The Biological Importance of Water

Gas exchange requires a moist surface, since gas exchange takes place in solution. In mammals, the alveoli in the lungs are moist with water, to allow gas exchange, and many plants have moist surfaces in their leaves (the mesophyll cells) for gas exchange.

Water also has many thermal properties as a temperature stabiliser. Water has a boiling point of 100oC and a melting point of  0 oC. Water also has a high specific heat capacity, the result of this is that it takes 4.2 Joules of energy to raise one gram of water by 1oC. This means that it takes a lot of heat energy to raise the temperature of water significantly, but once warm it cools slowly. This is essential to life where internal body temperature has to be maintained at a constant temperature and fluctuations can result in a breakdown of essential processes. Large bodies of water will remain at an almost constant temperature with only very gradual changes which makes temperature regulations for organisms far more straightforward.

Because of the large number of bonds holding water molecules together, it takes 2 kJ per gram of water which is a considerable amount of energy to separate the bonds and turn the liquid to vapour. Water is therefore described as having a high latent heat of evaporation. Animals use this property of water by using excess body heat to evaporate water from their surfaces, resulting in them transferring a lot of energy into the environment but only losing a little water. Sweating and panting are based on this principle.

Water also has a high latent heat of fusion from solid to liquid. It requires 300 J per gram of ice to melt it to water. This means that water stays liquid. This is vital in the case of cytoplasm in cells which is made of a high percentage of water because once frozen the cell would be irreparably damaged. The freezing point of water is also lowered by solutes because the soluble molecules disrupt the hydrogen bonds making the water freeze at a lower temperature and it easier to melt ice. As there are many solutes in cytoplasm the water will not freeze until well below 0 oC and the cells are protected until the temperature gets extremely low.
As water cools it's density increases and the hydrogen bonds between the water molecules take on a more latticed formation as ice. Yet ice floats on the surface of water which means it's density must be lower than that of water. Water is at it's most dense at 4oC which is when its bonds are closest together. When water freezes the lattice arrangement of its structure move apart slightly and it floats on the surface. This means that the layer of ice insulates the water below which stays at 4 oC and aquatic life can continue.

Water is the medium for many reactions, especially those which occur in cells. Water is key in condensation reactions where water is removed from molecules to bond them together, this occurs with many sugars and carbohydrates. Water is also used to split up molecules by adding water. This is called hydrolysis and is essential to animals and plants because it allows them to utilise stored foods which are in long chains by breaking off smaller molecules.

Respiration produces water as a by-product. A large amount of energy is produced by oxidising hydrogen, which contains so much energy it is an explosive gas, into water. Photosynthesis uses water as a source of hydrogen atoms which are needed to produce glucose which is then stored in the plant cells as starch or used for respiration. Without water these two essential reactions would not occur and life would not be able to continue on earth. An important property of water is also it's transparency, it allows sunlight to pass through it so aquatic plants can photosynthesis and on a larger scale, allow life on earth to begin, since life started in the oceans with small organisms which relied on sunlight for the reaction photosynthesis to take place.

The cohesive properties of water allows plants to pull up water through xylem vessels from the roots to the leaves, this is called the transpiration stream. It also means that the water molecules where the water meets the air will be tightly held together and the water molecules below them to form an elastic film known as surface tension. Because of water's strong hydrostatic forces water is incompressible. This provides support for soft bodied creatures such as worms, slugs and jellyfish which therefore do not require a supporting skeletal system. Water allows cells filled with water to become turgid and due to it's incompressibility plants can support themselves.

 A WATER MOLECULE

In conclusion water's unique properties make it perhaps the most biologically important substance on the planet. No other substance shares similar properties to water and in the way that one single molecule can possess such varied and essential characteristics.