Furan is an organic cyclic carbon molecule, formed of 5 components: 4 Carbon atoms and 1 Oxygen atom. It is an aromatic compound, so it has 2 unstable, delocalized double bonds in the ring. Furan is similar in structure to Pyran, which is similar, except it consists of 5 Carbon atoms and 1 Oxygen atom, has different bond angles, and does not contain any double bonds. 
Furans are important to the metabolism because of the many furanose sugars that exist in the body that are used to store energy. Furanose sugars do not contain any double bonds, unlike Furan. The term furan may also refer to any compound that is similar to Furan in structure.
- ↑ Biochemistry, 5th edition, Jeremy M Berg, John L Tymoczko, Lubert Stryer, W.H.Freeman &amp; Co, Pages 455-456