Metabolic effects of Insulin
Insulin is required by almost all of the body's cells but its major targets are liver cells, fat cells and muscle cells. For these cells, insulin does the following:

  • Stimulates liver and muscle cells to store glucose in glycogen
  • Stimulates fat cells to form fats from fatty acids and glycerol
  • Stimulates liver and muscle cells to make proteins from amino acids
  • Inhibits the liver and kidney cells from making glucose from intermediate compounds of metabolic pathways (gluconeogenesis) As such, insulin stores nutrients right after a meal by reducing the concentrations of glucose, fatty acids and amino acids in the bloodstream.
controlling blood-glucose levels

Insulin and glucagon have opposite effects on liver and other tissues for controlling blood-glucose levels
So, what happens when you do not eat? In times of fasting, or when there has been a long time after a meal and the blood levels of glucose drop significantly, your pancreas releases glucagon so that your body can produce glucose. Glucagon is another protein hormone that is made and secreted by the alpha cells of the pancreatic islets.

Metabolic Effects of Glucagon
Glucagon acts on the same cells as insulin, but has the opposite effects:

  • Stimulates the liver and muscles to break down stored glycogen (glycogenolysis) and release the glucose
  • Stimulates gluconeogenesis in the liver and kidneys

In contrast to insulin, glucagon mobilizes glucose from stores inside your body and increases the concentrations of glucose in the bloodstream
-- otherwise, your blood glucose would fall to dangerously low levels.

So how does your body know when to secrete glucagon or insulin?
Normally, the levels of insulin and glucagon are counter-balanced in the bloodstream. For example, just after you eat a meal, your body is ready to receive the glucose, fatty acids and amino acids absorbed from the food. The presence of these substances in the intestine stimulates the pancreatic beta cells to release insulin into the blood and inhibit the pancreatic alpha cells from secreting glucagon. The levels of insulin in the blood begin to rise and act on cells (particularly liver, fat and muscle) to absorb the incoming molecules of glucose, fatty acids and amino acids. This action of insulin prevents the blood-glucose concentration (as well as the concentrations of fatty acids and amino acids) from substantially increasing in the bloodstream. In this way, your body maintains a steady blood-glucose concentration in particular.

Thus, one can say that insulin is the key that opens the door to let glucose into the cells.