Water & pH

1. Water & pH

2. Water • The predominant chemical component of living organisms • The ability to solvate a wide range of organic and inorganic molecules • Dipolar structure • a high dielectric constant • Reflecting the number of dipoles • Capacity for forming hydrogen bonds

3. Water • Influences the structure of biomolecule • Water is a reactant or product in many metabolic reactions

4. Water • Water has a slight propensity to dissociate into hydroxide ions and protons • The acidity of aqueous solutions is generally reported using the logarithmic pH scale • Buffers normally maintain the pH of extracellular fluid between 7.35 and 7.45. • Suspected disturbances of acid-base balance are verified by measuring the pH of arterial blood and the CO2 content of venous blood

5. WATER IS AN IDEAL BIOLOGIC SOLVENT • Water Molecules Form Dipoles

7. WATER IS AN IDEAL BIOLOGIC SOLVENT • the strength of interaction F between oppositely charged particles is inversely proportionate to the dielectric constant ε of the surrounding medium. • Water therefore greatly decreases the force of attraction between charged and polar species relative to water-free environments with lower dielectric constants.

8. Water Molecules Form Hydrogen Bonds

12. Biomolecules,have functional groups

15. Water as solvent

18. Hydrophilic • Hydrophobic • The forces that hold the nonpolar regions of the molecules together are called hydrophobic interactions.

19. Most biomolecules are amphipathic; • Proteins • Phospholipid bilayer • Biomolecules fold to position polar & charged groups on their surfaces • It minimizes energetically unfavorable contact between water and hydrophobic groups

20. WATER IS AN EXCELLENT NUCLEOPHILE • Nucleophilic attack by water generally results in the cleavage of the amide, glycoside, or ester bonds • Hydrolysis

21. Water as a product • when monomer units are joinedtogether to form biopolymers • such as proteins or glycogen

24. The effect of solutes on osmolarity depends on the number of dissolved particles, not their mass • The high concentration of albumin and other proteins in blood plasma contributes to its osmolarity. • Cells also actively pump out ions such as Na into the interstitial fluid to stay in osmotic balance with their surroundings.

25. Regulation of water balance • Hypothalamic mechanisms that control thirst • Antidiuretic hormone (ADH) • Retention or excretion of water • Kidneys, and on evaporative loss • Nephrogenic diabetes • Inability to concentrate urine • Unresponsiveness of renal tubular osmoreceptors to ADH.

26. Acidity of aqueous solutions (pH) • pH = −log [H+ ] • Low pH values • Correspond to high concentrations of H+ • High pH values • Correspond to low concentrations of H+ • Acids • are proton donors • Bases • are proton acceptors

27. K w=[H+][OH−]=10−14 • pH+pOH=14 • pH = −log [H+ ]

28. pH • Strong acids • Have larger dissociation constants • Weak acids dissociate only partially • Strong bases (eg, KOH or NaOH) • the strong base KOH is completely dissociated in solution and that the concentration of OH ions is thus equal to that of the KOH • Weak bases (eg, Ca[OH]2) • Many biochemicals are weak acids

29. Many biochemicals possess functional groups that are weak acids or bases • We express the relative strengths of weak acids and bases in terms of their dissociation constants (Ka) • expressing the extent of ionization of water in quantitative terms. • The equilibrium constant is fixed and characteristic for any given chemical reaction at a specified temperature.

30. The tendency of any acid (HA) to lose a proton and form its conjugate base (A-) is defined by the equilibrium constant

31. pKais used to express the relative strengths of both acids and bases. • polyproteic compounds • Containing more than one dissociable proton, a numerical subscript is assigned to each in order of relative acidity • the pKa is the pH at which the concentration of the acid (R-NH3+) equals that of the base (R-NH2).

34. pKa of an acid group is the pH at which the protonated and unprotonated species are present at equal concentrations • The pKa Values Depend on • Molecular Structure • The presence of adjacent negative charge • Decreases with distance • Properties of the Medium

35. Buffer(s) • Solutions of weak acids or bases and their conjugates exhibit buffering • Maximum buffering capacity • Most effectively in the pH range pKa ± 1.0 pH unit. • Physiologic buffers • The value of pKa relative to the desired pH is the major determinant of which buffer is selected.

36. The pH of an aqueous solution can be approximately measured using • indicator dyes including • Litmus, phenolphthalein, and phenol red, which undergo color changes as a proton dissociates from the dye molecule • a glass electrode that is selectively sensitive to H+concentration

37. The pH of some aqueous fluids

38. Disturbances of acid-base balance • Measuring the pH of arterial blood • The CO2 content of venous blood • Acidosis (blood pH < 7.35) • Causes • include diabetic ketosis • Lactic acidosis • Alkalosis (pH > 7.45) • Vomiting of acidic gastric contents

40. Weak Interactions Are Crucial to MacromolecularStructure and Function • Macromolecules such as proteins, DNA, and RNAcontain so many sites of potential hydrogen bonding orionic, van der Waals, or hydrophobic interactions • the cumulative effect of the many small binding forcescan be enormous. • For macromolecules, the most stable(that is, the native) structure is usually that in whichweak-bonding possibilities aremaximized

41. The foldingof a single polypeptide or polynucleotide chain into itsthree-dimensional shape • The binding of an antigen to a specific antibody • Formationof anenzyme-substrate complex • The binding ofa hormone or aneurotransmitter to its cellular receptorprotein

43. Titration curve for an acid of the type HA. The heavy dot in the center of the curve indicates the pKa 5.0.

45. Water Is a Weak Electrolyte Protons that dissociate interact with oxygens of other water molecules to form clusters of water molecules. H+(H2O)n

47. The dissociation of an acid increases with increasing temperatures. • Keqwill be a • Small number if the degree of dissociation of a substance is small. • Large if the degree of dissociation is large

48. An extremely small number of water molecules actually dissociate • At 25°C the value of Keqfor dissociation of water is about 1.8 x 10-16