Explain why active transport, rather than diffusion, is needed for the uptake of glucose in the small intestine.

Glucose is transported from the intestinal lumen, where its concentration is low, into the enterocyte, where its concentration is higher. This movement is against the concentration gradient, so it cannot occur by simple diffusion, which only moves substances down a gradient. Instead, active transport is required. Active transport uses specific carrier proteins (e.g. SGLT1) embedded in the apical membrane of the enterocyte to bind glucose and move it across the membrane. The energy needed for this uphill transport is supplied by ATP generated in cellular respiration, which drives the conformational change of the carrier protein and allows glucose to be taken up even when the intracellular concentration is already higher than in the lumen.