21. Lipids and Carbohydrates   Previous PageNext Page
       Polysaccharides: Cellulose and Starch

Cellulose is the most plentiful organic compound on our planet. The purest natural form of cellulose is cotton, which is 90% cellulose. The woody parts of trees are cellulose, as are the supporting materials in plant stalks and leaves. All algae, except blue-green algae, have cell walls of cellulose. (The name cellulose means "cell sugar.") Although bacteria and animals generally do not depend on cellulose for structural support, two kinds of bacteria and a few marine invertebrates have a cellulose like polymer as an outer protection.

Cellulose is a simple, straight-chain, b-1,4 polymer of glucose, with 300 to 3000 glucose units per molecule, and a molecular weight of 50,000 to 500,000. The structure of one strand of cellulose is shown on the right. These strands then are bundled into fibrils and cross-linked by hydrogen bonds for strength. The fibrils are twisted into bundles, and the bundles into strong fibers that can be used for support.

As rope makers realize, fragile threads can gain great strength if they are twisted into strands, the strands are twisted separately into ropes, ropes into cables, and cables into hawsers. Nature anticipated the human rope maker by several hundred million years.


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