Water, H2O, Chemical characteristics explaining biological role
Water constitutes about 55% of the body weight; this percentage varies from 55% in men to 65% in women, according to their relative proportion of adipose tissue. The distribution of water in the body is about 70% intracellular and 30% extracellular, this latter divided into extravascular and intravascular compartments. There is relatively less water in obese than in normal subjects, this difference can modify the volume of distribution of drugs according to their water-soluble or liposoluble character.
Several hormones are involved in the regulation of water balance, but it is the antidiuretic hormone which plays the essential role, by inducing its renal reabsorption. A hypersecretion of the antidiuretic hormone, induced for example by drugs like carbamazepine, chlorpropamide, some psychotropic agents, can cause a water retention and a dilution of the compounds which it contains.
Water, H2O, is a polar molecule, the partial negative charge being placed on the oxygen atom and the positive partial charge distributed on the two hydrogen atoms. Thus water, neutral molecule, is a permanent dipole.
The water molecules bind together by hydrogen bonds established between the oxygen of a molecule and an hydrogen of another molecule. It is an intermolecular hydrogen bond whose energy is 4 kcal/mol approximately, whereas that of a covalent bond oxygen-hydrogen is of 110 kcal/mol. Each water molecule can establish up to 4 hydrogen bonds in a solid state, i.e. in ice. In water the statistical average number of hydrogen bonds is 3.4 for each molecule. These hydrogen bonds explain that the temperature of vaporization of water is high, 100°C , for a molecule of such a low molecular mass.
Pure water consists of molecules of H2O but also of H+ and OH- ions. The H+ ion or proton, is hydrated and can be written H3O+ or H9O4+ depending whether the positive charge H+ is distributed on one or four water molecules. OH- is combined with three water molecules to give H7O4- . The constant of Kd ionization or dissociation of water is written:
The water of the body contains various inorganic and organic molecules in solution and in suspension.
The polar molecules, more particularly ions, are water soluble and are called hydrophilic. When an ion is introduced into water, the water molecules are directed around it, the ions are hydrated. For example each sodium ion or potassium ion is surrounded by six water molecules.
The non polar molecules insoluble in water are hydrophobic; they are often lipophilic. In an aqueous medium, the lipophilic molecules are assembled in aggregates, which constitute the hydrophobic effect, at the origin of the formation of micelles, such as the lipid bilayer and of the structuration of proteins.
Water contains many molecules, electrolytes, organic compounds of more or less high molecular weight, at the origin of the osmotic pressure or osmolality. As a reference a solution of 9 g of NaCl per liter is iso-osmotic to plasma.
The cellular membranes are permeable to water whose movements depend on the differences in osmolality on both sides of the membranes. The transfer of water can be done by diffusion through the lipid bilayer of the plasma membrane but this transfer is slow and of low importance. The transfer of water is primarily made through specific channels called aquaporins.
Aquaporins are proteins inserted into the of plasma membrane bilayer of cells. They are tetramers. Each monomer contains a channel permeable to water molecules, in both directions, according to the osmotic gradient. Aquaporins are not permeable to ions, including protons. There are many types of aquaporins called aquaporins-0 (AQP-0), aquaporin-1 (AQP-1)… up to aquaporin-10 (AQP-10). They are permeable to water and, moreover, some of them are also permeable to small molecules like glycerol and urea and are called aquaglyceroporins.
The various types of aquaporins are not distributed in a homogeneous way in all the cells of the body. The AQP-1 are present especially in red cells, kidneys, choroid plexus; AQP-2, in the kidney (collecting trube) and are controlled by vasopressin, AQP- 4 in the brain, AQP-7 and AQP- 9 in adipocytes. The opening and closure of certain types of aquaporins are dependant on pH. Some contain cystein molecules which take part in their activity and are inhibited by mercurial derivatives.
For more information on aquaporins, see: Aquaporin Channels.