I. Introduction

Water makes up approximately 70% of most life forms. In biological systems, water is the omnipresent medium through which molecules (nutrients) must be transported and within which most biomolecular reactions must take place. With the central role of water in living things, it is not surprising that the chemistry of water has a profound influence on biomolecular reactions.

One of the most important reactions that water undergoes is the auto-ionization reaction. In this reaction, water forms protons (H⁺; the hydrated proton is written as the hydronium ion, H₃O⁺) and hydroxide ions (OH^–):

This reaction is assumed to be at equilibrium. Although for pure water, only 9 out of every billion (109) water molecules are ionized, the products of this ionization process have an enormous impact on biological systems. It is important to understand what governs the concentrations of these chemical species because living cells are extremely sensitive to the concentrations of H⁺ and OH^– in the solution.

Living organisms have limits to the acidity and alkalinity (defined as pH) that they can tolerate; thus acid rain can have potentially lethal consequences to fish and other wildlife. The control of the pH is important to proper cellular function, but not all cellular functions require the same pH. Proteins within blood require near neutral pH for optimal performance, while digestive enzymes require very low (highly acidic) pH (pH ~1 to 2). For this reason, it is important that living organisms maintain proper pH levels and the ability to moderate changes in pH.

This topic focuses on how different chemical species affect the concentrations of H⁺ and OH^– in solution, how the concentration of H⁺ and OH^– ions can be controlled, and methods that can be used to measure the concentration of these species.