Human Physiology, 14th edition (2016)

118 Chapter 5

significance of this process in conditions of fasting will be

discussed together with amino acid metabolism (section 5.4).

During exercise, some of the lactic acid produced by skel-

etal muscles may be transformed through gluconeogenesis in

the liver to blood glucose. This new glucose can serve as an

energy source during exercise and can be used after exercise

to help replenish the depleted muscle glycogen. This two-way

traffic between skeletal muscles and the liver is called the Cori

cycle ( fig. 5.11 ). Through the Cori cycle, gluconeogenesis

organs, the liver contains the enzymes needed to take pyruvic
acid molecules and convert them to glucose 6-phosphate, a pro-
cess that is essentially the reverse of glycolysis.
Glucose 6-phosphate in liver cells can then be used as an
intermediate for glycogen synthesis, or it can be converted to
free glucose that is secreted into the blood. The conversion
of noncarbohydrate molecules (not just lactic acid, but also
amino acids and glycerol) through pyruvic acid to glucose is
an extremely important process called gluconeogenesis. The

Figure 5.10 Glycogenesis and glycogenolysis. Blood glucose entering tissue cells is phosphorylated to glucose
6-phosphate. This intermediate can be metabolized for energy in glycolysis, or it can be converted to glycogen ( 1 ) in a process called
glycogenesis. Glycogen represents a storage form of carbohydrates that can be used as a source for new glucose 6-phosphate ( 2 ) in a
process called glycogenolysis. The liver contains an enzyme that can remove the phosphate from glucose 6-phosphate; liver glycogen
thus serves as a source for new blood glucose.

1 2

Glucose Glucose 6-phosphate

(blood)

Glucose

Liver (blood)

only

Many

tissues

Fructose 6-phosphate

GLYCOLYSIS

Glucose 1-phosphate

GLYCOGEN

Pi Pi

Pi ADP ATP

Figure 5.11 The Cori cycle. During exercise, muscle glycogen serves as a source of glucose 6-phosphate for the lactic
acid pathway (steps 1 through 3). This lactic acid is carried by the blood (step 4) to the liver, where it is converted back to glucose
6-phosphate (steps 5 and 6). This is next converted into free glucose (step 7), which can be carried by the blood (step 8) back to the
skeletal muscles. During rest, this glucose can be used to restore muscle glycogen (step 9).

Blood

Blood

Glucose

Skeletal muscles Liver

Glycogen

1

2

3

9

8

4

7

6

5

Rest

Glucose 6-phosphate

Pyruvic acid

Lactic acid

Exercise

Glycogen

Glucose 6-phosphate

Pyruvic acid

Lactic acid