Carbohydrate metabolism begins with digestion in the small
intestine where monosaccharides are absorbed into the blood stream.
Blood sugar concentrations are controlled by three hormones:
insulin, glucagon, and epinephrine. If the concentration of glucose
in the blood is too high, insulin is secreted by the pancreas.
Insulin stimulates the transfer of glucose into the cells, especially
in the liver and muscles, although other organs are also able
to metabolize glucose.
In the liver and muscles, most of the glucose is changed into
glycogen by the process of glycogenesis (anabolism). Glycogen
is stored in the liver and muscles until needed at some later
time when glucose levels are low. If blood glucose levels are
low, then eqinephrine and glucogon hormones are secreted to stimulate
the conversion of glycogen to glucose. This process is called
If glucose is needed immediately upon entering the cells to
supply energy, it begins the metabolic process called glycoysis
(catabolism). The end products of glycolysis are pyruvic acid
Since glycolysis releases relatively little ATP, further reactions
continue to convert pyruvic acid to acetyl CoA and then citric
acid in the citric acid cycle. The majority of the ATP is made
from oxidations in the citric acid cycle in connection with the
electron transport chain.
During strenuous muscular activity, pyruvic acid is converted
into lactic acid rather thatn acetyl CoA. Durlng the resting
period, the lactic acid is converted back to pyruvic acid. The
pyruvic acid in turn is converted back to glucose by the process
called gluconeogenesis (anabolism). If the glucose is
not needed at that moment, it is converted into glycogen by glycogenesis.
You can remember those terms if you think of "genesis"
as the formation-beginning.
These processes are summarized in the Metaboism Summary in
the graphic on the left. Each of these processes will be developed
in greater detail various pages of this module.