You are testing the rate of glucose transport into vesicles using the Na+-glucose pump.
A. In experiment 1, you employ liposomes that have the pump in the same orientation as that found in the plasma membrane in epithelial cells. These liposomes contain glucose but no Na+ ions. You then transfer the liposomes to a series of tubes with solutions containing the same glucose concentration as that inside the vesicle and 0, 1, 2, 3, or 10 mM Na+. You measure the initial rates of glucose transport and plot your results (Figure below). Why do the initial rates of glucose transport into the liposome reach a plateau as the concentration of Na+ increases?
B. In experiment 2, there is one new variable: you have included leaky Na+ channels in the liposomal membrane. You've plotted your results on the same graph as experiment 1 to compare. Explain the reason for obtaining such different results in experiment 2 from those in experiment 1.