 Dear students, in this module we shall discuss the phenomena of Donan equilibrium that was discovered by Friedrich Donan in 1911. This phenomena describes the unequal distribution of ions across the two sides of a differentially permeable membrane. It is defined as if diffusible solutes are separated by a membrane that is freely permeable to water and electrolytes, but is totally impermeable to one species of ions, the diffusible sards become unequally distributed between the two compartments. To explain Donan equilibrium we shall take an example of an experiment in which we take pure water in two compartments which are separated by a membrane and add potassium chloride to one of them. Potassium chloride ionizes into potassium and chloride ions. These ions diffuse through the membrane and after a while the concentration of potassium and chloride of both ions becomes equal in both compartments. At this stage we shall add another salt of calcium with a non-diffusible anion, let us say it Ka. It will also dissociate and produce the ions potassium ion and anion A. Potassium ion is diffusible but this anion A is not. So A will remain confined to compartment number one. Potassium and chloride ions quickly re-distribute and establish a new equilibrium, but due to NIN-A present in compartment 1, the equilibrium established in both compartments will have an unequal distribution. Similarly, potassium ions will be more concentrated in compartment number one which has anion A while chloride will be less concentrated in this compartment. We can explain this Donan equilibrium on the basis of distribution of anions and cations. This unequal distribution happens because we are considering gradients. But gradients are not only of chemical concentrations but gradients are also electrical in nature. Because we are discussing the ions here and considering the ions, the ions keep a chemical nature here and because they are charged, they are also electrical and the electrical gradients have to keep an eye on them. The reason is that in nature, when the ions are distributed in two compartments like this, then the electro-neutrality also keeps an eye on them and keeps them maintained. There should be equal charges in both the compartments of positive and negative ions. That is why in this effort, the unequal distribution of ions is done here. One effect of Donan equilibrium is that if a non-diffusible salt is present in a solution which is separated because of semi-permeable membrane, then in two leather compartments, the concentration of both solutions will never be equal. The concentration of solution that has non-diffusible solute remains high even at equilibrium. This is called Donan effect. Donan effect has greater implications for living cells as living cells have high concentration of non-diffusible and unequal ions in the form of proteins and organic phosphates that result in the unequal distribution of ions. It also implies that Donan effect creates an intracellular environment with more ions inside the cell than outside. The concentration of ions in the cell is always greater than the concentration of extracellular environment. The concentration of ions in the cell is much greater than the concentration of water molecules in the cell. As a result, hydrostatic pressure is created within the cell. Another effect of unequal distribution of ions is the development of an electrical potential across the plasma membrane.