Page 336 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch12_299-322.indd Page 313 9/3/10 6:13 PM user-f465
tiL12214_ch12_299-322.indd Page 313 9/3/10 6:13 PM user-f465 /Users/user-f465/Desktop

CH 2 OH CH 2 OH
A starch
H H H H
C O C O
OH H OH H
C C C C
O C C O C C O
H OH H OH

CH 2 OH
Cellulose
H
C O
H OH OH H
C C
H
C C O C C H O
C OH H C
H H OH
O C O
H
CH OH
2
FIGURE 12.22 Starch and cellulose are both polymers of
glucose, but humans cannot digest cellulose. The difference in
the bonding arrangement might seem minor, but enzymes must
fit a molecule very precisely. Thus, enzymes that break down
starch do nothing to cellulose.

chains, consisting of large numbers of glucose units. These
glucose units are arranged in a way that is very similar to the
arrangement of the glucose units of starch but with differences
in the bonding arrangement that holds the glucose units together
(Figure 12.22). This difference turns out to be an important one
where humans are concerned because enzymes that break down
starches do not affect cellulose. Humans do not have the neces-
sary enzymes to break down the cellulose chain (digest it), so
humans receive no food value from cellulose. Cattle and ter-
mites that do utilize cellulose as a source of food have protozoa
and bacteria (with the necessary enzymes) in their digestive sys-
tems. Cellulose is still needed in the human diet, however, for
fiber and bulk.
FIGURE 12.21 These plants and their flowers are made up
of a mixture of carbohydrates that were manufactured from carbon
dioxide and water, with the energy of sunlight. The simplest of the
carbohydrates are the monosaccharides, simple sugars (fruit sugar) Myths, Mistakes, & Misunderstandings
that the plant synthesizes. Food is stored as starches, which are
polysaccharides made from the simpler monosaccharide glucose. Fruit Juice Versus Cola
The plant structure is held upright by fibers of cellulose, another
form of a polysaccharide composed of glucose. Misconception: Fruit juices have fewer calories from sugar
(monosaccharides) than nondiet soft drinks.
In fact, apple juice has more sugar than the same amount
of cola. Did you also know that it is estimated that people
Plants store sugars in the form of starch polysaccharides,
in the United States consume more calories in the form of
and animals store sugars in the form of the polysaccharide
simple sugar than of meat, chicken, vegetables, and breads
glycogen. Glycogen is a starchlike polysaccharide that is synthe-
combined?
sized by the human body and stored in the muscles and liver.
Glycogen, like starch, is a very high molecular weight poly-
saccharide, but it is more highly branched. These highly
branched polysaccharides serve as a direct reserve source of
energy in the muscles. In the liver, they serve as a reserve source FATS AND OILS
to maintain the blood sugar level. Cereal grains and other plants also provide carbohydrates,
Cellulose is a polysaccharide that is abundant in plants, the human body’s preferred food for energy. When an excess
forming the fibers in cell walls that preserve the structure of amount of carbohydrates is consumed, the body begins to
plant materials (Figure 12.21). Cellulose molecules are straight store some of its energy source in the form of glycogen in the

12-15 CHAPTER 12 Organic Chemistry 313