The bodies of most living organisms comprises of at least 2/3 water. Water is the solvent that forms the basic aquatic medium of cytoplasm in cells and body fluids. It is also the transport medium in plants. Solutes which dissolve in water can be either inorganic (potassium, chloride, hydrogen) or organic (amino acids, glucose). Changes in water concentration lead to corresponding changes in solute concentrations in cells. The relative concentration of solutes determines the osmotic pressure. Water enters and leaves cells through osmosis and it follows the osmotic gradient (water moves from high concentration to low concentration). Hence the movement of water into and out of the cell depends directly on the concentration of solutions both inside and outside the cell.
Water provides the necessary medium in which all chemical reactions of metabolism can occur. Chemical reactions in cells can proceed only if the reactants are dissolved in water. Water and solute concentration in cells must be maintained at a relatively constant level, within a narrow range, so that these cellular reactions can take place.
Problems associated with change in water concentration
If the balance of water and solutes in cells is not maintained at an optimal concentration, too much water may enter the cell causing it to burst, or too much water may move out causing the cell to shrink and the cytoplasm too concentrated for cell functioning. The osmotic pressure of living tissue can also affect the pH in cells – for example too little water leads to an increase in the concentration of solutes such as carbon dioxide thus reducing the pH. Both osmotic pressure and pH must be maintained within a narrow range so that enzymes can function optimally, to allow for effective metabolism. Osmotic pressure also aids in providing structural support for many living organisms. For example in plant cells, the outward osmotic pressure from the vacuole is counteracted by the inward pressure of the cell – making the plant turgid. If water accumulates in high concentrations in cells, it may dilute the concentrations of solutes – slowing down the rate of metabolism; hence excess water must be excreted from living organisms.