148
chapter 9
Simple Carbohydrates
F I G U R E 9 -2 2
Linkages of glucose residues in glycogen. Glycogen is structurally similar to amylopectin but more highly branched.
residue (i.e., unsubstituted Q hydroxyl group), which is
the terminal unit on one of the chains. At each of the other
termini, the glucose residue has a free hydroxyl group at
C
4
, while the Ci hydroxyl group participates in the glyco-
sidic linkage. Synthesis and breakdown take place at these
termini.
A glycogen molecule contains about 10
5
glucose units.
It has no discrete molecular weight, since its size varies
NE
F I G U R E 9 -2 3
A diagrammatic representation of a portion of a glycogen molecule. The
glucose residues are shown by circles;
0 _ 0
indicates a(l —>■
4) linkages,
and
O —>■
O
indicates
a (
1
—►
6
) linkages that are branch points. The ratio
of 1,4 to 1,6 linkages varies between 12:1 and 18:1. Note that each
molecule of glycogen has many nonreducing (NE) termini; further addition
or cleavage of glucose units occurs at the nonreducing ends. Numbers 1
and 4 refer to Ci (reducing end) and C
4
(nonreducing end), respectively.
considerably depending on the tissue of origin and its
physiological state. In the human body, most glycogen is
found in liver and muscle. The functional roles of glycogen
in these two tissues are quite different: in muscle, glyco-
gen serves as an energy reserve mostly for contraction,
whereas liver glycogen supplies glucose to other tissues
via the blood circulatory system.
Cellulose, the most abundant carbohydrate on earth, is
an unbranched polymer with glucosyl residues joined to-
gether in/1(1 —> 4) linkages (Figure 9-24). Cellulose from
different sources varies in molecular weight, and the num-
ber of glucose units lies in the range of 2,500-14,000.
Cellulose is the structural polysaccharide in plants. Cellu-
lose microfibrils are tightly packed aggregates of cellulose
molecules, which are chemically inert and insoluble and
possess significant mechanical strength.
Because of its /3 linkages, the preferred conformation
of cellulose is one in which the ring oxygen of one residue
forms a hydrogen bond with the C
3
hydroxyl group of
the next residue (Figure 9-24). The 40 or more indi-
vidual cellulose molecules that aggregate to form mi-
crofibrils are held together by intermolecular hydrogen
bonds.
In many fungal cell walls and invertebrates (shells of
crustaceans and exoskeletons of insects), the main struc-
tural polysaccharide is
chitin,
which is a polymer of
N-acetylglucosamine linked in /1(1 —>• 4) glycosidic link-
age. In humans, cellulose is not digested in the small in-
testine but is digested in the large intestine to varying de-
grees by the microflora to yield short-chain fatty acids,
