Advanced Placement (AP) Biology Practice Exam

What part of the phospholipid bilayer is considered hydrophilic?

1. Fatty acid tails

2. Phosphate head

3. Cholesterol molecules

4. Transmembrane proteins

The correct answer is: Phosphate head

The hydrophilic nature of the phospholipid bilayer is attributed to the phosphate head of phospholipids. Each phospholipid molecule consists of a hydrophobic fatty acid tail and a hydrophilic phosphate head. The phosphate head interacts readily with water and other polar substances, which allows it to face outward towards the aqueous environment both inside and outside of the cell. This characteristic is crucial for the formation of the cell membrane, as it helps create a stable barrier between the internal and external environments of the cell while also facilitating communication and transport. The other components mentioned, such as the fatty acid tails, are hydrophobic and avoid contact with water. Cholesterol molecules play a role in stabilizing the membrane but do not inherently contribute to the hydrophilic properties of the bilayer. Transmembrane proteins also interact with both the hydrophobic and hydrophilic regions but are not themselves considered exclusively hydrophilic. Thus, the phosphate head is clearly identified as the hydrophilic part of the phospholipid bilayer.