Among other common disaccharides, lactose from milk is a dimer
of galactose and glucose. Maltose comes from partially digested
starch, and cellobiose from partially degraded cellulose).
These two disaccharides illustrate the key difference between
starch and cellulose, which helps the plant to keep them separated.
Both maltose and cellobiose are dimers Of D-glucose, and both are
formed by condensing the monomers at their C1 and C4 positions and
removing H20. The difference is that maltose is connected
through the a-position of the C1 carbon,
and cellobiose through the b-position.
Maltose therefore is an a-1,4 dimer,
and cellobiose a b-1,4
This may seem like a small difference, but these glucose monomers
are put together by enzymes tailored for a precise geometry. An
enzyme designed to make or break an a-1,4
bond cannot affect a b-1,4 bond, and
vice versa. They are like two locks, each with its own key.
All of the 1,4 bonds in starch are a-,
and all such bonds in cellulose are b-.
To the metabolic machinery in plants keyed to starch and b-1,4
bonds, cellulose is just as alien and unusable a substance as polyethylene.