Dispersion Of Lipid In H2o

*The arrows represent electric dipoles; there Is a partial negative charge (8 ) at the head of the arrow, a partial positive charge (8+; not shown here) at the tail.

fNote that polar molecules dissolve far better even at low temperatures than do nonpolar molecules at relatively high temperatures.

*The arrows represent electric dipoles; there Is a partial negative charge (8 ) at the head of the arrow, a partial positive charge (8+; not shown here) at the tail.

fNote that polar molecules dissolve far better even at low temperatures than do nonpolar molecules at relatively high temperatures.

Hydrophilic "head group"

Hydrophilic "head group"

H2o Molecule

"Flickering clusters" of H2O molecules in bulk phase

Hydrophobic alkyl group

"Flickering clusters" of H2O molecules in bulk phase

Hydrophobic alkyl group

Highly ordered H2O molecules form "cages" around the hydrophobic alkyl chains

FIGURE 2-7 Amphipathic compounds in aqueous solution. (a) Long-chain fatty acids have very hydrophobic alkyl chains, each of which is surrounded by a layer of highly ordered water molecules. (b) By clustering together in micelles, the fatty acid molecules expose the smallest possible hydrophobic surface area to the water, and fewer water molecules are required in the shell of ordered water. The energy gained by freeing immobilized water molecules stabilizes the micelle.

water, the polar, hydrophilic region interacts favorably with the solvent and tends to dissolve, but the nonpolar, hydrophobic region tends to avoid contact with the water (Fig. 2-7a). The nonpolar regions of the molecules cluster together to present the smallest hydrophobic area to the aqueous solvent, and the polar regions are arranged to maximize their interaction with the solvent (Fig. 2-7b). These stable structures of amphipathic compounds in water, called micelles, may contain hundreds or thousands of molecules. The forces that hold the nonpolar regions of the molecules together are called hydrophobic interactions. The strength of hydrophobic interactions is not due to any intrinsic attraction between nonpolar moieties. Rather, it results from the system's achieving greatest thermodynamic stability by minimizing the number of ordered water molecules required to surround hydrophobic portions of the solute molecules.

Many biomolecules are amphipathic; proteins, pigments, certain vitamins, and the sterols and phospho-lipids of membranes all have polar and nonpolar surface regions. Structures composed of these molecules are stabilized by hydrophobic interactions among the non-

Glucose Phosphate

Dispersion of lipids in H2O

Each lipid molecule forces surrounding H2O molecules to become highly ordered.

Clusters of lipid molecules

Only lipid portions at the edge of the cluster force the ordering of water. Fewer H2O molecules are ordered, and entropy is increased.

Micelles

All hydrophobic groups are sequestered from water; ordered shell of H2O molecules is minimized, and entropy is further increased.

Dispersion of lipids in H2O

Each lipid molecule forces surrounding H2O molecules to become highly ordered.

Clusters of lipid molecules

Only lipid portions at the edge of the cluster force the ordering of water. Fewer H2O molecules are ordered, and entropy is increased.

Micelles

All hydrophobic groups are sequestered from water; ordered shell of H2O molecules is minimized, and entropy is further increased.

polar regions. Hydrophobic interactions among lipids, and between lipids and proteins, are the most important determinants of structure in biological membranes. Hydrophobic interactions between nonpolar amino acids also stabilize the three-dimensional structures of proteins.

Hydrogen bonding between water and polar solutes also causes some ordering of water molecules, but the effect is less significant than with nonpolar solutes. Part

Hydrogen Bonds Biomolecules

Disordered water displaced by enzyme-substrate interaction

Enzyme-substrate interaction stabilized by hydrogen-bonding, ionic, and hydrophobic interactions

FIGURE 2-8 Release of ordered water favors formation of an enzyme-substrate complex. While separate, both enzyme and substrate force neighboring water molecules into an ordered shell. Binding of substrate to enzyme releases some of the ordered water, and the resulting increase in entropy provides a thermodynamic push toward formation of the enzyme-substrate complex.

of the driving force for binding of a polar substrate (re-actant) to the complementary polar surface of an enzyme is the entropy increase as the enzyme displaces ordered water from the substrate (Fig. 2-8).

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