Which statement about glycerol's structure is true?

Which statement about glycerol's structure is true?

• A. It is a monosaccharide.
• B. It is a fatty acid.
• C. It is a three-carbon molecule with three hydroxyl groups.
• D. It is a steroid.

Answer: (C)

Glycerol is a propane-1,2,3-triol: a three-carbon backbone with a hydroxyl group on each carbon. That makes it a three-carbon molecule with three hydroxyl groups, which is exactly what the statement describes. It’s not a monosaccharide because sugars have carbonyl activity (aldehyde or ketone) in addition to multiple hydroxyls and a larger carbon backbone. It isn’t a fatty acid, which would be a long hydrocarbon chain ending in a carboxyl group. It isn’t a steroid, which features four fused rings in a specific cycloalkane framework. The presence of three hydroxyl groups also explains glycerol’s role as the backbone for triglycerides, where three fatty acids esterify to the three OH groups.

Glycerol is a propane-1,2,3-triol: a three-carbon backbone with a hydroxyl group on each carbon. That makes it a three-carbon molecule with three hydroxyl groups, which is exactly what the statement describes. It’s not a monosaccharide because sugars have carbonyl activity (aldehyde or ketone) in addition to multiple hydroxyls and a larger carbon backbone. It isn’t a fatty acid, which would be a long hydrocarbon chain ending in a carboxyl group. It isn’t a steroid, which features four fused rings in a specific cycloalkane framework. The presence of three hydroxyl groups also explains glycerol’s role as the backbone for triglycerides, where three fatty acids esterify to the three OH groups.