Which of the following is polar?
**Core Concept**
The question is testing the student's understanding of the properties of lipid bilayers and the terms polar and nonpolar. In a lipid bilayer, polar molecules are those with a charge or a partial charge, typically found on the head group of phospholipids, while nonpolar molecules are hydrophobic and found in the lipid tails.
**Why the Correct Answer is Right**
The correct answer is a phospholipid, specifically the head group, which contains a phosphate group and a charged amino group. These groups are polar due to the presence of a charge or a partial charge, allowing them to interact with water and other polar molecules. The phospholipid head group is amphipathic, meaning it has both polar and nonpolar regions, which enables it to form a bilayer with the nonpolar lipid tails facing inward and the polar head groups facing outward.
**Why Each Wrong Option is Incorrect**
**Option A:** Cholesterol is a nonpolar molecule, as it has a large nonpolar steroid ring structure that allows it to interact with the nonpolar lipid tails in the bilayer.
**Option B:** A glycolipid is also nonpolar, as it has a nonpolar carbohydrate moiety that interacts with the nonpolar lipid tails.
**Option C:** A triglyceride is a nonpolar molecule, as it consists of three nonpolar fatty acid chains attached to a glycerol backbone.
**Clinical Pearl / High-Yield Fact**
The amphipathic nature of phospholipids is crucial for the formation of cell membranes, allowing them to maintain a stable structure while interacting with both polar and nonpolar molecules.
**Correct Answer:** C. Triglyceride is a nonpolar molecule.