Junction impermeable/permeable

The apical side of the cell faces the lumen this side is also the mucosal side in many tissues, The basal side faces the interstitial fluid and the capillaries this side is also called the serosal side. The sides of the cell facing adjacent cells are called the lateral sides. Nutrients entering and exiting the basal and lateral membranes (basolateral membrane) can freely enter and exit the permeable membranes of nearby capillaries. Thus, the Na secreted by the basolateral membrane can enter the circulatory system. Potassium leaking from the capillaries can be taken up by the Na,K-ATPase of the basolateral membrane. The cells are connected to each other by impermeable junctions called tight junctions. 

The affinity-purified connexin-32 was incorporated into unilamellar liposomes as before. The connexin-32 induced a sucrose permeability in liposomes, as assayed by the density-shift technique, and gave results essentially identical to those in Figure 2. Liposomes that were sucrose-permeable did not retain the dye Lucifer Yellow (retained by the sucrose-impermeable liposomes), which is near the upper size-permeability limit for gap junction channels (19, 108, 109). The fraction of the liposomes that were permeable to sucrose decreased by a factor of 4 when the pH in the gradients was changed from 7.5 to 6.0. This effect was partially reversible. 

Although this terminology (permeable/impermeable) is frequently used in the field of junction thermodynamics, it will not be used in the remainder of this book. In fact, it is easier to apply a common treatment for electrochemical chains where different kinds of interfaces are involved. This is notably true when treating kinetic phenomena occurring at these interfaces. If you consider all these interfaces to be impermeable interfaces, whether reactive or not, then this unification is made possible (see next example). In the case of a permeable junction, the two exchanging species (e.g., Ag in solid AgCI and Ag in solution) will simply be considered as two different species the first one is transformed into the second one by a reaction at the interface. 

In a study of guinea-pig cecal epithelium, Mora-Galindo (Mora-Galindo 1986) showed that cells at the surface and in middle regions of crypts possess TJ impermeable to lanthanum, whereas junctions between cells located at the bottom of crypts often were permeable to the tracer, indicating that permeability decreases as the epithelial cells mature. 

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