Which scenario illustrates active transport in the small intestine?

• A. Absorption of nutrients from the lumen of the small intestine into epithelial cells against their concentration gradient
• B. Diffusion of oxygen into blood capillaries
• C. Osmosis of water into intestinal lumen
• D. Passive transport of glucose down its gradient through a transporter

Answer: (A)

The main idea here is active transport in the small intestine—moving substances from the intestinal lumen into epithelial cells against their concentration gradient using energy and a transport protein. The described scenario fits this because nutrients move into the epithelial cells even when their concentration is lower in the cell than in the lumen, which requires energy input and a carrier mechanism. This is exactly how certain nutrients, like glucose in the intestine, are taken up via active transport systems that use cellular energy (for example, coupling with a sodium gradient maintained by the Na+/K+ ATPase).

The other options reflect passive processes: diffusion of oxygen into blood capillaries happens down its gradient without energy; osmosis of water into the lumen is a passive water movement driven by osmotic differences; and glucose moving down its gradient through a transporter is facilitated diffusion, not energy-requiring active transport.