Tutoring Biology 12, you talk about the hormones insulin and glucagon, which are used to control blood sugar.

Insulin is much more widely spoken of than glucagon, but in fact they are opposite sides of the blood sugar equation.  Your body uses insulin to lower blood glucose (blood sugar), and glucagon to raise it.  The appropriate blood glucose level is 1 mg/ml, or 0.1%.

When blood sugar rises above normal (just after a meal, for instance), the pancreas releases insulin.  Insulin causes liver, muscle, and fat cells to take in glucose from the blood.  The result:  blood sugar is lowered.  The liver cells convert the glucose to glycogen.  The muscle cells do the same, or else use the glucose to power their efforts if they are working.  The fat cells use the glucose to form fat.  No matter which type of cell takes up the glucose, the glucose gets consumed.

When blood sugar drops below normal (such as between meals), the pancreas releases glucagon.  Glucagon causes the liver to break down glycogen into glucose.  The liver releases that glucose into the bloodstream, raising blood sugar.  Glucagon also causes fat cells to break down fat into glycerol and fatty acids, which then get released in the blood.  The liver takes up the glycerol and fatty acids and converts them into glucose.  The resulting glucose can be released into the blood to increase blood sugar.  Furthermore, glucagon causes the liver to burn fat and protein for its own energy needs, rather than burning glucose.  Hence, more glucose is left in the blood.

Insulin and glucagon are both produced and secreted by sites in the pancreas called the islets of Langerhans.

Source:  Inquiry into Life, 11th Edition, by Sylvia S. Mader.  McGraw-Hill:  2006.