Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
Solutes alter the properties of water

Aqueous solutions exhibit four important properties known as colligative properties, which are based on the alteration of Water's physical constants under METABOLISM/18.html">The Influence of dissolved solutes. These properties include: 1) freezing point, 2) boiling point, 3) vapor pressure, and 4) osmotic pressure. The term "colligative" means "tied together" or "bound by a common origin." Colligative properties of solutions share a common basis and vary under the influence of solutes in a predictable manner.

A solution of 1.00 mole of any ideal, nonvolatile solute in 1000 g of water (or any other solvent) is called a molal (1 m) solution. In such a solution, The Effect of the solute is manifested by the fact that at a pressure of 760 mm Hg, the freezing point of water (0°C for pure water) is lowered to -1.86°C, the boiling point (normally 100°C) is elevated to 100.543°C, and the osmotic pressure, measured using specialized apparatus (Fig. 4-8), reaches 22.4 atm. An ideal solute is defined as one that does not dissociate into two or more components or undergo association leading to a decrease in the total number of dissolved particles. Colligative properties depend solely on the number of dissolved particles per unit volume of solvent and are independent of their chemical Structure. This is because one mole of any non-ionized compound contains a strictly defined number (6.02∙10²³) of molecules (Avogadro's number). It follows that 1 m solutions of glycerol (molecular weight 92) and glucose (molecular weight 180) must have identical freezing points (-1.86°C), boiling points (100.543°C), and osmotic pressures (22.4 atm), since both solutions contain the same number of molecules in 1000 g (1 L) of water. The freezing point of a 0.100 m glucose solution should be 10 times lower than that of a 1.0 m solution, i.e., -0.186°C, because the number of molecules in this solution (per 1 L of water) is 10 times smaller than in a 1 m solution. A 0.100 m NaCl solution, in which all molecules are completely dissociated into Na⁺ and Cl^- ions, should freeze at -0.372°C because the number of dissolved particles in it (per 1 L of water) is twice that in a 0.100 m glucose solution. All these rules, which allow for the anticipation of colligative properties and the prediction of the numerical values of the corresponding constants, are strictly observed only in the case of dilute aqueous solutions.

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Fig. 4-8. Osmosis and osmotic pressure. A. Initial state. Water flows from the external space through the membrane into the solution, tending to equalize water concentrations on both sides of the membrane. B. Final state. Water has penetrated the solution of a substance whose molecules are unable to pass through the membrane, resulting in the dilution of the solution. At equilibrium, the pressure of the solution Column of height h exactly balances the osmotic pressure, which reflects the tendency of water to flow into a zone of lower water concentration. C. Osmotic pressure is the force that must be applied to the piston to prevent the opposite-directed osmotic flow of liquid. It is numerically equal to the hydrostatic pressure of a liquid column of height h.

The aforementioned ability of water to alter its properties under the influence of dissolved solutes is of profound biological significance. For example, it allows freshwater fish to remain active in water at its freezing point, since the total concentration of all solutes in the fish's Blood is high enough to keep its freezing point below that of water. Furthermore, due to the presence of dissolved solutes in the blood—specifically Proteins that cannot pass through capillary membranes—a higher osmotic pressure is generated in the blood than in the interstitial fluid. As a result, water diffuses from the interstitial fluid into the blood capillaries, which helps fill The Vascular System and protects it against collapse.

Another reason solutes affect The properties of water is that these substances tend to disrupt Hydrogen Bonds between water molecules. The presence of ionic substances such as NaCl in water leads to a noticeable change in The structure of liquid water. This occurs because each ion (specifically, Na⁺ and Cl^- ions) is surrounded by a Hydration shell consisting of dipole water molecules, and the geometry and properties of such hydrated ions differ somewhat from the geometry and properties of clusters formed by water molecules via hydrogen bonds; hydrated ions possess a more ordered and regular structure. Thus, dissolved salts tend to disrupt the normal structure of liquid water and alter its properties as a solvent. As we will see below, the Solubility of proteins decreases sharply with an increase in the concentration of neutral salts, such as NaCl, Na₂SO₄, and (NH₄)₂SO₄, which alter the properties of water and diminish its capacity to dissolve proteins. This effect of dissolved neutral salts can be exploited for the Fractionation of Protein mixtures, as many proteins differ in their susceptibility to precipitation from salt solutions.