Cellulose is a type of carbohydrate found in plants. Cellulose is like other carbohydrates made of glucose, however, the circular glucose has the hydrogen and -OH groups in an opposite conformation on the 1 carbon of the glucose ring. This switch gives a molecule called β-glucose, as distinct from α-glucose, which is the form of glucose found in most other biochemical reactions (Figure 1).

Figure 1: The structures of the α (left) and β (right) forms of glucose, with the difference in conformation around the 1 carbon highlighted by the red circles. α glucose is the form involved in cellular metabolism in animals, whereas β glucose is the form which produces cellulose when it polymerizes. The differing conformation of the -OH and -H groups around this carbon atom is what gives each version of glucose their unique properties required for cellular metabolism and cellulose formation respectively.

Because of the different orientation of hydrogen and -OH groups at this carbon atom, the polypeptide chain is not able to form a straight chain with the 6 carbons of the glucose rings all pointing the same direction, which is the case in polysaccharides composed of α-glucose. Instead, when the β-glucose polymerizes, the 6 carbons point in opposite directions alternatively throughout the length of the chain, as the misalignment of the –OH groups required to form the glycoside bonds causes the molecules to form a twist when they polymerise. This means that each subunit of the cellulose polymer can be thought of as two glucose rings with the 6 carbons of each ring pointing in opposite directions (Figure 2), as it is this pattern, with opposite oriented glucose molecules and 6 carbon atoms, is what is repeated throughout the cellulose chain.

Figure 2: Polymerization of two β-glucose molecules forming a disaccharide, showing the distinction between the cellulose polymer and other polysaccharides. Because of the difference in conformation of the -OH and -H groups around the 1 carbon of the glucose ring, the polymerization introduces a twist in the polymer chain, whereby the 6 carbons in the chain point in alternate directions each glucose unit in the chain. As a consequence, this twisting means that a cellulose molecule can be thought of as a repeating chain of this β glucose disaccharide.

As the cellulose polymer chains elongate, the significance of these opposite pointing 6 carbons becomes apparent (Figure 3). The -OH groups on the 6 carbons are able to form hydrogen bonds between adjacent cellulose chains, and because these 6 carbons are found on both sides of the chain in cellulose, but not in other polysaccharides, the cellulose is able to form a strong, cross-linked structure, unlike other polysaccharides formed by α-glucose, which have a globular structure. Adjacent polysaccharide chains are crosslinked by hydrogen bonds, forming a rigid structure, which is the basis behind the plant cell wall. This rigid structure gives plants their strength, and protects plant cells from damage.

Figure 3: Cellulose polymer chains aligned side by side. When side by side, the importance of the alternating position of the 6 carbons becomes apparent. This alternation means that a cellulose chain is able to form hydrogen bonds between adjacent cellulose chains on either side of the chain, via the -OH groups on the 6 carbons. This ability to form strong interactions with adjacent molecules gives cellulose its enormous strength, due to the strong cross-linking between the cellulose chains.

Cellulose is not found in animal cells. Animal cells do not have a cell wall, which means their cell walls are not rigid like those found in plant cells. Cellulose is also more difficult for animal digestive systems to digest, which means that cellulose is one of the main constituents of dietary fibre. Some animals, such as cattle and other ruminants are able to digest cellulose though. They do this by having bacteria in their stomachs which are able to break down the cellulose. In other animals, the cellulose content passes through digestion intact, though it does have an impact on health, as the dietary fibre causes numerous changes to the digestive process that improves wider bodily health.