The Endocrine System
Catalyze the oxidation of glucose for ATP production
Join glucose molecules together to form glycogen
Convert glucose to fat (particularly in adipose tissue)
As a rule, energy needs are met ﬁrst, followed by glycogen for-
mation. Finally, if excess glucose is still available, it is converted
to fat. Insulin also stimulates amino acid uptake and protein
synthesis in muscle tissue. In summary, insulin “sweeps” glu-
cose out of the blood, causing it to be used for energy or con-
verted to other forms (glycogen or fats); insulin also promotes
protein synthesis and fat storage.
Factors That Inﬂuence Insulin Release
Elevated blood glu-
cose levels are the chief factor that stimulates pancreatic beta
cells to secrete insulin. Other stimuli include rising blood levels
of amino acids and fatty acids, and release of acetylcholine by
parasympathetic nerve ﬁbers. As body cells take up glucose and
Enhances membrane transport of glucose (and other simple
sugars) into most body cells, especially muscle and fat cells.
Inhibits the breakdown of glycogen to glucose.
Inhibits the conversion of amino acids or fats to glucose.
±ese inhibiting eﬀects counter any metabolic activity that
would increase plasma levels of glucose.
needed for glucose entry into liver, kidney, and
brain tissue, all of which have easy access to blood glucose re-
gardless of insulin levels. However, insulin does have impor-
tant roles in the brain—it participates in neuronal development,
feeding behavior, and learning and memory.
Insulin activates its receptor (a tyrosine kinase enzyme), which
phosphorylates speciﬁc proteins, beginning the cascade that pro-
motes glucose uptake and insulin’s other eﬀects. A²er glucose en-
ters a target cell, insulin binding triggers enzymatic activities that
Normal blood glucose level (about 90 mg/100 ml)
uptake by cells
Insulin and glucagon from the pancreas regulate blood glucose levels.