Epithelial cells possess the ability to perform vectorial transport due to the presence of tight junctions, which divide the plasma membrane into two distinct domains - apical and basolateral. This means that ions or molecules can be transported from one side of the epithelial sheet to the opposite side. To achieve vectorial transport, specific membrane transport proteins must be targeted to and remain in either the apical or basolateral membrane domain. In the example shown in Figure 2.11, the Na+ channel is exclusively present in the apical membrane, while the Na+,K+-ATPase and K+ channels are limited to the basolateral membrane. The activity of the Na+,K+-ATPase channel and the passive efflux of K+ across the basolateral membrane establishes a significant electrochemical gradient, facilitating the entry of Na+ into the cell through the apical membrane via the Na+ channel (intracellular [Na+] < extracellular [Na+], and Vm with an intracellular negative charge relative to the extracellular environment). Subsequently, Na+ is pumped out of the cell by the Na+,K+-ATPase, enabling vectorial transport from the apical side to the basolateral side of the epithelium. This transport process is referred to as absorption or reabsorption, depending on the direction. For instance, the absorption of nutrients from the gastrointestinal tract lumen and the reabsorption of NaCl and water from the renal nephron lumen. Conversely, transport from the basolateral side to the apical side of the epithelium is known as secretion.
Because the tight junction divides the plasma membrane into two domains (i.e., apical and basolateral), epithelial cells are capable of vectorial transport, whereby an ion or molecule can be transported from one side of the epithelial sheet to the opposite side (
Fig. 2.11 ). The accomplishment of vectorial transport requires that specific membrane transport proteins be targeted to and remain in one or the other of the membrane domains. In the example shown in
Fig. 2.11 , the Na+ channel is present only in the apical •Fig. 2.10 Ciliaareapicalmembranespecializationsofsomeepithelialcells.Ciliaare5to10µminlengthandcontainarraysofmicrotubules,asdepictedinthesecross-sectiondiagrams.Left, Theprimaryciliumhasnineperipheralmicrotubulearrays.Itisnonmotileandservesasamechanoreceptor(e.g.,cellsoftherenalcollectingduct).Cellsthathaveaprimaryciliumhaveonlyasinglecilium.Right, Thesecondaryciliumhasacentralpairofmicrotubulesinadditiontothenineperipheralmicrotubulearrays.Alsointhesecondarycilium,themotorproteindyneinisassociatedwiththemicrotubulearraysandthereforeismotile.Asinglecellcanhavethousandsofsecondaryciliaonitsapicalsurface(e.g.,epithelialcellsoftherespiratorytract).(RedrawnfromRodat-DespoixL,DelmasP.Ciliaryfunctionsinthe nephron.Pflugers Archiv.2009;458:179.) membrane, whereas the Na+,K+-ATPase and the K+ channels are confined to the basolateral membrane. The operation of the Na+,K+-ATPase channel and the leakage of K+ out of the cell across the basolateral membrane sets up a large electrochemical gradient for Na+ to enter the cell across the apical membrane through the Na+ channel (intracellular [Na+] < extracellular [Na+], and Vm which is oriented with the cell’s interior electrically negative with respect to the cell’s exterior). The Na+ is then pumped out of the cell by the Na+,K+-ATPase, and vectorial transport from the apical side of the epithelium to the basolateral side of the epithelium occurs. Transport from the apical side to the basolateral side of an epithelium is termed either absorption or reabsorption: For example, the uptake of nutrients from the lumen of the gastrointestinal tract is termed absorption, whereas the transport of NaCl and water from the lumen of the renal nephrons is termed reabsorption. Transport from the basolateral side of the epithelium to the apical side is termed secretion.