides support for soft bodied creatures such as worms, slugs and jellyfish who do not require a strong supporting skeletal system. Water confers turgidity to plant cells, maintaining maximum leaf surface area, maximum light absorption and so maximum photosynthesis to occur.
In conclusion, water’s unique biological properties make it perhaps the most biologically important substance on the face of the earth, and without it life as we know it would cease to exist.
