1. What would be the consequences of incorporating porins into the plasma membrane instead of into the outer membrane of bacteria?

The presence of porins in the plasma membrane would allow the free diffusion of ions and small molecules between the cytosol and extracellular fluids—a disaster for the cell.

2. The concentration of K⁺ is about 20 times higher inside squid axons than in extracellular fluids, generating an equilibrium membrane potential of -75 mV. What would be the expected equilibrium membrane potential if the K⁺ concentration were only 10 times higher inside than outside the cell? Why does the actual resting membrane potential (-60 mV) differ from the K⁺ equilibrium potential of -75 mV?

Given C_o/C_i = 10, the K⁺ equilibrium potential calculated from the Nernst equation is –58 mV. The actual resting membrane potential differs from the K⁺ equilibrium potential because the plasma membrane is not completely impermeable to other ions.

3. An important function of the Na⁺-K⁺ pump in animal cells is the maintenance of osmotic equilibrium. Why is this unnecessary for plant cells?

The plant cell wall prevents cell swelling and allows the buildup of turgor pressure.

4. An anticipated problem in gene therapy for cystic fibrosis is that normal CFTR genes would probably be successfully transferred to only a fraction of the cells in the patient's respiratory epithelium. How is this potential pitfall affected by the presence of gap junctions?

Because gap junctions allow ions to diffuse freely between adjacent cells, expression of normal CFTR in one epithelial cell would provide a functional Cl^– channel to its neighbors.

5. What is the importance of selectively targeting different glucose transporters to the apical and basolateral domains of the plasma membrane of intestinal epithelial cells? What is the role of tight junctions in this process?

The correct localization of transporters mediating active transport and facilitated diffusion is necessary for the polarized function of epithelial cells in transferring glucose from the intestinal lumen to the blood supply. Tight junctions prevent the diffusion of these transporters between domains of the plasma membrane, as well as sealing the spaces between cells of the epithelium.