How Is Glucose Taken Into the Cells?
-
Passive Transporters
-
Most animal cells take up glucose from the extracellular fluid by passive transport through uniporter glucose carrier proteins. When glucose is ingested or created in the body, it concentrates in the extracellular fluid outside of these cells. Inside the cell, the concentration of glucose is relatively low. This concentration imbalance allows glucose carriers to passively transport glucose across the cell membrane. There are many types of these glucose carriers, but all belong to the same family of proteins with similar characteristics. All of them are uniporters, meaning they transport a single type of molecule from one side of the membrane to the other at a characteristic speed for that particular type of carrier.
Active Transporters
-
In intestinal and kidney cells, the extracellular glucose concentrations are low and passive transporters would not function. In these cells, the glucose carriers are coupled transporters. These carriers operate by transporting another solute, the sodium ion, in the same direction (symport) across the membrane. Since the concentration of sodium is higher on the outside of the cell than the inside, the concentration difference allows sodium to pass easily to the inside of the cell (moving down its electrochemical gradient), and the glucose carriers exploit sodium's electrochemical gradient to carry glucose along with it.
-
Carrier Functionality
-
The exact details by which any carrier protein moves a solute across the membrane is unknown. However, they are thought to expose a binding site first to the side of the membrane where the desired solute--in this case, glucose--is. When glucose attaches to the binding site--and in the case of coupled transporters, when the sodium also attaches--the carrier reversibly changes its configuration, opening itself to the other side of the membrane and exposing the glucose to that side. The glucose molecule then detaches from the carrier and floats freely in the cytoplasm until taken into another chemical process, such as glycolysis.
-
