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. The disaccharide cellobiose (reload initial scene) is a breakdown product of cellulose which shows the beta 1,4 linkage between two glucose molecules 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, starches (specifically the linear form amylose) 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 starches 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 biosynthesis)[1]. 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 enzyme throughout.
Once secreted, individual cellulose chains self-assemble 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 model of a cellulose type I beta micro-fibril shows a tightly packed structure. The model was made using cellulose builder (https://cces-sw.iqm.unicamp.br/cces/admin/cellulose, [2]) and is based on a fiber-diffraction study by Nishiyama et al [3]. The individual chains of cellulose form layers, and multiple layers stack to form a 3D structure without any gaps. While interactions within layers are dominated by hydrogen bonding, interactions between layers are hydrophobic[4]. You can use the buttons below to explore the 1D, 2D and 3D assembly of the microfibril model.
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