Transport across Cell Membrane

Three-dimensional structure of a recombinant cardiac gap junction membrane channel determined by electron crystallography. These channels allow the direct exchange of ions and small molecules between adjacent cells. Each channel is formed by association of six connexin subunits, each of which contains four α helices, in one plasma membrane, with a similar structure in the plasma membrane of an adjacent cell

The plasma membrane is a selectively permeable barrier between the cell and the extracellular environment. Its permeability properties ensure that essential molecules such as glucose, amino acids, and lipids readily enter the cell, metabolic intermediates remain in the cell, and waste compounds leave the cell. In short, the selective permeability of the plasma membrane allows the cell to maintain a constant internal environment. In several earlier chapters, we examined the components and structural organization of cell membranes (see Figures 3-32 and 5-30). Thephospholipid bilayer — the basic structural unit of biomembranes — is essentially impermeable to most water-soluble molecules, such as glucose and amino acids, and to ions. Transport of such molecules and ions across all cellular membranes is mediated by transport proteins associated with the underlying bilayer. Because different cell types require different…