Concept 4 Review
Facilitated Diffusion and Active Transport of Glucose
Whether a cell uses facilitated diffusion or active transport depends on the specific needs of the cell. For example, the sugar glucose is transported by active transport from the gut into intestinal epithelial cells, but by facilitated diffusion across the membrane of red blood cells. Why? Consider how different these two environments are.
Epithelial cells lining the gut need to bring glucose made available from digestion into the body and must prevent the reverse flow of glucose from body to gut. We need a mechanism to ensure that glucose always flows into intestinal cells and gets transported into the bloodstream, no matter what the gut concentration of glucose. Imagine what would happen if this were not so, and intestinal cells used facilitated diffusion carriers for glucose. Immediately after you ate a candy bar or other food rich in sugar, the concentration of glucose in the gut would be high, and glucose would flow "downhill" from the gut into your body. But an hour later, when your intestines were empty and glucose concentrations in the intestines were lower than in your blood and tissues, facilitated diffusion carriers would allow the glucose in blood and tissues to flow "downhill," back into the gut. This would quickly deplete your short-term energy reserves. Because this situation would be biologically wasteful and probably lethal, it is worth the additional energy cost of active transport to make sure that glucose transport is a one-way process.
By contrast, erythrocytes (red blood cells) and most other tissues in your body move glucose by facilitated diffusion carriers, not by active transport. Facilitated diffusion makes sense in this context because the environment is different for red blood cells and the gut. Whereas the gut experiences a constantly fluctuating concentration of glucose that can be either higher or lower than the glucose concentration inside gut cells, glucose concentration in the blood is carefully regulated so that it is normally higher than intracellular concentrations. Glucose is transported across erythrocyte membranes by a uniporter, a type of facilitated diffusion protein. As soon as glucose enters the cell, it is converted into other chemicals needed for by the cell for energy production or biosynthesis, so the intracellular concentration of glucose remains much lower than the 5 mM glucose level normally maintained in blood. In this situation, diffusion alone can ensure a constant flow of glucose into the erythrocyte, so it would be wasteful and unnecessary for erythrocytes to use active transport for glucose.