Chemistry Reference
In-Depth Information
regular supply of carbohydrates. Herbivorous animals get carbohydrates
directly, when they eat and digest plants; carnivorous animals acquire car-
bohydrates indirectly, from the meat of other animals, and omnivorous
animals, including humans, acquire them from both plants and animals.
Although animals are unable to produce carbohydrates, most animals can
combine molecules of monosaccharides into larger polysaccharide mole-
cules, such as those of
glycogen
, which serves as a reserve store of energy
in their bodies.
FIGURE 66
Cellulose.
Cellulose, a strong yet flexible material, is the major
component of all plant tissues, constituting about half of a plant's dry weight.
The cell walls of wood, for example, are made up of a mesh of cellulose fibers
surrounded by and interlinked with supporting materials, namely, hemicellulose
and lignin. The molecules of cellulose are long, unbranched polymers made up
of many molecules of glucose (a carbohydrate) linked together. The figure illus-
trates just four of as many as 1500 glucose units that may make up a cellulose
polymer. Because of the length of its molecules, cellulose makes excellent natural
fibers as, for example, those of cotton and linen.
Plants synthesize two different types of carbohydrates, generally known
as
simple
and
complex carbohydrates
. All the simple carbohydrates, which are
also known by the generic name sugars, are sweet-tasting crystalline solids,
soluble in water; their molecules basically consist of either one or two
combined molecules of carbohydrates: those of the
monosaccharides
- of
which glucose (as in ripe grapes and in honey) and fructose (fruit sugar)
are examples - include only a single carbohydrate molecule. The molecules
of the
disaccharides
include two monosaccharide molecules joined together;
the molecules of sucrose (table sugar), for example, a disaccharide synthe-
sized by sugarcane, sugarbeet, and many ripe fruits, are made up of one mol-
ecule of glucose combined with one of fructose. Plants also join together
simple carbohydrate molecules into long linear or branching polymeric
