 Dear students, in this module we shall discuss the unequal distribution of ions across the plasma membrane. The plasma membrane maintains different concentrations of ions inside and outside the cell. This results in potential difference across the membrane. The major ions that are unequally distributed across the membrane include the sodium, potassium, calcium and chloride ions. First we shall discuss the distribution of potassium ions. The concentration of potassium ions is maintained 10 to 30 times higher inside the cell than outside. This happens because the plasma membrane is more permeable to potassium ions. As a result of this, potassium ions are the most concentrated ions inside the cell. As compared to potassium ions, the sodium ions are kept 10 to 20 times lower inside the cell as compared to the outside. This lower concentration of sodium ions is maintained because of lower permeability of plasma membrane to sodium ions. In comparison to these ions, the permeability varies for the chloride ions. Some of the membranes are more permeable for the chloride ions and some are less. The intercellular concentration of calcium ions is maintained several orders of magnitude below inside the cell in comparison to their concentration outside the cell. This difference is due to two major factors. One is that the calcium ions are actively transported out and the second factor is that the mitochondria and endoplasmic reticulum. These two organelles sequester the calcium ions present in the cytoplasm and take them out. This is why the concentration of calcium ions is high in these two organelles but in cytoplasm it becomes extremely high. In cytoplasm, the concentration of calcium ions is usually 10 raised to power minus 6 moles. The unequal distribution of ions has a basis in the donor effect and donor equilibrium. As cytosol has many non-diffusable peptides and proteins that have carboxyl and other anionic groups, due to these anions the donor effect is produced that results in unequal distribution of ions across the plasma membrane. Inside the cell, many anions are present and all diffusable ions are unequally distributed across the cell membrane. As we can see in this figure, all major species of ions, i.e. sodium, potassium, calcium and chloride are unequally distributed inside the cell or outside the cell. Because of the presence of an anion A that is highly concentrated inside the cell.