Interrelationship among Salts, Ions, and Gases (With Statistics)

Salts are found in all cells and exist in their ionized forms. The ions may be dissolved in the cell fluid or specifically bound to other molecules such as proteins or lipids.

Salts have two broad roles in the cell. One role is osmotic, in that the total concentration of dis­solved salts affects the flux of water across the cell’s membranes.

The second is the more specific role that certain ions play in contributing to the structure and function of cellular particles and macromolecules. The specific cellular roles of a number of ions are given in Table 3-5.

Gases that enter the cell from the environment or are produced by the metabolism of a cell dissolve in the cytoplasmic water. The most common gases in air are nitrogen (N₂), oxygen (O₂), and carbon dioxide (CO₂). In the atmosphere at sea level, N₂ accounts for 78.03% of all gases, O₂ 20.99% and CO₂ 0.03%. Both nitrogen and oxygen dissolve in water in their molecu­lar form, but their solubility in water is low.

For exam­ple, at 25°C and at 1 atmosphere pressure, only 2.83 ml of oxygen and 1.43 ml of nitrogen dissolve in 100 ml of distilled water. The behavior of carbon dioxide in water is quite different.

Some of the dissolved carbon dioxide reacts with water, forming carbonic acid, which in turn dissociates into ions:

Most of the carbon dioxide in cells is in the bicarbon­ate and carbonate forms. CO₂ gas is more soluble in water than are O₂ and N₂. For example, 75.9 ml of CO₂ dissolves in 100 ml of water at 25°C and 1 atmosphere pressure.