The Biological importance of water with reference to structure and bonding

Water (H2O) is one of the most abundant molecules on earth, and it forms the basis of the fluids within living organisms; it is therefore essential to their survival. Due to its physical and chemical properties, water can also be a habitat for some organisms.

Structurally water contains one oxygen that is covalently bonded to two hydrogens. This means there is a shared pair of electrons between each hydrogen atom and the lone oxygen.  However, electrons in the covalent bond are not shared equally, this is because the oxygen atom has a greater electronegativity, meaning that it has a greater pull on the electrons. Due to this, each water molecule has slightly negative and slightly positive regions. The two hydrogen atoms are positively charged whilst the oxygen atom is negatively charged therefore the bonded molecule ( H2O), is a polar molecule because the charges do not cancel. This means hydrogen atoms remain slightly positive and oxygen atoms slightly negative. As a result, there is an attraction between the oppositely charged atoms and this attraction is referred to as a hydrogen bond. This is because it is stronger than normal other electrostatic attractions.The chemical structure (hydrogen bonding mainly) of water means that it has many physical properties that make it incredibly diverse.
Water has a very high boiling point and specific heat capacity, many compounds with molecules similar to the size of water are usually gases. Since each water molecule can form hydrogen bonds with up to 4 other water molecules, water is a liquid at room temperature.

Liquid	Molecular formula	Bpt / oC	Mpt / oC	Specific heat capacity / kJ kg-1 oC -1
Water	H2O	100	0	4.18
Ethanol	C2H5OH	79	-117	2.46
Benzene	C6H6	80	6	1.05
Tetrachloromethane	CCl4	77	-23	0.86

The table shows water in comparison to other compounds containing molecules of a similar size. The high specific heat capacity is an advantage to cells particularly because many of the reactions occurring within them are catalysed by enzymes. Enzyme activity is sensitive to temperature and reactions only occur in a narrow range of temperatures. It also has a large enthalpy of vaporisation (heat energy required to convert a liquid to a gas) and this results in a high boiling point. When it evaporates, water draws thermal energy out of the surface it’s on, which can be observed in sweating. In many living organisms, including humans, the evaporation of sweat, which is 90 percent water, allows the organism to cool so that homeostasis of body temperature can be maintained.  In a similar pattern water also happens to have a low melting point as can be seen above, Water’s exceptionally high melting point helps protect Earth’s glaciers from melting. It also keeps oceans and most lakes from freezing completely. Both features are critical for aquatic life. As water cools, its temperature decreases until it reaches its freezing point (0°C). Further cooling, however, will not lower the temperature until all of the water freezes. Because oceans and large lakes are too big to freeze completely solid, their temperatures will never drop below water’s melting point which results in the preservation of aquatic life.

  Hydrogen bonding in water accounts for more than just its thermal properties, because of it, frozen water (ice) is less dense than liquid water. For most other substance the reverse is true however when water freezes it forms a giant structure with every oxygen atom at the centre of a tetrahedral arrangement of hydrogen atoms (two are covalently bonded and two are hydrogen-bonded). This makes a Giant lattice that has many gaps and therefore a lower density (mass per unit volume) than that of the liquid version. This means that lakes freeze from the top down to the bottom which is important for animals that live on ice, as their habitats would be greatly reduced or not exist at all if ice sank. E.G- Seals or Polar bears.

  Although previous properties have been unique and of importance, perhaps the most important features of water is that it is:

     a)      A liquid at room temperature

     b)      the “universal solvent

Water being a liquid at room temperature provides a marine environment for organisms to live in, but also provides a liquid environment inside cells, which holds significant importance, because metabolic reactions that are key to life, take place in solution. It is described as the ‘universal solvent’, because it dissolves much more substances than most common solvents. This is due to H2O being dipolar, which means that the charged areas of the hydrogen and oxygen atoms will attract polar and ionic substances that are dissolved in it, this allows the water molecules to form a layer around each charged ion, keeping the substance in solution. Water being such a successful solvent enables the transport of nutrients and the removal of waste products within living organisms.

Another property caused by water molecules being dipolar is that water has both cohesion and adhesion properties. Cohesion refers to the fact that water sticks to itself very easily, and adhesion means that water also sticks very well to other things. Both of these properties can be observed in transpiration but are also beneficial in other situations. Water cohesion results in a high surface tension of water which is essential for the transfer of energy from wind to the water surface to create waves. Waves are necessary for rapid oxygen diffusion in lakes and seas. The cohesion of water also directly affects us in our daily lives because it results in Capillary action. This is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. This assists the pumping action of your heart to help keep the blood moving in your blood vessels.

In conclusion, water is of great biological importance mainly because of its chemical structure. This results in unique thermal and structural properties. It is the most diverse substance and as yet no living organism has been found to successfully function without it.