The role of insulin and the beta cells of the pancreas

• Insulin is a globular protein made up of 51 amino acids, and it is released as a hormone by the beta cells of the islets of Langerhans in the pancreas. This response occurs when the blood glucose concentration is too high, as detected by the receptors on these beta cells.
• Most body cells have glycoprotein receptors on their cell-surface membranes which enable the binding of insulin molecules. However, red blood cells are an exception to this trend.
• One of the major changes that insulin brings about when it binds with receptors is a change in the tertiary structure of the glucose transport carrier proteins. They change shape and open, allowing more glucose into the cells by facilitated diffusion.
• One of the major changes that insulin brings about when it binds with receptors is an increase in the number of carrier proteins responsible for glucose transport in the cell-surface membrane. At low insulin concentrations, the protein from which these channels are made is part of the membrane of vesicles. A rise in insulin concentrations results in these vesicles fusing with the cell-surface membrane so increasing the number of glucose transport channels.
• One of the major changes that insulin brings about when it binds to receptors is the activation of the enzymes that convert glucose into glycogen and fat.
• As a result of the effects insulin has, the blood glucose concentration is lowered in several different ways.
• One way in which blood glucose concentration is lowered due to insulin is by increasing the rate of absorption of glucose into body cells, especially muscle cells.
• One way in which blood glucose concentration is lowered due to insulin is by increasing the respiratory rate of the cells, which therefore use up more glucose, thus increasing their uptake of glucose from the blood.
• One way in which blood glucose concentration is lowered due to insulin is by increasing the rate of conversion of glucose into glycogen (glycogenesis) in the liver and muscle cells.
• One way in which blood glucose concentration is lowered due to insulin is by increasing the rate of conversion of glucose to fat.
• The effect of the processes triggered by insulin is to remove glucose from the blood and so return its concentration to the optimum. This lowering of the blood glucose concentration causes the beta cells to reduce their secretion of insulin, creating a negative feedback loop.