Cell wall

As we have just noted, not all eucaryotes possess a cell wall; among those that do are fungi, algae and plants. Whilst the function, like that of procaryotes, is to give strength to the cell, the chemical composition is very different, generally being a good deal simpler. The cell walls of plants, algae and lower members of the fungi are based on cellulose (Figure 3.17a), a repeating chain of glucose molecules joined by f-1,4 linkages, and may also include pectin and hemicellulose, both also polymers of simple sugars. Most fungi such as yeasts and mushrooms contain chitin, a polymer of N-acetylglucosamine (Figure 3.17b: we have encountered N-acetylglucosamine before, as a component of peptidoglycan in bacterial walls.) Chitin is also to be found as the major component of insect and crustacean exoskeletons, where the function is also to provide strength and rigidity. As in procaryotes, the cell wall plays little part in the exchange of materials between the cell and its environment, a role fulfilled by the plasma membrane.

Some protozoans and unicellular algae are surrounded by a flexible pellicle made of protein.

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