The absorption process where nutrients need a protein carrier, but not energy, to pass across the intestinal wall is called:

• A. Osmosis
• B. Passive diffusion
• C. Facilitated diffusion
• D. Diffusion

Answer: (C)

Facilitated diffusion is the process where a substance crosses the intestinal wall with the help of a carrier protein but without using cellular energy. The carrier protein binds the nutrient and undergoes a shape change to shuttle it across the membrane, allowing movement down its concentration gradient. This is different from simple diffusion, which doesn’t involve a carrier, and from active transport, which requires energy to move substances against their gradient. In the intestine, many nutrients rely on this carrier-mediated, energy-free path to move into cells once there’s a favorable gradient. So when a nutrient needs a protein carrier but no energy to cross the intestinal wall, facilitated diffusion is the best description.