Question

Lipid vesicles (small spherical bilayers) provide an ideal in vitro system for the study of pumps and channels. You have prepared lipid vesicles that contain Na+/K+ pumps. The pumps are oriented such that the portion of the pump that normally faces the cytoplasm is facing the outside of the vesicle. Predict what would happen under each of the following conditions (for simplicity, assume that the ions are pumped in a 1:1 ratio).

a. The solution on the inside and outside of the vesicles contains both ions, but no ATP.

b. The solution inside the vesicle contains both ions; the solution outside contains both ions and ATP.

c. The solution inside contains sodium ions; the solution outside contains sodium ions plus ATP.

d. The solution as in part B, but the pumps are randomly oriented, some facing outward and some facing inward.

Answer 1

a) There is no transport of Na⁺or K⁺, therfore the pump depends upon the hydrolysis of ATP. Na⁺will be loaded into its binding pocket.

b) Na⁺will be pumped into the vesicles and K⁺will be pumped out of the vesicle. One ATP molecule will be hydrolyzed for each pumping cycle.

c) Each pump will go through half a cycle with Na⁺being pumped into the vesicle and one ATP molecule will be hydrolyzed. The cycle cannot be completed without the presence of K⁺inside the vesicle. The cycle cannot be completed without K⁺inside the vesicle.