Carbohydrates are a diverse group of compounds composed of the elements carbon (C), hydrogen (H), and oxygen (O) with the empirical formula [CH₂ O]_n, where n represents the number of CH₂ O units in the compound. The ultimate source of all carbohydrates is photosynthesis, in which the energy of sunlight is used to chemically fix atmospheric CO₂ into carbohydrate. Carbohydrates constitute as much as 80 percent of the dry weight of a plant. This is largely due to the presence of cell walls made of complex carbohydrates surrounding each plant cell.

Sugars, also called saccharides, are carbohydrates. Common sugars occurring in nature have from three (glyceraldehyde) to seven (sedoheptulose) carbon atoms bonded together to form the molecule's backbone. Sugars can be classified chemically as either aldehydes or ketones, and contain OH (hydroxyl) groups attached to their carbon backbones. Glucose (an aldehyde sugar) and fructose (a ketose sugar) are...