Introduction

A living organism consists of a very large number of interdependent fluid compartments. These compartments are bounded by lipid barriers (membranes) and they contain mainly water and exist in an environment that is mostly water. Communication and exchanges between these compartments are carried out by molecules or ions that are able to move through the water and the lipid phases. In order to study these compartmental exchanges it is thus necessary to examine the way in which molecules and ions move through homogeneous liquid phases. We shall consider homogeneous liquid phases because the membrane lipids are in a liquid state and can be treated in the same way as water – that is, as a homogeneous liquid phase (see Chapter 5). Let us first start by an examination of the way in which ions are formed and then go on to analyse the way in which they move through aqueous solutions under the influence of an electric field.

Formation of ions

Ions are formed when neutral molecules are dissociated or when salts are dissolved. To dissolve a salt, or dissociate a molecule, requires a medium with a high dielectric constant (see Appendix 24 where dielectric constants are discussed) which is then able to weaken ionic bonds. An example of an ionic bond is the bond that holds Na+ and Cl ions together in a NaCl crystal. Methane, for example (CH4), is held together by covalent bonds. Ionic and covalent bonds are extreme cases; most chemical bonds are a mixture of the two types.