Organic Chemistry MCAT Practice Exam

Which component of phospholipids is hydrophilic?

1. Hydrophobic tails

2. Hydrophilic head

3. Saturated fatty acid

4. Cholesterol

The correct answer is: Hydrophilic head

Phospholipids are fundamental molecules that comprise the cell membrane, and their structure includes both hydrophilic and hydrophobic components. The hydrophilic head of a phospholipid is composed of a phosphate group, which is polar and interacts favorably with water. This polar nature allows the head to form hydrogen bonds with water molecules, making it water-soluble. In contrast, the hydrophobic tails consist of long hydrocarbon chains, which are nonpolar and repel water. This unique arrangement of hydrophilic heads and hydrophobic tails is critical for the formation of lipid bilayers in biological membranes. The hydrophilic heads face outward towards the aqueous environment, while the hydrophobic tails point inward, away from water. The options related to saturated fatty acids and cholesterol do not pertain to the hydrophilic nature of phospholipids. Saturated fatty acids are components of the hydrophobic tails, while cholesterol is a sterol that helps to maintain membrane fluidity but is not itself a part of the phospholipid structure. Therefore, the hydrophilic head is indeed the component that interacts with water, making it the correct answer.