8885dc02_0517/25/0311:52 AM Page51mac76mac76:385reb: Chapter 2 Water TABLE 2-2 Some Examples of Polar, Nonpolar, and Amphipathic Biomolecules(Shown as lonic Forms at pH 7) Polar Glucose CHO CH3(CH2)-CH-CH-(CH2)-CH2 O CH3(CH2)--CH-CH-(CHa)7-CH Glycine+NH3-CH2-COO NH3 CHo-CH-CO0- OoC—CH2-CH-CO0 Phosphatidylcholine Lactate CH3-CH-CO0 CH3(CH2)15CH2-C-O-CHa CH3(CHg)sCH, 0-CH N(CH3)3 CH2-0-P--0-CH2-CH2 HOCH2-CH-CH2OH Polar groups Nonpolar groups For water at 25C, 8(which is dimensionless) is 78.5 and for the very nonpolar solvent benzene, a is 4.6. Thus Entropy Increases as Crystalline Substances Dissolve ionic interactions are much stronger in less polar As a salt such as Nacl dissolves, the Na and Cl ions ronments. The dependence on r- is such that ionic leaving the crystal lattice acquire far greater freedom of tractions or repulsions operate only over short motion(Fig. 2-6). The resulting increase in entropy ances--in the range of 10 to 40 nm(depending on the (randomness) of the system is largely responsible for electrolyte concentration) when the solvent is water. the ease of dissolving salts such as Nacl in water. In Hydrate Na+ion 8a ob oo d Na of the water molecules q=)8 E 2-6 Water as solvent Water dissolves many crystalline salts charges are partially neutralized, and the electrostatic attractions nec- by hydrating their component ions. The NaCl crystal lattice is disrupted essary for lattice formation are weakened as water molecules cluster about the Cl and Na ions the ionFor water at 25
C,
(which is dimensionless) is 78.5, and for the very nonpolar solvent benzene,
is 4.6. Thus, ionic interactions are much stronger in less polar envi￾ronments. The dependence on r2 is such that ionic at￾tractions or repulsions operate only over short dis￾tances—in the range of 10 to 40 nm (depending on the electrolyte concentration) when the solvent is water. Entropy Increases as Crystalline Substances Dissolve As a salt such as NaCl dissolves, the Na and Cl ions leaving the crystal lattice acquire far greater freedom of motion (Fig. 2–6). The resulting increase in entropy (randomness) of the system is largely responsible for the ease of dissolving salts such as NaCl in water. In Chapter 2 Water 51 TABLE 2–2 Some Examples of Polar, Nonpolar, and Amphipathic Biomolecules (Shown as Ionic Forms at pH 7) + Hydrated Na+ ion Note the orientation of the water molecules Hydrated Cl– ion H2O Na+ Cl– + – + – + – – – – + + + + – – – – – – – – – + – – FIGURE 2–6 Water as solvent. Water dissolves many crystalline salts by hydrating their component ions. The NaCl crystal lattice is disrupted as water molecules cluster about the Cl and Na ions. The ionic charges are partially neutralized, and the electrostatic attractions nec￾essary for lattice formation are weakened. H HO CH2OH O OH OH OH CH2 NH3 COO CH2 OOC COO H H H H NH3 CH CH OH OH CH3 CH COO HOCH2 CH2OH CH3(CH2)7 CH CH (CH2)6 CH2 C CH3(CH2)7 CH CH (CH2)7 CH2 CH2 CH GNH3 GN(CH3)3 O O COOJ CH3(CH2)15CH2 CH2 O CH2 CH2 O OJ C CH3(CH2)15CH2 O CH O O CH2 P C O O Polar groups Nonpolar groups Polar Glucose Glycine Aspartate Lactate Glycerol Nonpolar Typical wax Amphipathic Phenylalanine Phosphatidylcholine 8885d_c02_051 7/25/03 11:52 AM Page 51 mac76 mac76:385_reb: