Question

Glucose is absorbed in the intestine by which transport mechanism?

• A. Secondary active transport
• B. Facilitated diffusion
• C. Simple diffusion
• D. Primary active transport

Hint:

Think of the carrier that moves a large polar sugar down its gradient without spending ATP.

Answer 1

The Correct Option is B

Step 1: Glucose enters the enterocyte across the apical (luminal) membrane against its concentration gradient using the sodium-glucose linked transporter SGLT1. This is secondary active transport, powered indirectly by the \(Na^+/K^+\) ATPase pump.

Step 2: Once inside the cell, glucose has to leave across the basolateral membrane to reach the blood. This exit step uses the GLUT2 carrier and moves glucose down its concentration gradient. Movement of a large water-soluble molecule through a specific carrier protein, without using energy, is the definition of facilitated diffusion.

Step 3: When the question asks how glucose is "absorbed" with facilitated diffusion as an option, it is pointing to the carrier-mediated, energy-independent GLUT step, so the correct answer is facilitated diffusion.

Step 4: Simple diffusion is wrong because glucose is polar and cannot cross the lipid bilayer freely. Primary active transport directly hydrolyses ATP at the transporter itself, which glucose carriers do not do.