Glucose, the building block of cellulose and starch, can form six-membered rings with two distinct stereoisomers called the alpha and beta anomer. The only difference between alpha and beta glucose is at carbon C1. Cellobiose (reload ) is a breakdown product of cellulose which shows the beta 1,4 linkage also present in cellulose. "beta 1,4" refers to a glycosidic link between the anomeric carbon () in beta configuration of one glucose molecule with carbon 4 () of the other glucose molecule. . In contrast, starch can be broken down to maltose, a stereoisomer of cellobiose showing an alpha 1,4 linkage. Thus, it is the type of glycosidic linkage that distinguishes cellulose from starch at the molecular level.
Longer chains of beta 1,4 linked glucoses are found in cellulose. When cellulose is synthesized, these chains are made individually (cellulose chain during ). Again, the linkages are all of the beta 1,4 type (). In this structure, monomers are added to polymer chain inside the cell and secreted through the membrane, surrounded by the throughout.
Once secreted, individual cellulose chains aggregate to from semi-crystalline cellulose microfibrils. There are multiple forms of cellulose (I alpha and beta, II, III) which differ in the orientation and the detailed interactions between linear polymers. A shows how inter-chain hydrogen bonding and hydrophobic interactions lead to a highly ordered structure. The model was made using cellulose builder (http://cces-sw.iqm.unicamp.br/cces/admin/cellulose, [1]) and is based on a fiber-diffraction study by Nishiyama et al [2].
