Association with the extracellular matrix 4. Association with the cytoskeleton 5. Cell-cell associations, including tight junctions, which maintain cell polarity and serve as a barrier restricting the movements of molecules across cell sheets 6. Glycocalyx, the carbohydrate-rich zone of the external plasma membrane. In polarized epithelial cells and endothelial cells, the apical and basolateral membranes have distinctive membrane compositions.
Tight junctions form a boundary between the apical and basolateral membranes.
Barrier function has multiple roles including generation and maintenance of ion gradients between the cytoplasm and the extracellular space. These ionic differences result in the inside of the plasma membrane being negatively charged relative to the outside of the plasma membrane. Lipids exhibit some membrane solubility. Binding causes a conformational change of the carrier and this results in the transport of the solute across the membrane. Carriers are used for the transport of a wide array of molecules.