Biochemistry, 4th Edition P9. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | What Are Buffers and What Do They Do 43 HUMAN BIOCHEMISTRY The Bicarbonate Buffer System of Blood Plasma The important buffer system of blood plasma is the bicarbonate carbonic acid couple H2CO3 --H HCO3 The relevant pKa pK1 for carbonic acid has a value far removed from the normal pH of blood plasma pH . The pK1 for H2CO3 at 25 C is Table but at 37 C pK1 is . At pH the concentration of H2CO3 is a minuscule fraction of the I CO3 concentration thus the plasma appears to be poorly protected against an influx of OH ions. HCO3 pH logjo H H2CO3 JICO lira 6761 For example if 11ICO3 24 mM then H2CO3 is only fiM X 10 11M and an equivalent amount of OH its usual concentration in plasma would swamp the buffer system causing a dangerous rise in the plasma pH. How then can this bicarbonate system function effectively The bicarbonate buffer system works well because the critical concentration of H2CO3 is maintained relatively constant through equilibrium with dissolved CO2 produced in the tissues and available as a gaseous CO2 reservoir in the lungs. Gaseous CO2 from the lungs and tissues is dissolved in the blood plasma symbolized as CO2 d and hydrated to form H2CO3 CO2 g --CO2 d CO2 d H2O -- H2CO3 H2CO3 -- H HCO3 Thus the concentration of H2CO3 is itself buffered by the available pools of CO2. The hydration of CO2 is actually mediated by an enzyme carbonic anhydrase which facilitates the equilibrium by rapidly catalyzing the reaction H2O CO2 d --H2CO3 Under the conditions of temperature and ionic strength prevailing in mammalian body fluids the equilibrium for this reaction lies far to the left such that more than 300 CO2 molecules are present in solution for every molecule of H2CO3. Because dissolved CO2 and H2CO3 are in equilibrium the proper expression for H2CO3 availability is CO2 d H2CO3 the so-called total carbonic acid pool consisting primarily of CO2 d . The overall equilibrium for the bicarbonate buffer system then is Kh .
