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.