Which three of the statements are true?
A Hydrophobic molecules do not readily dissolve in water in part because water molecules involved in hydration cannot participate in normal hydrogen bonding with one another:
B Nonpolar molecules are capable of forming micelles_ The tendency of water to minimize its contact with nonpolar substances is called the hydrophobic effect:
C Hydrophobic molecules are individually hydrated in water; increasing the entropy of the system:
D Amphipathic (amphiphilic) lipids are the structural basis of biological bilayer membranes
The correct answer and explanation is:
The three true statements are:
A. Hydrophobic molecules do not readily dissolve in water in part because water molecules involved in hydration cannot participate in normal hydrogen bonding with one another.
B. Nonpolar molecules are capable of forming micelles. The tendency of water to minimize its contact with nonpolar substances is called the hydrophobic effect.
D. Amphipathic (amphiphilic) lipids are the structural basis of biological bilayer membranes.
Explanation:
- Statement A: Hydrophobic molecules and water
- Water is a highly polar solvent, and it forms hydrogen bonds with other polar molecules or ions. Hydrophobic (nonpolar) molecules do not dissolve well in water because they do not form hydrogen bonds with water molecules. Instead of interacting with water molecules, they disrupt the hydrogen-bond network, leading to an unfavorable entropic situation. Water molecules then surround the hydrophobic molecule in an ordered arrangement, which decreases the overall entropy of the system. This is why hydrophobic molecules prefer not to dissolve in water.
- Statement B: Hydrophobic effect and micelle formation
- The hydrophobic effect refers to the tendency of water molecules to minimize their contact with nonpolar substances. When nonpolar molecules, such as fatty acids or detergents, are placed in water, water molecules form an ordered shell around them. To minimize this structured water arrangement, the nonpolar molecules aggregate, forming structures like micelles. A micelle is a spherical arrangement where the hydrophobic parts of the molecules are shielded from water, and the hydrophilic parts face outward. This helps increase the entropy of the system, making it thermodynamically favorable.
- Statement D: Amphipathic lipids in biological membranes
- Amphipathic lipids, such as phospholipids, have both a hydrophobic tail and a hydrophilic head. These properties make them ideal for forming biological membranes. The hydrophilic heads interact with water, while the hydrophobic tails avoid water, leading to the formation of lipid bilayers. These bilayers are the structural foundation of cell membranes, where the hydrophobic tails face inward, shielded from water, and the hydrophilic heads face outward toward the water inside and outside the cell. This bilayer arrangement is crucial for the cell’s integrity and function.
Incorrect Statement:
Statement C: Hydrophobic molecules are individually hydrated in water, increasing the entropy of the system.
- This is incorrect because when hydrophobic molecules are placed in water, they do not become individually hydrated in a way that increases entropy. Instead, water molecules become organized around hydrophobic molecules, reducing entropy. The overall entropy of the system would increase if the hydrophobic molecules aggregated to minimize their exposure to water.