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descrive the biological importance of water

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Making up between 70 and 95% of the mass of a cell, and covering over three quarters of the planets surface, water is one of the most important compounds on this planet. A single water molecule is made up of one oxygen atom covalently bonded to two hydrogen atoms. Covalent bonds are formed by sharing electrons between the outer shells of the oxygen and hydrogen atoms. However, what makes water so unique is the fact that it remains a liquid at room temperature. Many similar sized molecules (ammonia has a molecular mass equal to that of water-18) remain in their gaseous form at this temperature. The reasons for this unique thermal property are hydrogen bonds. The nucleus of an oxygen atom is larger and therefore contains many more protons that that of a hydrogen atom. Therefore, the electrons shared in the covalent bond between the oxygen and hydrogen atoms have a greater affinity for the oxygen atom than either hydrogen atom. This pulls the electrons closer to the oxygen atom and away from the hydrogen atoms resulting in the oxygen atom having a slightly negative charge and the hydrogen atoms developing slightly positive charges. ...read more.


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. ...read more.


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. ...read more.

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Response to the question

A very well done essay. The candidate can clearly relate biological function to deeper chemical aspects of water in great depth. This shows great scientific understanding. They also discuss a wide variety of factors that make water a very ...

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Response to the question

A very well done essay. The candidate can clearly relate biological function to deeper chemical aspects of water in great depth. This shows great scientific understanding. They also discuss a wide variety of factors that make water a very important substance in good scientific depth and in a concise and clear way.

Level of analysis

Their introduction is good, but it really should expand to include some of the aspects they are going to talk about in the essay as well as stating basic facts about the importance of water. The candidate starts by outlining well the chemical properties of water and how this relates to its function and this continues in depth across a number of examples. Their conclusion is adequate based on the information they have discussed and it ties up loose ends well.

Quality of writing

Punctuation, grammar and spelling all done to a very high level. Format and tone of the essay are also well done

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Reviewed by skatealexia 16/09/2012

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