Carbohydrates Carbohydrates are substances whose molecules contain the elements carbon, hydrogen, and oxygen in which the ratio of hydrogen to oxygen is 2 to 1. However, carbohydrates are not simply hydrated carbon atoms. The ratio of H to O might be the same as water, but otherwise, water is not a component of a carbohydrate molecule. Carbohydrate molecules can vary widely in their complexity, and in the speed with which they are digested and metabolized. Sugars are a class of carbohydrates.
Sugars consist of single units, or two units bound together with covalent bonds.
A simple sugar contains a single unit. Examples of monosaccharides include glucose, fructose, and galactose. Two monosaccharides may even contain the exact same chemical formula. The difference is in the way the atoms are arranged in the molecule. Substances that have the same chemical formula, but differ in structure are called isomers.