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.
Some epithelial cells specialize in the transfer of ions and water in either direction across the epithelium, the process known as transcellular transport. Apical tight junctions prevent paracellular diffusion or back-flow between the cells. Epithelia of kidney tubules are key sites or ion and water transport, maintaining the body’s overall balance of salts and water. The apical surface at the tubule lumen is freely permeable to Na⁺. The basal membrane of these cells is elaborately folded, with mitochondria located between the folds to supply ATP or Na⁺/K⁺ pumps.
All cells can also internalize extracellular molecules and fluid using endocytosis and formation of cytoplasmic, membrane-bound vesicles. This activity is clearly observed in the simple squamous epithelial cells lining blood and lymphatic capillaries or body cavities. 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.