Water is a simple molecule, yet it is fundamental to life. In active living cells, two-thirds, or often more, of the area is occupied by water, and two-thirds of the globe is covered in water. Water is therefore extremely abundant, and in biological terms it has great importance both inside cells, and externally, for example as a habitat
The simple molecule of H2O has many interesting physical and chemical properties which contribute to its importance. It is the only substance which can be found naturally in all three states - solid (ice), liquid and gas (water vapor). Water molecules bond to each other by means of hydrogen bonds, and this raises its melting and boiling points substantially. If these bonds did not exist, its boiling point would be in the region of -120C, rather than the actual value of 100C. Water is also very good at ionizing substances due to its structure, and is therefore a good solvent. These properties, and many others, will be considered by looking at the main areas in which water is of biological significance.
Water is the most abundant component of any organism. Humans are 60% water, and most organisms are 60-90% water. The lowest water content can be found in plant seeds (20%), and the highest in jellyfish (99%), and this is the cause of their transparency. The water is found mainly in the protoplasm, and here it plays vital roles in many functions, for example in metabolism in all organisms, and photosynthesis and support in plants.
Water molecules have the well-known formula H2O, each molecule
containing two hydrogen atoms and one oxygen atom.
These atoms are covalently bonded together, sharing electrons to hold them together.
However, these electrons are not evenly distributed within the molecule.
Oxygen has an affinity towards electrons, and therefore draws them away from the
hydrogen atoms in the molecule.
This attraction makes the oxygen part of the molecule very slightly negatively charged, and it is said to be electronegative. Consequently, the electrons are slightly further away from the hydrogen atoms in the molecule that they would usually be,
and this part of the molecule is electropositive, possessing a small positive charge.
The molecule is said to be polar, and as opposite charges 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. This is the reason behind the unexpectedly high melting and boiling points of water, when compared to similar compounds such as H2S (hydrogen sulphide - gaseous at room temperature), sulphur being in the same group as oxygen in the Periodic Table.
The diagram, illustrates the polar nature of the water molecule.
The H and O represent hydrogen and oxygen atoms respectively,
and the black circles represent the electrons which are shared
(one from each hydrogen atom, and two from the oxygen atom, one for each bond),
and they are shown nearer to the oxygen atom,...