Optometry Admission Test (OAT) Practice

Which region of a phospholipid is identified as hydrophilic?

1. The fatty-acid region

2. The cholesterol region

3. The phosphoric-acid region

4. The protein region

The correct answer is: The phosphoric-acid region

The phosphoric-acid region of a phospholipid is hydrophilic due to its polar nature. This part of the molecule contains a phosphate group, which carries a negative charge and can form hydrogen bonds with water molecules. Because of this polar characteristic, the phosphoric-acid region interacts favorably with the aqueous environment, making it hydrophilic. In contrast, the fatty-acid region is comprised of long hydrocarbon chains that are nonpolar and do not interact well with water, making them hydrophobic. The cholesterol region is primarily hydrophobic as well, as cholesterol consists mainly of carbon and hydrogen atoms; it helps stabilize the cell membrane but does not contribute to hydrophilicity. The protein region's hydrophilicity or hydrophobicity depends on the specific proteins involved and cannot be generalized like the phosphoric-acid region. Thus, the distinct polar nature of the phosphoric-acid region crucially defines its hydrophilic characteristics in the context of phospholipid structure and function.