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Cellulose is an important component of plant cell walls. It consists of a long chain of glucose units and is a very stable compound. Some fungi can degrade cellulose, either alone or in partnerships. For example, cows can only digest grass because they have anaerobic microorganisms in one of their stomachs that can decompose the long cellulose molecules.
Explanation for children:
Some microorganisms can eat cellulose, i.e. use it as a substrate. Cellulose is like a complicated sugar that humans cannot digest. It is mainly found in plants. All living beings that eat plants need the help of microorganisms, because only these can break down cellulose. For example, they live in the stomachs of herbivores and decompose cellulose there.