Carb 6

• Glucose entry into most cells is driven by concentration and is independent of sodium, unlike absorption from the digestive tract.
• Normal glucose concentration in peripheral blood is 5.6 Mm (normal range: 4–6 mM).
• There are four glucose transporters, called GLUT 1 through GLUT 4.
• GLUT 2 and GLUT 4 are the most significant of these because they are located only in specific cells and are highly regulated.
• GLUT 2 is a low-affinity transporter in hepatocytes and pancreatic cells.
• After a meal, blood traveling through the hepatic portal vein from the intestine is rich in glucose.
• GLUT 2 captures the excess glucose primarily for storage.
• When the glucose concentration drops below the K for the transporter, much of the remainder leaves the liver and enters the peripheral circulation.
• The K of GLUT 2 is quite high (~15 mM), meaning that the liver will pick up glucose in proportion to its concentration in the blood (first-order kinetics).
• In other words, the liver will pick up excess glucose and store it only after a meal, when blood glucose levels are high.
• In the β-islet cells of the pancreas, GLUT 2, along with the glycolytic enzyme glucokinase, serves as the glucose sensor for insulin release.