Lecture 2

What is the main driving force for absorption in absorptive epithelia?

Na⁺ transport, generated by the Na⁺/K⁺‑ATPase on the basolateral membrane.

Why can Na⁺ enter the apical membrane passively in absorptive epithelia?

Because the electrochemical gradient (chemical + electrical) strongly favors Na⁺ entry.

Secondary active transport depends on what primary mechanism?

The Na⁺ gradient created by Na⁺/K⁺‑ATPase (primary active transport).

Which apical transporter mediates Na⁺‑coupled glucose uptake?

SGLT1 or SGLT2 (sodium‑glucose cotransporters).

Why does Cl⁻ move paracellularly in leaky epithelia?

Na⁺ absorption makes the lumen negative and interstitium positive, driving Cl⁻ toward the interstitium.

Which apical channel mediates Na⁺ entry in tight epithelia (e.g., cortical collecting duct)?

ENaC (Epithelial Na⁺ Channel).

Which aquaporin is inserted into the apical membrane of tight epithelia under hormonal control?

AQP2, regulated by vasopressin (ADH).

Why is mediated transport saturable but diffusion is not?

Because mediated transport depends on a finite number of transport proteins, whereas diffusion depends only on concentration gradients.