Chapter 24
Nutrition, Metabolism, and Body Temperature Regulation
925
24
Carbo loading is standard practice among marathon runners
and distance cyclists, because studies have shown that it im-
proves performance and endurance.
Gluconeogenesis
When too little glucose is available to stoke
the “metabolic furnace,” glycerol and amino acids are converted
to glucose.
Gluconeogenesis
, the process of forming new (
neo
)
glucose from
noncarbohydrate
molecules, occurs in the liver.
Gluconeogenesis takes place when dietary sources and glucose
reserves have been used up and blood glucose levels are begin-
ning to drop. Gluconeogenesis protects the body, especially the
nervous system, from the damaging effects of low blood sugar
(
hypoglycemia
) by ensuring that ATP synthesis can continue.
Check Your Understanding
14.
What name is given to the chemical reaction in which
glycogen is broken down to its glucose subunits?
15.
What does carbo loading accomplish?
For answers, see Appendix H.
When more glucose is available than can immediately be oxi-
dized, rising intracellular ATP concentrations eventually inhibit
glucose catabolism and cause glucose to be stored as glycogen
or fat. Because the body can store much more fat than glycogen,
fats account for 80–85% of stored energy.
Glycogenesis
When high ATP levels begin to “turn off” gly-
colysis, glucose molecules are combined in long chains to form
glycogen, the animal carbohydrate storage product. ±is pro-
cess is called
glycogenesis
(
glyco
5
sugar;
genesis
5
origin)
(
Figure 24.13
, right side).
Glycogenesis begins as glucose entering cells is phosphor-
ylated to glucose-6-phosphate and then converted to its isomer,
glucose-1-phosphate
. ±e terminal phosphate group is cleaved
off as the enzyme
glycogen synthase
catalyzes the attachment of
glucose to the growing glycogen chain. Liver and skeletal mus-
cle cells are most active in glycogen synthesis and storage.
Glycogenolysis
On the other hand, when blood glucose levels
drop, glycogen lysis (splitting) occurs. ±is process is known as
glycogenolysis
(gli
0
ko-jĕ-nol
9
ĭ-sis) (Figure 24.13, le² side). ±e
enzyme
glycogen phosphorylase
oversees phosphorylation and
cleavage of glycogen to release glucose-1-phosphate, which is
then converted to glucose-6-phosphate, a form that can enter
the glycolytic pathway to be oxidized for energy.
In muscle cells and most other cells, the glucose-6-phosphate
resulting from glycogenolysis is trapped because it cannot cross
the cell membrane. However, hepatocytes (and some kidney
and intestinal cells) contain
glucose-6-phosphatase
, an enzyme
that removes the terminal phosphate, producing free glucose.
Because glucose can then readily diffuse from the cell into the
blood, the liver can use its glycogen stores to provide blood
sugar for other organs when blood glucose levels drop. Liver
glycogen is also an important energy source for skeletal muscles
that have depleted their own glycogen reserves.
Athletes and Carbohydrates
A common misconception is
that athletes need to eat large amounts of protein to improve
their performance and maintain muscle mass. Actually, a diet
rich in complex carbohydrates results in more glycogen storage
in muscle, and is more effective than high-protein meals for
sustaining intense muscle activity.
Notice that the emphasis is on
complex
carbohydrates. Eating
a candy bar before an athletic event to provide “quick” energy
does more harm than good because it stimulates insulin secre-
tion, which favors glucose use and retards fat use at a time when
fat use should be maximal. Building muscle protein or avoiding
its loss requires not only extra protein, but also extra (protein-
sparing) complex carbohydrate calories to meet the greater en-
ergy needs of increasingly massive muscles.
Endurance athletes—long-distance runners in particular—
are well aware of the practice of glycogen loading, popularly
called “carbo loading,” for endurance events. Carbo loading
“tricks” the muscles into storing more glycogen than they nor-
mally would. It generally involves eating a carbohydrate-rich
diet (75% of energy intake) for three to four days before an en-
durance event while decreasing activity. ±is practice increases
muscle glycogen stores to as much as twice the normal amount.
ATP
P
i
P
i
Cell exterior
Hexokinase
(all tissue cells)
Cell interior
Mutase
Glycogenesis
Glycogenolysis
Mutase
ADP
Glucose-6-
phosphatase
(present in liver,
kidney, and
intestinal cells)
Glycogen
synthase
Glycogen
phosphorylase
Pyrophosphorylase
2
Blood glucose
Glucose-6-phosphate
Glucose-1-phosphate
Glycogen
Uridine diphosphate
glucose
Figure 24.13
Glycogenesis and glycogenolysis.
When glucose
supplies exceed demands, glycogenesis (conversion of glucose to
glycogen) occurs. Falling blood glucose levels stimulate glycogenoly-
sis (breakdown of glycogen to release glucose).
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