Why do phospholipids spontaneously form bilayers when mixed with $H2O$?

• A. Because they are completely hydrophobic and repel $H2O$.
• B. Because they are completely hydrophilic and dissolve in $H2O$.
• C. Because their hydrophobic tails repel $H2O$ and their hydrophilic heads attract $H2O$.
• D. Because their hydrophilic tails repel $H2O$ and their hydrophobic heads attract $H2O$.

Answer: (C)

Verified step by step guidance

1. Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts.
2. The structure of a phospholipid includes a hydrophilic 'head' that is attracted to water and hydrophobic 'tails' that repel water.
3. When phospholipids are mixed with water, the hydrophilic heads interact with the aqueous environment, while the hydrophobic tails avoid water.
4. To minimize the energy of the system, phospholipids arrange themselves into a bilayer, with the hydrophobic tails facing inward, shielded from water, and the hydrophilic heads facing outward, interacting with the water.
5. This spontaneous formation of a bilayer is driven by the need to reduce the unfavorable interactions between the hydrophobic tails and water, while maximizing the favorable interactions of the hydrophilic heads with water.