Why do phospholipids spontaneously form a bilayer when placed in water?

• A. Because the phosphate head groups are nonpolar and pack together in the bilayer interior.
• B. Because covalent bonds form between phospholipid tails, creating a permanent two-layer sheet.
• C. Because their amphipathic structure drives hydrophobic tails to avoid water while hydrophilic heads interact with water, minimizing free energy.
• D. Because water molecules are nonpolar and preferentially dissolve the fatty acid tails, forcing heads outward.

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the structure of phospholipids. Phospholipids have an amphipathic nature, meaning they contain both hydrophilic (water-attracting) 'head' groups and hydrophobic (water-repelling) fatty acid 'tails'.
2. Step 2: Recognize the behavior of hydrophilic heads and hydrophobic tails in an aqueous environment. The hydrophilic heads interact favorably with water molecules, while the hydrophobic tails avoid water to minimize unfavorable interactions.
3. Step 3: Explain how this amphipathic property leads to spontaneous bilayer formation. When phospholipids are placed in water, the hydrophobic tails aggregate together away from water, while the hydrophilic heads face outward toward the water, forming a bilayer structure.
4. Step 4: Connect this arrangement to thermodynamics. This organization minimizes the system's free energy by reducing the exposure of hydrophobic tails to water and maximizing favorable interactions between hydrophilic heads and water.
5. Step 5: Clarify why other options are incorrect. For example, phosphate head groups are polar (not nonpolar), covalent bonds do not form between tails to create the bilayer, and water molecules are polar, not nonpolar, so they do not dissolve fatty acid tails.