Cellulose
Cellulose is the most abundant natural biopolymer on Earth because it is the main component in plant cell walls. Cellulose provides plants with structural support, hence wood and paper are mostly composed of cellulose. Cellulose is made up of glucose monomers that are linked by β 1,4 glycosidic bonds.
Figure 1: In cellulose, glucose monomers are linked in unbranched chains by β 1,4 glycosidic bonds.
Because of the way the glucose subunits are joined, every glucose monomer is flipped relative to the next one resulting in a linear, fibrous structure. This gives cellulose its rigidity and high tensile strength — which is so important to plant cells.
Human digestive enzymes cannot break down the β 1,4 glycosidic bonds. However, herbivores such as cows, koalas, buffalos, and horses possess specialized gut flora which can digest cellulose and use it as a food source. In these animals, certain species of bacteria and protists reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase.
Cellulose-digesting bacteria also live in the appendix of grazing animals. The appendix is therefore important in the digestive systems of ruminants.
Termites also contain microorganisms in their body which can break down cellulose.
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