Ganong's Review of Medical Physiology, 23rd Edition

22 SECTION ICellular & Molecular Basis of Medical Physiology

glycogenin is one of the factors determining the amount of

glycogen synthesized. The breakdown of glycogen in 1:4α

linkage is catalyzed by phosphorylase, whereas another en-

zyme catalyzes the breakdown of glycogen in 1:6α linkage.

FACTORS DETERMINING THE

PLASMA GLUCOSE LEVEL

The plasma glucose level at any given time is determined by

the balance between the amount of glucose entering the

bloodstream and the amount leaving it. The principal deter-

minants are therefore the dietary intake; the rate of entry into

the cells of muscle, adipose tissue, and other organs; and the

glucostatic activity of the liver (Figure 1–25). Five percent of

ingested glucose is promptly converted into glycogen in the

liver, and 30–40% is converted into fat. The remainder is me-

tabolized in muscle and other tissues. During fasting, liver gly-

cogen is broken down and the liver adds glucose to the

bloodstream. With more prolonged fasting, glycogen is de-

pleted and there is increased gluconeogenesis from amino ac-

ids and glycerol in the liver. Plasma glucose declines modestly

to about 60 mg/dL during prolonged starvation in normal in-

dividuals, but symptoms of hypoglycemia do not occur be-

cause gluconeogenesis prevents any further fall.

FIGURE 1–23 Directional flow valves in energy production
reactions. In carbohydrate metabolism there are several reactions that
proceed in one direction by one mechanism and in the other direction by
a different mechanism, termed “directional-flow valves.” Five examples
of these reactions are illustrated (numbered at left). The double line in ex-
ample 5 represents the mitochondrial membrane. Pyruvate is converted
to malate in mitochondria, and the malate diffuses out of the mitochon-
dria to the cytosol, where it is converted to phosphoenolpyruvate.

Pyruvate

Pyruvate

Oxaloacetate

Malate Malate

Oxaloacetate

5. Phosphoenolpyruvate


6. Fructose 6-phosphate

Phosphoenolpyruvate

carboxykinase

Fructose 1,6-

Fructose 1,6- biphosphate

biphosphatase

Phospho-

fructokinase

3. Glucose 1-phosphate Glycogen
   Phosphorylase

Glycogen synthase

2. Glucose


3. Glucose entry into cells and glucose exit from cells

Glucose 6-phosphate

Glucose 6-phosphatase

Hexokinase

Pyruvate kinase

ADP ATP

FIGURE 1–24 Glycogen formation and breakdown. Glycogen is the main storage for glucose in the cell. It is cycled: built up from glucose
6-phosphate when energy is stored and broken down to glucose 6-phosphate when energy is required. Note the intermediate glucose
1-phosphate and enzymatic control by phosphorylase a and glycogen kinase.

CH 2 OH

O

CH 2 OH

O

O

CH 2 OH

O

O

O

CH 2 OH

O

CH 2 O

O

O

CH 2 OH

O

CH 2 OH

O

O

CH 2

O

CH 2 OH

O

CH 2 OH

O

O O

1:6α linkage

1:4α linkage

PO

O−

O−

PO

O−

O−

Glucose

1-phosphate

Glucose

6-phosphate

Uridine

diphospho-

glucose

Glycogen

Phosphorylase a

Glycogen

synthase