Question

Contrast the effects of glucagon and insulin. Which hormone is released during the fasted state? Which hormone will cause glycogen to break down into glucose?

Is insulin a lipid or a protein. Would you expect to find their receptors inside the cell or on the cell surface?

How does activating insulin receptors, in fat or muscle cells, alter glucose levels in the blood? Make sure you explain how it alters glucose transport.

Explain how insulin facilitates the entry of glucose into liver cells.

Answer 1

Both Insulin and glucagon are hormone released by pancreas.

.

Insulin Insulin is secreted by beta cells It is a protein made of 2 peptide chains- A and B chains. It has cysteine residues in structure. It binds with glycoproteins called insulin receptors of the target cell membrane. It is secreted when food reaches for digestion and when there is a high blood glucose level. Insulin acts to reduce blood glucose level.

Glucagon Glucagon is secreted by alpha cells of pancreatic islets. It is a monomeric protein devoid of cysteine in structure. It binds to specific receptors on the plasma membranes of hepatocytes and adipocytes which activates adenylate cyclase for the formation of cAMP . cAMP is the second messenger for glucagon. It is secreted when there is a short supply of fuel substances like glucose and fatty acids. It prevents hypoglycemia and maintain fuel supply to the tissues by increasing blood sugar level and blood fatty acid level.

In fasting stage, glucagon is secreted for increasing the blood glucose level by the depletion of glycogen (glycogenolysis) from hepatic and adipose tissues. Glucagon acts on glycogen to convert it to glucose.

Insulin is a protein and the receptors are found on the cell membrane of the target cell. The receptor-insulin complex is aggregated to form patches on the membrane and then internalized into the cell by adsorptive pinocytosis.

Insulin increases the glucose uptake from the extracellular fluid by the muscles, adipocytes. It stimulates the translocation of glucose transporters from the intracellular pool to the plasma mambrane where they participate in the facilitated diffusion of D-glucose across the membrane, Insulin thus increases the number of glucose transporters in the membrane and increses the velocity of facilitated diffusion of glucose into these cells.

But in hepatocytes, insulin increases the hepatic uptake of glucose by inducing the synthesis of glucokinase, the enzyme involved in glycolysis that phosphorylates glucose into glucose-6-phosphate. Thus insulin lowers the intracellular concentration of glucose and enhances its simple diffusion down the inward concentration gradient.