1. The solvent power of water
Water does not dissociate easily and hence, is an excellent solvent. It is also so because of its H-bonding capabilities and very high dielectric constant. Thus, it acts like a electrostatically charged buffer that screens out charges and contributes to its extensive solvent power.
As a matter of fact, the substances that are polar in nature are soluble in water. However, compounds ranging from uncharged (non-ionizable) organic molecules (e.g., sucrose) to salts that are completely dissociated into ions, even in their solid crystalline state, get dissolved in water.
Hydrocarbons are insoluble in water, but with substituted functional groups like alcohol or hydroxyl group (-OH), carboxy1 (> C=O) or keto group, carboxyl group (-COOH) and amino group (-NH2) they become more readily soluble in water. It is primarily for the presence of these functional groups, that biologically important molecules such as proteins, nucleic acids and carbohydrates have bound water associated with them.
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Besides, the water molecules tend to be oriented in an electric field. When salts, organic and inorganic, dissolve in water they dissociate into anions and cations. Water molecules get oriented and packed around ions forming on aqueous layer called hydration shell or water of hydration. In this way water screens out the interaction between the charged ions and permits them to be dispersed in aqueous solution. This is also true for larger charged objects like proteins and so also cellular membranes.
2. Dissociation and dissociating effects of water
Although water is a stable compound it can dissociate occasionally and is also able to enhance the dissociation of other substances called electrolytes.
The dissociation of water involves two molecules of water and proceeds with the formation of hydroxyl ion (OH-) and hydronium ion (H3O+). The latter, in fact, is a hydrated proton (H2O+H+), generally represented in shorthand form as H+ (proton or the free hydrogen ion). This happens because the protons, being relatively light and mobile particles, manage to hop over the oxygen of a water molecule at infrequent intervals.
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As such water acts as both an acid and a base, one accepting a proton and other donating a proton. The H3O+ concentration [H+] is expressed as pH (potential of hydrogen). The pH of a solution is the negative logarithm to the base 10 of hydrogen ion concentration [H+]. The pH scale is logarithmic that ranges from 0 to 14. At equilibrium the product of the concentration of H+ and OH– ions in pure water is constant.
The tendency to maintain a constant pH is called buffer action which plays an important role in the homeostasis of living organisms.
The dissociating effect of water is important for the proper functioning of macromolecules (proteins, nucleic acids, phospholipids, etc.) because they are usually weak polyelectrolytes.
3. Hydrolysis and condensation reactions
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It is rather surprising to learn that among substances which ionise in water is water itself, through to a very slight extent (about one in 550 million molecules). Thus, water can act as a reagent and does take part in many biochemical reactions within cells, but not in the uncontrolled manner of an unstable compound.
The hydrogen and hydroxyl parts of water can spit apart and later rejoin. They are the source of H+ and OH– ions which make many a reaction possible. Even by donating electron to chlorophyll it gets oxidised into oxygen. Thus, it is the source of H+ and oxygen in living cells. It also takes part in enzyme controlled reactions.
The large molecules are split by addition of water (Hydrolysis). The covalent bonds between parts of molecules are broken and H+ and OH– ions from water become attached to the component subunit molecules. Such reactions produce the small molecules that are used by cells for synthesis of large molecules. Hydrolysis of ATP yields the energy needed to support energy requiring reactions in cells.
The large molecules are formed by condensation reaction which is reverse of hydrolytic reaction. The H+ and OH– ions removed from the functional groups of molecules undergoing condensation reaction combine to form a molecule of water. In the process, the component molecules (monomers) are joined by covalent bond (s) into large molecule (polymer), that are less soluble and more stable (long-lived).