Monosaccharides

A monosaccharide is a single sugar molecule. Glucose, fructose and galactose are hexoses: each contains six carbon atoms. Ribose is a pentose, containing five carbon atoms. These names count all the carbon atoms in the molecule, not just those in a ring.

Glucose and ribose ring structures

Glucose commonly forms a six-membered ring containing five carbon atoms and one oxygen atom. Its sixth carbon lies outside the ring. A hydroxyl group (−OH) is attached to carbon 1. In alpha-glucose, this hydroxyl group is on the opposite side of the ring from the carbon-6 group. In beta-glucose, it is on the same side. This small difference affects the chains that glucose molecules can form.

Ribose commonly forms a five-membered ring containing four carbon atoms and one oxygen atom. Its fifth carbon lies outside the ring.

Glucose in water and respiration

Glucose has hydroxyl groups that interact with water. This makes glucose soluble, so it can be transported in solution. Cells can also use glucose as a substrate in respiration to release energy.