Epithelial cells transport solutes directionally through selective apical and basolateral transporter expression. The Na⁺-K⁺-ATPase on the basolateral membrane establishes ion gradients that drive secondary active transport. Tight junctions seal the paracellular pathway, forcing transport through transcellular routes. Coordinated apical and basolateral transporter opening controls both rate and direction of net transport for secretion or absorption.
From your study of tissue types, you know that epithelial cells form sheets that line and cover body surfaces, and that their defining feature is polarity — the apical surface faces a lumen or exterior while the basolateral surface faces the underlying tissue and blood supply. This polarity is not just anatomical; it is functional. The two faces of an epithelial cell carry completely different sets of transport proteins, and it is this asymmetry that gives epithelia their remarkable ability to move substances in a controlled direction — a property called vectorial transport.
The engine driving nearly all epithelial transport is the Na⁺-K⁺-ATPase, which sits exclusively on the basolateral membrane and continuously pumps three Na⁺ out of the cell in exchange for two K⁺ in, using ATP. This creates a low intracellular Na⁺ concentration and a negative interior charge — an electrochemical gradient that acts like a battery. Transporters on the apical membrane exploit this gradient through secondary active transport: for example, SGLT1 in the intestine co-transports glucose inward along with Na⁺ moving down its gradient. The cell doesn't spend energy directly on pulling glucose in — it spends energy maintaining the Na⁺ gradient, and glucose hitchhikes. Once inside the cell, glucose exits across the basolateral membrane via GLUT2, a facilitated transporter, down a concentration gradient into the bloodstream.
Tight junctions are the structural partner to this scheme. These protein complexes — built from claudins, occludin, and scaffolding proteins — form a continuous seal around each epithelial cell where neighboring cells meet. They do two things: they prevent the cell's apical and basolateral membrane proteins from diffusing into each other's domains (maintaining polarity), and they block paracellular transport — movement of ions and solutes through the spaces between cells. By sealing the paracellular route, tight junctions ensure that absorbed material must pass through the transcellular pathway, where it is subjected to the cell's transport machinery. The tightness of these junctions varies by tissue: the kidney proximal tubule is "leaky," allowing substantial paracellular flow of water and ions, while the urinary bladder epithelium is extremely tight, preventing any back-diffusion of urine.
The same machinery can operate in reverse to produce secretion. In a secretory epithelium, ion transporters are arranged so that Cl⁻ or HCO₃⁻ accumulates inside the cell and then exits through apical channels into the lumen. Water follows osmotically. Cystic fibrosis — a disease caused by defective CFTR, the apical Cl⁻ channel in airway and pancreatic epithelia — illustrates how loss of a single apical transporter disrupts secretion across multiple organ systems, causing the thick, sticky secretions that characterize the disease. Understanding vectorial transport explains not just normal physiology but the logic behind a wide range of absorptive and secretory disorders.