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After studying this chapter, you should be able to:

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  • Describe the properties of water that account for its surface tension, viscosity, liquid state at ambient temperature, and solvent power.
  • Use structural formulas to represent several organic compounds that can serve as hydrogen bond donors or acceptors.
  • Explain the role played by entropy in the orientation, in an aqueous environment, of the polar and nonpolar regions of macromolecules.
  • Indicate the quantitative contributions of salt bridges, hydrophobic interactions, and van der Waals forces to the stability of macromolecules.
  • Explain the relationship of pH to acidity, alkalinity, and the quantitative determinants that characterize weak and strong acids.
  • Calculate the shift in pH that accompanies the addition of a given quantity of acid or base to the pH of a buffered solution.
  • Describe what buffers do, how they do it, and the conditions under which a buffer is most effective under physiologic or other conditions.
  • Illustrate how the Henderson–Hasselbalch equation can be used to calculate the net charge on a polyelectrolyte at a given pH.

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Water is the predominant chemical component of living organisms. Its unique physical properties, which include the ability to solvate a wide range of organic and inorganic molecules, derive from water's dipolar structure and exceptional capacity for forming hydrogen bonds. The manner in which water interacts with a solvated biomolecule influences the structure both of the biomolecule and of water itself. An excellent nucleophile, water is a reactant or product in many metabolic reactions. Regulation of water balance depends upon hypothalamic mechanisms that control thirst, on antidiuretic hormone (ADH), on retention or excretion of water by the kidneys, and on evaporative loss. Nephrogenic diabetes insipidus, which involves the inability to concentrate urine or adjust to subtle changes in extracellular fluid osmolarity, results from the unresponsiveness of renal tubular osmoreceptors to ADH.

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Water has a slight propensity to dissociate into hydroxide ions and protons. The concentration of protons, or acidity, of aqueous solutions is generally reported using the logarithmic pH scale. Bicarbonate and other 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. Causes of acidosis (blood pH <7.35) include diabetic ketosis and lactic acidosis. Alkalosis (pH >7.45) may follow vomiting of acidic gastric contents.

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Water Molecules Form Dipoles

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A water molecule is an irregular, slightly skewed tetrahedron with oxygen at its center (Figure 2–1). The two hydrogens and the unshared electrons of the remaining two sp3-hybridized orbitals occupy the corners of the tetrahedron. The 105° angle between the hydrogen differs slightly from the ideal tetrahedral angle, 109.5°. Ammonia is also tetrahedral, with a 107° angle between its hydrogens. The strongly electronegative oxygen atoms in water attract electrons away from the hydrogen nuclei, leaving them with a partial positive charge, while its two unshared electron ...

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