Questions: Epithelial Vectorial Transport and Secretion

Question 1

SGLT1 on the apical membrane of intestinal epithelial cells co-transports glucose into the cell along with Na⁺ flowing down its electrochemical gradient. What is the primary source of this Na⁺ gradient?

• A. SGLT1 itself hydrolyzes ATP to actively pump Na⁺ out and maintain a low intracellular Na⁺ concentration
• B. The Na⁺-K⁺-ATPase on the basolateral membrane continuously pumps three Na⁺ out for every two K⁺ in, keeping intracellular Na⁺ low
• C. Tight junctions prevent extracellular Na⁺ from re-entering the cell through paracellular spaces, passively building the gradient
• D. Na⁺ diffuses passively out of the cell through GLUT2 on the basolateral membrane

Answer: (B)

The Na⁺-K⁺-ATPase on the basolateral membrane uses ATP to pump three Na⁺ out of the cell in exchange for two K⁺ in, creating a low intracellular Na⁺ concentration and a negative interior charge. SGLT1 exploits this electrochemical gradient: Na⁺ flows down its gradient into the cell, and glucose is co-transported in the same direction without directly consuming ATP. The energy investment is made by the Na⁺-K⁺-ATPase (primary active transport); SGLT1 uses that stored gradient energy for glucose uptake (secondary active transport). This indirect coupling allows one ATPase to drive many different apical co-transporters.

Question 2

If tight junctions between intestinal epithelial cells were completely disrupted, what would be the most direct consequence for vectorial transport?

• A. The Na⁺-K⁺-ATPase would stop functioning without the structural support of tight junctions
• B. Glucose would be trapped inside epithelial cells because basolateral GLUT2 would be removed from the membrane
• C. Solutes and ions could bypass the transcellular machinery by moving paracellularly, eliminating the directional control that defines vectorial transport
• D. Apical membrane proteins would diffuse laterally to the basolateral membrane, reversing the direction of transport

Answer: (C)

Tight junctions do two things: they maintain cell polarity by preventing apical and basolateral proteins from intermixing, and they seal the paracellular space to force transport through the transcellular route. Without tight junctions, ions and solutes could move freely between cells along their concentration gradients, completely bypassing the cell's transport machinery. Directional control depends on tight junctions ensuring that everything moving from lumen to bloodstream must pass through the cell's regulated channels and carriers — otherwise there is no way to impose direction.

Question 3

Epithelial vectorial transport can occur even if the apical and basolateral membranes express identical transporter proteins, as long as electrochemical gradients are correctly maintained.

• T. True
• F. False

Answer: False

Asymmetric transporter expression is the structural basis of directionality — it cannot be replaced by gradient effects alone. If both membrane faces expressed the same transporters, any solute entering through one face would be equally likely to exit from the same face; there would be no preferred direction. The net movement from lumen to bloodstream (for absorption) requires that different proteins perform different jobs on each face: SGLT1 on the apical face captures glucose from the lumen; GLUT2 on the basolateral face releases it to the bloodstream. Identical proteins on both faces cannot produce sustained net directional transport.

Question 4

The Na⁺-K⁺-ATPase powers secondary active transport indirectly — it does not directly move the co-transported solute but creates the electrochemical gradient that secondary transporters exploit.

• T. True
• F. False

Answer: True

Primary active transport uses ATP directly (the Na⁺-K⁺-ATPase). Secondary active transport couples movement of one solute down its gradient (Na⁺) to drive another solute (glucose, amino acids) against its gradient, without directly consuming ATP. The energy was stored in the Na⁺ gradient by the ATPase. This indirect coupling is efficient: one ATPase can power many different secondary transporters simultaneously, all exploiting the same Na⁺ gradient. Cystic fibrosis illustrates what happens when a single apical transporter (CFTR for Cl⁻) is lost: even though the Na⁺ gradient is intact, secretion fails across multiple organs because the apical exit channel for Cl⁻ is missing.

Question 5

Why is it essential that the Na⁺-K⁺-ATPase is located on the basolateral membrane rather than the apical membrane for intestinal glucose absorption to work correctly?

Model answer: The Na⁺-K⁺-ATPase creates a low intracellular Na⁺ concentration by pumping Na⁺ out toward the basolateral side. This gradient drives SGLT1 on the apical membrane to co-transport Na⁺ and glucose into the cell from the lumen. If the ATPase were on the apical membrane instead, it would pump Na⁺ into the lumen rather than toward the bloodstream, eliminating the inward Na⁺ gradient that drives apical uptake. Basolateral placement ensures Na⁺ is always being cleared toward the bloodstream, sustaining the apical-to-basolateral gradient that powers glucose entry.

The spatial segregation of the pump (basolateral) from the secondary co-transporter (apical) is what creates directionality. Glucose moves lumen → cell via SGLT1 (apical), then cell → bloodstream via GLUT2 (basolateral). Both steps depend on the Na⁺ gradient maintained by the basolateral ATPase. If the ATPase were mislocated to the apical face, the gradient would reverse or collapse, SGLT1 would lose its driving force, and net glucose absorption would fail. The entire absorptive scheme is a spatial circuit: apical entry, basolateral exit, with the ATPase continuously maintaining the gradient that drives the circuit.