How does a phospholipid behave when placed in water?

• A. It spontaneously forms a bilayer with hydrophobic tails facing inward and hydrophilic heads facing outward.
• B. It aggregates with hydrophilic heads facing inward and hydrophobic tails facing outward.
• C. It remains as individual molecules evenly dispersed in the water.
• D. It dissolves completely, forming a homogeneous solution.

Answer: (A)

Verified step by step guidance

1. Understand the structure of a phospholipid: A phospholipid consists of a hydrophilic (water-attracting) head group and two hydrophobic (water-repelling) fatty acid tails. This amphipathic nature is key to its behavior in water.
2. Recognize the principle of hydrophobic and hydrophilic interactions: In an aqueous environment, hydrophilic regions interact with water, while hydrophobic regions avoid water and tend to cluster together.
3. Consider the behavior of amphipathic molecules in water: Amphipathic molecules like phospholipids spontaneously organize themselves to minimize the exposure of hydrophobic regions to water while maximizing the interaction of hydrophilic regions with water.
4. Visualize the formation of a bilayer: Phospholipids arrange themselves into a bilayer structure, where the hydrophilic heads face outward toward the water, and the hydrophobic tails face inward, shielded from the water. This arrangement is energetically favorable.
5. Eliminate incorrect options: Based on the amphipathic nature of phospholipids, they do not dissolve completely, remain as individual molecules, or aggregate with hydrophilic heads facing inward. The correct behavior is the spontaneous formation of a bilayer.