Carbohydrate structure

Carbohydrates are molecules of carbon, hydrogen, and oxygen with the general formula Cn(H2O)n — literally "hydrated carbon." Their simplest units are monosaccharides: single sugars like glucose, fructose, and galactose, each with a carbon backbone bearing hydroxyl groups and one carbonyl group. If the carbonyl is an aldehyde the sugar is an aldose (glucose); if it is a ketone the sugar is a ketose (fructose).

Monosaccharide structure carries several layers of stereochemistry. Sugars are classified D or L by the orientation of the hydroxyl on the chiral carbon farthest from the carbonyl, and nearly all biological sugars are D. In water, five- and six-carbon sugars cyclize: the carbonyl reacts with a hydroxyl to form a ring, creating a new chiral center at the anomeric carbon. The ring can close in two ways, giving α and β anomers, which interconvert through mutarotation. Sugars differing at a single chiral center, like glucose and galactose, are epimers.

Monosaccharides join through glycosidic bonds formed by dehydration. Two sugars make a disaccharide — maltose (glucose + glucose), sucrose (glucose + fructose), lactose (galactose + glucose) — and many make polysaccharides. Linkage geometry determines function: glycogen and starch use α linkages that human enzymes digest, storing energy in animals and plants respectively, while cellulose uses β linkages we cannot break, making it structural fiber.

The MCAT tests carbohydrate structure in its biochemistry foundations. High-yield points include distinguishing aldoses from ketoses, identifying anomers and epimers, drawing ring forms, and explaining why α- versus β-glycosidic linkages separate digestible glycogen and starch from indigestible cellulose.