Many cells have the ability to actively transport certain ions against concentration and electrical potential gradients. An important example is the extrusion of Na+ from cells by the transmembrane protein Na+/K+-ATPase, also called the Na+/K+ pump, which allows cells to maintain the required low intracellular sodium concentration (5-15 mmol/L vs ~140 mmol/L in extracellular fluid).
Some epithelial cells specialize in the transfer of ions (by ion pumps) and water (via the membrane channels called aquaporins) in either direction across the epithelium, the process is known as transcellular transport (Figure 1). Apical tight junctions prevent paracellular diffusion or backflow between the cells.
Fig1. Ion and water absorption and secretion.
Epithelia of kidney tubules are key sites for ion and water transport, maintaining the body’s overall balance of salts and water. Cells of the proximal renal tubules are specialized structurally for transcellular transport. The apical surface at the tubule lumen is freely permeable to Na+, and the basolateral cell membranes have sodium pumps for the active extrusion of Na+ into the interstitial fluid outside the tubules. Osmotic and electrical balance is maintained by the passive transfer of chloride ions (Cl−) and water into the cell. The basal membrane of these cells is elaborately folded, with mitochondria located between the folds to supply ATP for Na+/K+ pumps (Figure 2). Lateral membrane folds inter digitating between the cells further increase the surface area for transport. Regulated transfer of ions and water by various epithelial cells along the renal tubules maintains the ionic balance within the body and allows excretion of excess water and salts in the urine.
Fig2. Features of absorptive cells.
All cells can also internalize extracellular molecules and fluid using endocytosis and formation of cytoplasmic, mem brane-bound vesicles. This activity is clearly observed in the simple squamous epithelial cells lining blood and lymphatic capillaries (endothelia) or body cavities (mesothelia). These thin cells have few organelles other than the abundant pinocytotic vesicles, which cross the thin cells in both directions and release their contents on the opposite side by exocytosis. This process of transcytosis also occurs between the apical and basolateral membranes domains in cells of simple cuboidal and columnar epithelia and is important in many physiologic processes.
