Chapter 2: Water

Water serves as the fundamental solvent and medium for all biochemical processes, comprising 60 to 90 percent of cellular mass and dictating the structure and behavior of biological macromolecules. The molecule's distinctive V-shaped geometry with a bond angle of 104.5 degrees, combined with oxygen's high electronegativity, creates a permanent dipole that enables water to form up to four hydrogen bonds with neighboring molecules. This capacity for hydrogen bonding produces water's remarkable physical properties, including exceptional specific heat and heat of vaporization, along with the counterintuitive density decrease upon freezing that allows ice to float. As a solvent, water's polarity makes it exceptionally effective at dissolving both polar and ionic compounds through solvation sphere formation, while simultaneously excluding nonpolar molecules through the hydrophobic effect, a thermodynamically favorable process driven by entropy rather than molecular attraction. The interactions between water and dissolved substances reveal four fundamental types of noncovalent forces: electrostatic charge-charge interactions or salt bridges, hydrogen bonds, van der Waals forces consisting of London dispersion and repulsive components, and hydrophobic interactions that organize nonpolar molecules. Water functions as a nucleophile capable of hydrolyzing biological polymers into constituent units, though the rate of degradation inside cells remains negligibly slow, preserving macromolecular stability. Cellular biosynthesis of large molecules overcomes water's hydrolytic tendency by coupling condensation reactions to adenosine triphosphate hydrolysis. The ionization of water produces hydronium and hydroxide ions in equal concentrations at 10^-7 molar, establishing the neutral pH reference point of 7 and enabling quantification of acidity and basicity through logarithmic pH calculations. Weak acids dissociate only partially, characterized by their acid dissociation constants and pKa values, relationships elucidated by the Henderson-Hasselbalch equation and titration curves that reveal half-dissociation at pH equivalence with pKa. Buffer systems, particularly the carbon dioxide-carbonic acid-bicarbonate buffer in mammalian blood, maintain pH stability by resisting changes when strong acids or bases are introduced, with maximal buffering capacity occurring within one pH unit of the buffer acid's pKa value.