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
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.