Glucose entry into a cultured cell rises as extracellular glucose climbs, then plateaus even though the concentration gradient keeps increasing. What does the plateau reveal about glucose transport?

Core Concept

Mediated transport depends on carriers that bind glucose, shuttle it across the membrane, and reset for the next molecule. When extracellular glucose is abundant, every carrier cycles at its maximum rate, and a steeper gradient cannot add throughput, so the curve flattens. Simple diffusion would keep rising with concentration rather than saturating, which is why the plateau marks a protein-limited pathway.