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

Bacteria and protozoa thrive in the warm culture vat of the rumen, and the excess population spills over into the lower stomachs of the cow, where they are digested as another rich food source. A cow is an admirably efficient chemical factory. We can duplicate the digestive talents of a cow in the laboratory with acid or cellulase, but the method so far is not a practical proposition.The results cannot yet stand up in either economic or aesthetic competition with prime rib or sirloin. Cows are still cheaper than chemists.

For all except a few bacteria and protozoa and their hosts, starch is the normal source of glucose. Amylose, the simplest form of starch, is an unbranched a-1,4 polymer of 250~300 glucose units per molecule (right). The more common amylopectin has around 1000 such units in a branched chain, as shown on the right.

Branching occurs by connecting the a-position of a C1 carbon with the C6 -OH of another glucose molecule in an a-1,6 bond. This is a one-way junction that leads to the curved, branched trees shown. Amylopectin has one a-1,6 branch per twenty or so glucose units. The smaller amylose is somewhat water, soluble, but amylopectin is insoluble and hence is a safer energy-storage molecule for plants. Amylopectin as we obtain it from plant sources in, the laboratory probably already is somewhat degraded; the natural starches originally had molecular weights in the millions.


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