You have prepared lipid vesicles (spherical lipid bilayers) that contain Na+-K+

Question

You have prepared lipid vesicles (spherical lipid bilayers) that contain Na+-K+ pumps as the sole membrane protein. All of the Na+-K+ pumps are oriented in such a way that the portion of the molecule that normally faces the cytosol is on the inside of the vesicle and the portion of the molecule that normally faces the extracellular space is on the outside of the vesicle. Assume that each pump transports one Na+ ion in one direction and one K+ ion in the other direction during each pumping cycle.

1. The solutions inside and outside the vesicles contain both Na+ and K+ ions but no ATP.

2. The solution outside the vesicles contains both Na+ and K+ ions; the solution inside contains both Na+ and K+ ions and ATP.

3. The solution outside contains Na+; the solution inside contains Na+ and ATP.

Explanation / Answer

i). The solutions inside and outside the vesicles contain both Na+ and K+ ions but no ATP.

....................Without any ATP to provide energy for the Na+-K+ pumps, no ions will be pumped.

(II). The solution outside the vesicles contains both Na+ and K+ ions; the solution inside contains both Na+ and K+ ions and ATP.

...............The pump will bind a molecule of Na+, causing the ATPase activity to hydrolyze ATP and transfer the phosphate group onto the pump. A conformational change will occur, leading to release of Na+ from the vesicle. K+ will be pumped into the vesicles, while Na+ is pumped out. Na+K+ pump is responsible for high extracellular Na ion concentration outside and high K ion concentration intracellular inside.

(iiI). The solution outside contains Na+; the solution inside contains Na+ and ATP.

............The pump will bind a molecule of Na+, causing the ATPase activity to hydrolyze ATP and transfer the phosphate group onto the pump. A conformational change will occur, leading to release of Na+ from the vesicle. However, since there is no K+ outside of the vesicle, the pump will get stuck at that step and subsequent steps of the cycle will not occur.

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