You discover a light-activated symport that transports Na+ and pyruvate into the

Question

You discover a light-activated symport that transports Na+ and pyruvate into the cell. You call this symport Symp1. You create an artificial vesicle where the only protein in the lipid is Symp1. Symp1 is oriented such that the cytoplasmic portion is inside the vesicle and the extracellular portion is on the outside of the vesicle (and thus similar to the arrangement of Symp1 on the plasma membrane of a cell). Your starting conditions are: A. 50 mM of Na+ inside and outside the vesicle. B. All the pyruvate is on the outside of the vesicle and there is no pyruvate inside the vesicle. C. Test tube in the dark

3. You discover a light-activated symport that transports Na' and pyruvate into the cell. You call thi symport Sympi You create an artificial vesicle where the only protein in the lipid is Symp1. Symp1l is oriented such that the cytoplasmic portion is inside the vesicle and the extracellular portion is on the outside of the vesicle (and thus similar to the arrangement of Sympl on the plasma membrane of a e Your starting conditions are: A. 50 mM of Nat inside and outside the vesicle B. All the pyruvate is on the outside of the vesicle and there is no pyruvate inside the vesicle C. Test tube in the dark a) Given these starting conditions, is there a concentration gradient of pyruvate? (2 points) b) You take the vesicles at the starting conditions (as described above in A, B, and C above) add light and examine the vesicles after 5 minutes. Will the concentration of Na outside the vesicle become higher than 50 mM, lower than 50 mM, or stay at 50 mM? Explain. (3 points) c) You create vesicles that have Sympl (oriented as described above) and a Na leak channel. You place the vesicles at starting conditions (as described in A, B, and C above), add light, and examine the vesicles after 5 minutes. In which direction do you predict pyruvate will be transported? (2 points) d) You examine the vesicles from 3c for Na 5 minutes after exposure to light. Will the concentration of Na outside the vesicle become higher than 50 mM, lower than 50 mM, or stay at 50 mM? Explain (5 points) e) You create a third set of membranes that have SympI (oriented as described above) and a voltage gated Na channel (but no Na leak channels). Do you predict that the voltage-gated Na' channel will open with if these vesicles are at the starting conditions (as described in A, B, and C above) above? Explain. (5 points)

Explanation / Answer

3. (a) A concentration gradient is formed if there is difference in concentration of the molecule across a membrane or surface. In this condition, concentration of pyruvate is considerably large towards outside of the vesicle as compared to inside. So, there is evidence of concentration gradient of Pyruvate across the vesicle.

(b) As the protein symp1 is light-activated and a symport (transport of two different molecules together in the same direction). On providing light, the protein will get activated and will transport the pyruvate along its concentration gradient, that is, from outside of vesicle to inside. As the transporter protein is symport, it will carry a molecule of Na+ along with it towards inside of vesicle. So the concentration of Na+ on outside the vesicle will become less than 50 mM.

(c) Na+ leak channel will alter the concentration of Na+ across the vesicle. Pyruvate will be transported along its concentration gradient from outside to inside of vesicle. But, later, it will be transported along with Na+ from concentration gradient of Na+, that is, from inside to outside.

(d) Initially, the concentration of Na+ on outside will be lower than 50mM, but on initiation of symp1 symport of Na+ and pyruvate towards inside of cell, Na+ will be transported back to outside along its concentration gradient, so the concentration on outside will become more than 50 mM.

(e) Yes, the voltage gated Na+ channel will open, as concentration of Na+ towards outside will be less than inside due to its transport along with pyruvate towards inside. The concentration diffference of Na+ across the membrane will activate the voltage sensitive gate.

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