GLUT4 Transporters
Glucose is too large to travel on its own from the blood to the insides of cells. This means that cells need to use special proteins that transport glucose from the blood to the inside of the cell. Some tissues, such as the liver and the brain, are constantly taking in glucose and so do not require insulin to transport glucose into the cell. As the Colorado State University Library of Hyper Texts explains, muscle and adipose (fat) cells use a special transporter protein called GLUT4. This protein, however, is normally kept on the inside of the cell. In order for it to get transported to the cell membrane (which allows the cells to take in glucose from the blood), muscle and fat cells need insulin.
Insulin Mechanism
Insulin is a hormone which is made by cells in the pancreas when glucose levels in the blood rise. Insulin's effects begin when it binds to its receptor, which is a special protein on the outside of insulin-sensitive cells that detects the presence of insulin. As the Insulin Signaling Pathway chart from Kegg Pathways shows, once insulin binds to its receptor, a number of different chemical signals are generated. Two different proteins, called Akt and Protein Kinase C are activated, which causes them to generate signals that send the GLUT4 transporters to the cell membrane. Once the GLUT4 transporters have been sent to the membrane, they can shuttle glucose into the cell.
Liver
As mentioned previously, liver cells are constantly taking in glucose from the blood. By the same token, however, liver cells are also able to pump glucose out of the cells into the blood. Insulin works to change the net movement of glucose in liver cells by activating an enzyme called hexokinase. Hexokinase chemically changes glucose so that it cannot be pumped out of the liver cells. Insulin also activates other enzymes which are needed to turn glucose into glycogen, which is a way for the liver to store extra glucose for times when blood glucose levels are lower.
