Lecture 2 pH and its Relevance to Biochemistry_organized.pdf

Water Water is the predominant chemical component of living organisms. It has unique physical properties, which include the ability to dissolve a wide range of organic and inorganic molecules. Water can be ionized. The products of water’s dissociation are a hydrogen ion or proton (H+) and a hydroxid...

Water Water is the predominant chemical component of living organisms. It has unique physical properties, which include the ability to dissolve a wide range of organic and inorganic molecules. Water can be ionized. The products of water’s dissociation are a hydrogen ion or proton (H+) and a hydroxide ion (OH-) According to the law of mass action, the ionization of water can be described by a dissociation constant, K, which is equivalent to the concentrations of the reaction products divided by the concentration of un-ionized water: Kw is the ionization constant of water: Kw is 10-14. In a sample of pure water, [H+] = [OH-], so [H+] and [OH-] must both be equal to 10-7M: The concentration of [H+] + [OH-] in any solution must be equal to 10-14. So, if hydrogen ion concentration is more than 10-7M in a solution. The concentration of hydroxide ion concentration is less than 10-7M. pH A solution in which [H+] = 10-7M is said to be neutral, a solution with [H+] higher than 10-7M is acidic and a solution with [H+] less than 10-7M is basic. To describe such solutions more easily, the hydrogen ion concentration is expressed as a pH pH = - log [H+] Definition of pH It is the negative logarithm of hydrogen ion concentration (- log H+). It refers to the concentration of hydrogen ions in a solution. The term, widely used in chemistry, and biology, translates the values of the concentration of the hydrogen ion which ranges from 0–14. A neutral solution has a pH of 7, an acidic solution has a pH < 7, and a basic solution has a pH >7. Medical Importance of pH pH is biologically important because it affects the structure and activity of macromolecules and enzymes. For example, the catalytic activity of enzymes is strongly dependent on pH pH is an important homeostatic process. For example, most animals breathe not because they lack oxygen, but because CO2 buildup in the blood increases the blood acidity beyond normal levels. Body pH Must Be Kept Constant Normal metabolic activities generate acids because of degradation of amino acids, the incomplete oxidation of glucose and fatty acids, and the ingestion of acidic groups. These acids must be buffered so that they do not change the pH of blood. Buffers A buffer is a solution that can resist pH change upon the addition of an acidic or basic components. It is able to neutralize small amounts of added acid or base, thus maintaining the pH of the solution relatively stable. The Bicarbonate Buffer System The bicarbonate buffer system is an acid-base homeostatic mechanism involving the balance of carbonic acid (H2CO3), bicarbonate ion (HCO−3), and carbon dioxide (CO2) in order to maintain pH in the blood , and other tissues, to support proper metabolic function. Catalyzed by carbonic anhydrase, carbon dioxide (CO2) reacts with water (H2O) to form carbonic acid (H2CO3), which in turn rapidly dissociates to form a bicarbonate ion (HCO−3 ) and a hydrogen ion (H+). When stress is applied to a system in equilibrium, the reaction will shift in a direction that will reduce stress. Buffering Organs in Human Body Lung: Excess hydrogen ions in the body can be buffered and eliminated from the body. H+ combines with HCO - to re-form H2CO3, which dissociates into CO2 + H2O. Some CO2 can be expired in the lungs. Change in pulmonary function can adjust blood pH for minutes to hours (by decreasing or increasing CO2 in expiration). Kidney Long-term adjustments of hours to days are made by the kidneys, which have mechanisms to excrete or retain H+, bicarbonate, and other ions. kidneys play a major role in the buffering of metabolic acids: Major Buffer Systems of the Human Body Bicarbonate buffer CO2 + H2O Hemoglobin Phosphate buffer Protein Hb-H H2PO4 Pr-H H2CO3 H+ + HCO3 In blood plasma Hb + H+ + H + HPO4 +  Pr + H Interior of red blood cells 2 Most important in urine Intracellular fluid Measuring pH pH Probe: A pH probe is an instrument that has an arm with two small electrodes in it. The arm is placed in a substance and the electrodes respond to the pH of the solution. The pH is typically displayed on a small screen. There are both large and small portable models of this instrument. Litmus paper: Litmus paper is a thin strip of paper that changes color based on the pH of the solution it is dipped in to. This pH indicator can be found at some pet supply stores to test the pH of aquariums. Chemical Indicators: Chemical indicators are chemicals you can place into solution that will change the color of the solution as the pH changes. Some common ones are phenolphthalein.