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Biochemistry

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Hydrophobic Effect: What Drives Solubility?

BIOCHM-FRSLSK

The hydrophobic effect can be described as the tendency of nonpolar molecules to aggregate and minimize contact with polar water molecules.

The hydrophobic effect influences the shapes of many molecules and structures and cellular processes including the solubility of salts and insolubility of fats in cells, receptor-ligand interactions, the folding of proteins and their insertion into biological membranes, as well as the overall structure and orientation of amphipathic molecules forming biological membranes.

Thinking about each of these structures or processes, identify which of the following descriptions does NOT accurately describe the hydrophobic effect.

A

NaCl dissolves in water because the ionic forces between the ions are weaker than the interactions formed with the polar dipoles of the water molecules. In this case, the entropy of the solute increases and the free energy change (${ \Delta G }$ ) is highly negative.

B

Nonpolar substances, such as fats, aggregate together away from water driven largely by internal hydrophobic interactions among the long-chain hydrocarbons. This is driven by an increase in entropy of the solute.

C

Amphipathic molecules (e.g., phospholipids) tend to form micelles or bilayer structures in water due to the hydrophobic effect in which the nonpolar lipid tails aggregate away from water and the polar phosphate head groups H-bond with water.The formation of this structure is driven by an increase in entropy of the surrounding water molecules.

D

Native protein structure is driven by the hydrophobic effect in which nonpolar side chains aggregate to the interior of the protein driven by the resulting increase in entropy of surrounding water molecules which are "released" from each nonpolar functional group.

E

Adding oil to water does not result in the formation of a spontaneous solution; therefore, the ${ \Delta G }$ value is highly positive. Adding additional nonpolar molecules to the solution would result in an increase in entropy of the water molecules, but the overall ${ \Delta G }$ remains positive.