How do membrane phospholipids interact with water?

• A. Both the heads and tails of phospholipids are hydrophobic and avoid water.
• B. Their hydrophobic tails face outward toward the water, while their hydrophilic heads face inward.
• C. Their hydrophilic heads face outward toward the water, while their hydrophobic tails face inward away from the water.
• D. Phospholipids dissolve completely in water due to their amphipathic nature.

Answer: (C)

Verified step by step guidance

1. Understand the structure of a phospholipid: Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. The hydrophilic head contains a phosphate group, while the hydrophobic tail consists of fatty acid chains.
2. Recognize how amphipathic molecules interact with water: The hydrophilic heads of phospholipids are attracted to water and will orient themselves toward the aqueous environment. In contrast, the hydrophobic tails avoid water and will orient themselves away from it.
3. Visualize the arrangement of phospholipids in water: When placed in water, phospholipids spontaneously form structures like micelles or bilayers. In a bilayer, the hydrophilic heads face outward toward the water on both sides, while the hydrophobic tails face inward, away from the water, creating a stable barrier.
4. Clarify why phospholipids do not dissolve completely in water: Due to their amphipathic nature, phospholipids do not dissolve but instead organize into specific structures to minimize the exposure of hydrophobic tails to water while maximizing the interaction of hydrophilic heads with water.
5. Relate this concept to biological membranes: In cell membranes, phospholipids form a bilayer that serves as a selective barrier, with hydrophilic heads interacting with the aqueous environment inside and outside the cell, and hydrophobic tails creating a nonpolar interior that regulates the passage of substances.