text: The functioning of a signal transduction pathway is based on extra-cellular signaling that in turn creates a response that causes other subsequent responses, hence creating a chain reaction, or cascade. During the course of signaling, the cell uses each response for accomplishing some kind of a purpose along the way. Insulin secretion mechanism is a common example of signal transduction pathway mechanism.
  Insulin is produced by the pancreas in a region called Islets of Langerhans. In the islets of Langerhans, there are beta-cells, which are responsible for production and storage of insulin. Insulin is secreted as a response mechanism for counteracting the increasing excess amounts of glucose in the blood.
  Glucose in the body increases after food consumption. This is primarily due to carbohydrate intake, but to a much lesser degree protein intake ([http://jcem.endojournals.org/content/89/6/3048.long])([http://genesdev.cshlp.org/content/24/24/2717.long]). Depending on the tissue type, the glucose enters the cell through facilitated diffusion or active transport. In muscle and adipose tissue, glucose enters through GLUT 4 receptors via facilitated diffusion ([http://ajpendo.physiology.org/content/299/2/E169.long]). In brain, retina, kidney, RBC, placenta and many other organs, glucose enters using GLUT 1 and GLUT 3. In the beta-cells of the pancreas and in liver cells, glucose enters through the GLUT 2 receptors 
   (process described below).