The mammalian cell comprises 70% water, 15% protein, 7% nucleic acids, 2% carbohydrate, 2% lipid, 1% inorganic ions and 3% other small molecules. The predominant substance, water, is made up of two atoms of hydrogen and one of oxygen and is described as ‘polar.’ A polar molecule has an overall charge, or an uneven distribution of charge on its surface. The most polar of small particles are ions (e.g. Na + , K + ) which have lost an electron from their outer shell. However, even covalently bound molecules (e.g. water) demonstrate polarity by virtue of the uneven distribution of their electron ‘cloud.’ In water, the oxygen atom pulls electrons towards itself so the electron cloud is more dense over that part of the molecule, which is thus relatively more negative than it is over the hydrogen atoms. The partial negative charge over the oxygen atom is attracted to the partial positive charge over the neighbouring hydrogen atoms, and this weak ‘hydrogen bonding’ keeps the molecules together (Figure 1) and gives water its principal characteristic (i.e. a liquid, which, for the size of its molecule, has a relatively high boiling point).