Organic Chemistry MCAT Practice Exam

What structure do phospholipids primarily form in aqueous environments?

1. Lipid micelles

2. Lipid monolayers

3. Lipid bilayers

4. Lipid vesicles

The correct answer is: Lipid bilayers

Phospholipids primarily form lipid bilayers in aqueous environments due to their amphipathic nature. Each phospholipid molecule contains a hydrophilic (water-attracting) "head" and two hydrophobic (water-repelling) fatty acid "tails." When placed in water, phospholipids spontaneously arrange themselves so that their hydrophilic heads face the surrounding water, while the hydrophobic tails point inward, away from the water. This orientation minimizes the unfavorable interactions between the hydrophobic tails and water, allowing for a stable arrangement. The resulting structure is a bilayer, where two layers of phospholipids are packed together, with the tails of one layer facing the tails of the other. This bilayer structure is fundamental to the formation of cell membranes, providing a barrier that separates the inside of the cell from the external environment and allows for compartmentalization. Other structures mentioned—such as lipid micelles, lipid monolayers, and lipid vesicles—are formed under different conditions. Micelles typically occur with molecules having a single hydrophobic tail and are more common in the digestion of lipids. Monolayers are less stable since they would involve a single layer of phospholipids instead of