Chemical composition of the cell; water and salts

Water is the most abundant substance in the cell: about 80 % on average in a multicellular organism, over 90 % in plant cells and only a few per cent in tooth enamel. The water molecule is polar (a dipole): the oxygen side is slightly negative and the hydrogen side slightly positive, so molecules are linked by hydrogen bonds. For this reason water is a good solvent for polar and ionic substances, transports substances, takes part in hydrolysis reactions, absorbs and releases heat slowly (keeping temperature stable) and provides cell turgor. Mineral salts occur as ions (K⁺, Na⁺, Ca²⁺, Cl⁻) in excitability and membrane processes, and in solid form (calcium phosphate, carbonate) in bones and shells. Inside the cell H₂PO₄⁻ and HPO₄²⁻ anions, and outside H₂CO₃ and HCO₃⁻, act as buffers and keep the pH of the medium nearly constant.