glycosidic bonds. The degree of branching
varies from nearly linear to highly
branched. The branches usually are
polymers of arabinose or uronic acid, but
both types of branches will not be present
on the same molecule of hemicellulose.
The interactions of hemicellulose and
lignin are numerous and varied. Exposure
of hemicellulose to gastric digestion
improves its digestibility by enteric
microorganisms.
Pectin is a water-soluble polymer com-
posed primarily of galacturonic acid linked
by 1–4 glycosidic bonds. Pectin serves as
a ‘cement’ that holds the various com-
ponents of the cell wall together. The
carboxyl moieties of galacturonic acid are
cross-linked by Ca2+. Phosphates in the
lumen of the alimentary canal chelate the
Ca2+ and solubilize the pectin. Enteric
microorganisms rapidly ferment pectin, but
the 1–4 bonds are not a substrate for
amylases. Galactose, being a C4-epimer of
glucose, makes the C4-hydroxy of pectin
axial rather than equatorial as in glucose.
Thus the 1–4 galactosidic bond is similar
in character to a 1–4 glucosidic bond.
Pectin, however, is not a flat molecule like
cellulose but tends to coil like amylose.
Interspersed in the galacturonic chains are
rhamnose moieties inserted with 1–4 and
2–4 rhamnosyl galacturonate linkages. The
vicinal linkages with rhamnose cause
sharp bends in the pectin polymer.
Various gums are found in the cell
wall of the seeds of many plants. The gums
are polymers of various sugars linked 1–4
with 1–3 branch points. The branching
prevents these molecules from packing
together, rendering them as open structures
that are soluble in water or form viscous
gels in water. The gums are indigestible by
mammalian enzymes but are fermented
readily by enteric microorganisms.
Another important component of the
cell wall is lignin. Lignin is not a carbo-
hydrate and is, for the most part, indigest-
ible, but lignin does affect the digestion of
cell wall carbohydrates. Lignin is an
extremely variable structure made primarily
of polymers of phenylprope(a)noid
monomers. Phenylpropenoid compounds
are strong electron sinks that polymerize
by free-radical condensation. Lignin
influences digestion by encrustation of
potentially digestible cell wall carbo-
hydrates. Lignins can also complex with
digestive enzymes or glycoprotein sub-
strate to inhibit digestion.
Chitin is the second most abundant
organic compound on earth. Chitin forms
the cell walls of lower plants and the
exoskeleton of arthropods. Chitin is a
linear polymer of N-acetyl-glucosamine
linked by 1–4 glycosidic bonds.
Chitinase, the hydrolytic enzyme for
chitin, is found in a wide variety of
animals as well as being produced by
enteric microflora (Stevens and Hume,
1995).Procurement of Carbon
Direct absorbersDirect digestion and absorption of dietary
carbohydrate is the most efficient method
of obtaining carbohydrate if large quanti-
ties of substrate are available in the diet
(Table 6.2). Swine and poultry are the
major groups of farm animals that rely pre-
dominantly, but not solely, on the direct
digestion and absorption of dietary carbo-
hydrates. Rabbits and grain-fed fish would
also be included in this category. This
group contains wide diversity in the struc-
ture of the alimentary canal.Swine
We will begin with a discussion of swine
because the structure of the alimentary
canal in swine most closely resembles that
of humans. The natural diet of swine
consists primarily of starch foods such as
roots, tubers and nuts that are harvested by
rooting with their rigid snout (Ensminger,
1991) and chewed thoroughly prior to
swallowing. Grains are the source of starch
for the Western swine industry, although
swine are still used as a part of the ‘garbage
disposal’ system in China and developing
countries. While being chewed, ingesta is
insalivated with a mixture of mucus and124 R.W. Russell and S.A. Gahr