Which of the following is a small molecule?

Glycerol is classified as a small molecule because it is a simple compound made up of three carbon atoms, each bonded to hydroxyl (-OH) groups, making it a type of sugar alcohol. Small molecules are typically low in molecular weight and consist of a limited number of atoms, which allows them to easily move across cell membranes and participate in a variety of biochemical reactions.

In contrast, starch is a complex carbohydrate made up of many glucose units linked together, making it a large molecule. Proteins are composed of long chains of amino acids and also represent large biomolecules with complex structures. Fats, or lipids, consist of glycerol as well but are larger molecules formed by the addition of fatty acids to glycerol, which creates triglycerides and other lipids. This distinction among molecular sizes is essential for understanding biological processes and the role of different molecules in metabolism.