Metabolism of Water and Inorganic Ions

Abstract

Metabolically active tissues consist of 85–95% water. This substance possesses unique physicochemical characteristics which determine the feature of living systems in many ways. In the physiological temperature range, water is a fluid with relatively low viscosity, a high dielectric constant (dissociation constant = 10⁻¹⁴) and minimum quantum absorption below 850 nm. Because of its small size and dipolar nature, H₂O is an excellent solvent for an exceptionally broad spectrum of substances from strongly polar to moderately apolar, particularly for ions. The polar structure of the water molecule (Fig. 16.1, top) allows the hydration of cations and anions, including macromolecules such as proteins and nucleic acids. As a solvent, water is chemically rather inert and is thus an ideal medium for diffusion and for the chemical interactions of other substances. Water is, because of its extremely high latent heat of vaporisation (44 kJ · mol⁻¹ at 25°C), high heat capacity and high heat conductivity, an ideal medium for thermal regulation.