Which of the following compounds is classified as a carbohydrate?

Glucose is classified as a carbohydrate because it is a simple sugar, or monosaccharide, which serves as a fundamental energy source for living organisms. Carbohydrates are characterized by their carbon, hydrogen, and oxygen composition, often in a ratio that approximates 1:2:1. Glucose specifically has the chemical formula C6H12O6, fitting this carbohydrate definition perfectly. It plays a vital role in metabolic processes, such as cellular respiration, where it is broken down to produce ATP, the energy currency of cells.

The other compounds listed do not fit the definition of carbohydrates. Adenine is a nitrogenous base, a component of nucleotides in DNA and RNA, and is not classified as a carbohydrate. Cholesterol is a type of lipid, specifically a steroid, which plays crucial roles in cell membrane structure and hormone synthesis but does not meet the criteria of a carbohydrate. Hemoglobin, on the other hand, is a protein responsible for oxygen transport in blood and is composed of amino acids rather than sugars, distinguishing it from carbohydrates.