Glucose Transport
Cells of all kinds are surrounded by a membrane that is made up of lipids and proteins. These membranes are designed to keep large molecules from moving in and out of the cell in an unregulated manner. Because glucose is a large molecule, it cannot pass through the cell membrane without help. Cells import glucose using special proteins called glucose transporters that bind to glucose and then ship it into the cell. Some tissues (such as the liver and brain tissues) have a constant supply of these transporters at their surface. Other tissues, such as fat and muscle tissue, can vary their glucose transport.
Insulin Mechanism
As the Hypertext Library at Colorado State explains, one compound that can increase glucose transport is insulin. Insulin is a hormone that is made by the pancreas in response to elevated glucose levels in the blood. Insulin exerts its effects on muscle and fat tissue by increasing the amount of a glucose transporter called GLUT4. When insulin is not present, the GLUT4 proteins are found inside the cell (as opposed to the cell membrane) where they cannot transport glucose. Insulin binds to special recognition proteins (called receptors) on the outside of muscle and fat cells. The binding of insulin to its receptors causes a chemical signal inside of the cell that causes it to shuttle the GLUT4 proteins to the membrane. The more GLUT4 proteins there are, the more glucose can be transported.
Metformin
Some diabetes medications (such as metformin) work to increase glucose transport using a pathway that is independent of insulin. According to an article in the Journal of Endocrinology, metformin works by stimulating muscle cells to move a different glucose transporter, called GLUT1, to the cell membrane. Again, increased glucose transportation proteins leads to increased glucose transport.
