Acids, bases, and pH
Only a minute proportion of water molecules, something like one in every 5 × 108, is present in its dissociated form, but as we have already seen, the H+ and OH− ions play an important part in cellular reactions. A solution becomes acid or alkaline if there is an imbalance in the amount of these ions present. If there is an excess of H+ , the solution becomes acid, whilst if OH− predominates, it becomes alkaline. The pH of a solution is an expression of the molar concentration of hydrogen ions:
pH= −log10[H+ ]
In pure water, hydrogen ions are present at a concentration of 10−7M, thus the pH is 7.0. This is called neutrality, where the solution is neither acid or alkaline. At higher concentrations of H+ , such as 10−3M (1 millimolar), the pH value is lower, in this case 3.0, so acid solutions have a value below 7. Conversely, alkaline solutions have a pH above 7. You will see from this example that an increase of 104 (10 000)-fold in the [H+ ] leads to a change of only four points on the pH scale. This is because it is a logarithmic scale; thus a solution of pH 10 is 10 times more alkaline than one of pH 9, and 100 times more than one of pH 8. Figure 2.9 shows the pH value of a number of familiar substances.
Most microorganisms live in an aqueous environment, and the pH of this is very important. Most will only tolerate a small range of pH, and the majority occupy a range around neutrality, although as we shall see later on in this book, there are some startling exceptions to this. Most of the important molecules involved in the chemistry of living cells are organic, that is, they are based on a skeleton of covalently linked carbon atoms. Biological molecules have one or more functional groups attached to this skeleton; these are groupings of atoms with distinctive reactive properties, and are responsible for many of the chemical properties of the organic molecule. The possession of a functional group(s) frequently makes an organic molecule more polar and therefore more soluble in water.
Some of the most common functional groups are shown in Table 2.4. It can be seen that the functional groups occur in simpler organic molecules as well as in the macromolecules we consider below.