Question

You are studying a newly discovered bacterium. This bacterium has an antiport called PmpZ. PmpZ is located on the bacterium plasma membrane, and uses energy from light to pump ions across the membrane. PmpZ normally transports 2 potassium (K^+) ions inward for every 1 magnesium (Mg^2+) ion transported outward. You first create an artificial lipid bilayer that contains no proteins. (Recall that lipid bilayers will spontaneously form spherical lipid vesicles spontaneously in an aqueous environment.) If you create a situation where the bilayers have the same concentrations of K^+ and Mg^2+ inside and outside of the vesicle, would you expect either ion to freely move across the membrane? Briefly explain. Now you create an artificial lipid bilayer that contains many molecules of PmpZ, oriented as it normally is within the plasma membrane. You start with a a situation where the bilayers are in an aqueous environment and have the same concentrations of Na^+ K^+ Cl^-, and Mg^2+ inside and outside of the vesicle. You place these vesicles in the presence of bright sunlight and examine what happens after 20 minutes. Which positively charged ion do you expect to be more abundant inside the vesicle (relative to the concentration on the outside)? Which ion do you expect to be more abundant outside the vesicle (relative to the concentration on the inside)? Will this situation set up a membrane potential (i.e., a voltage across the membrane)? Given the situation described in part 3b, if you were to add some Na^+ leak channel to this artificial lipid bilayer, in which direction would you expect Na^+ to flow? Explain.

Answer 1

3a. The lipid bilayer is not permeable to many ions like K+ and Mg+2, and it requires specific membrane proteins to transport these ions across the membrane. When you create a lipid bilayer without any membrane proteins, there would be no movement of ions across the membrane.

b.

i. In the presence of sunlight, K+ ions concentration would be more inside relative to the outside.

ii. Mg+2 ions would be more outside relative to inside.

iii. There would not be any membrane potential as the Na+ and Cl- ions movement are still constant.