Importance of Water Ionization and Buffers

As organisms typically contain 70-90% water, water is an indirect participant in biological reactions. The "bent" structure of the water molecule is well-suited to hydrogen-bond formation (with a natural induced dipole charge) and in that formation, water can serve as either a proton (H+) donor or a proton acceptor. Also, the natural ability of water to form hydration shells around ionic and hydrophobic solutes, as well as ice, influences how various reactions will occur (or not occur). The formation of membranes by fatty acids is influenced by the formation of micelles by amphipathic molecules (both partially hydro-phobic and partially hydrophilic) in aqueous solutions.

Water shows a slight tendency to form ions, a property that allows it to conduct electricity. The ions formed from one water molecule are one proton (H+) and one hydroxyl (OH"), with the former being immediately hydrated to form a hydronium ion (H3O+), although by convention we still refer to the proton concentration in solution and not the hydronium ion concentration.

At equilibrium, the concentration of free protons in water is equal to the concentration of free hydroxyl ions. The product of the two ion concentrations is called the ion product of water (Kw). When a solution is acidic, it contains a higher concentration of free protons and a lower concentration of free hydroxyl ions. Likewise, a basic solution contains more free hydroxyl ions than free protons. To simplify the expression of proton concentrations, the negative logarithm of the free proton concentration (-log [H+]) has been defined as the "power of hydrogen," or pH, where pH + pOH = 14. For example, the pH of bleach, a strong basic solution, is 12.6, whereas the pH of vinegar, a weak acid, is 2.9. The pH of blood is 7.4. Solutions with a pH of 7 are said to be neutral; that is, neither acid nor base.

Other substances that are able to dissociate into ions in water are called electrolytes, because the addition of their ions to those already present in water increases the ability of the solution to conduct electricity. Substances that dissociate almost completely into ions in water are called strong electrolytes. So-called "strong" acids and bases are examples of strong electrolytes. Similarly, substances that only slightly dissociate into ions in water are called weak electrolytes. Again, so-called weak acids and bases are examples of weak electrolytes. The extent to which a particular substance forms ions in water is called the ionization constant (Ka).

Solutions that resist changes in their pH when acids or bases are added are called buffers. A buffer is most effective when the pH of the solution is within one unit of the pKa of the buffer solute (i.e., potassium phosphate). Since enzyme activity is very sensitive to changes in pH, maintenance of cellular pH is critical to the survival of living organisms. Intracellular pH is maintained by phosphate and histidine buffer systems. The bicarbonate buffer system controls the pH of blood and extracellular fluids.

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