Hemicellulose: Polysaccharides with a random, amorphous structure that
are components of plant cell walls. Unrelated to cellulose structurally,
they are composed of a variety of monosaccharides, including some
acidic sugars, with xylose being the most prevalent.
Insoluble fibers: Components of plant cell walls that are insoluble in water
and not broken down by the body’s digestive enzymes.
Lignins: Aromatic polymers formed by the irreversible dehydration of sug-
ars. Because of their structure, they cannot be broken down by the diges-
tive enzymes and make up part of the stool bulk.
Mucilaginous: Having a characteristic that is like the viscous and sticky
nature of glue.
Pectins: One of the soluble fibers in the diet composed primarily of poly-
mers of galacturonic acid with varying amounts of other hexose and pen-
Soluble fibers: Mucilaginous fibers such as pectin and true plant gums that
are soluble in water and digestible by the enzymes of the intestinal tract.
By absorbing water and forming viscous gels, they decrease the rate of
D ISC U SSIO N
Simply stated, dietary fiber is that part of food that remains intact and not
absorbed following the digestive process in humans. It consists of all of the
components of the cell walls of plants that are not broken down by the body’s
digestive enzymes. Dietary fiber can be grouped into two main categories,
those that are soluble and those that are insoluble in water. The soluble fibers
include pectins, gums, some hemicelluloses, and storage polysaccharides
(starch and glycogen). The insoluble fibers include cellulose, most hemicellu-
loses, and lignins.
Cellulose is a major structural component of plant cell walls. Cellulose is a
long, linear polymer of glucose (P-
-glucopyranose) units that are joined by
P(1—>4) glycosidic bonds (Figure 25-1a). Cellulose molecules have an
p i-> 4
Figure 25-1a. The molecular structure of cellulose, indicating the repeating
disaccharide unit, cellobiose.