Question

Hi, I am struggling to understand this worksheet my professor gave us for practice. Could someone make any sense of this?

The scenario: Sodium is found largely in the extracellular compartment with concentrations between 130-145 mM with intracellular sodium concentrations between 3.5-5mM. This chemical difference gives sodium a large concentration gradient, which when permitted (by opening of a channel or through facilitated transport) will move down its concentration gradient to enter the cell. Sodium also has a favorable electrical gradient; the cellular membrane potential sits at -70mV with the extracellular space sitting at 0 mV. Sodium being a cation, also has an electrical gradient that favors the inward movement of sodium when permitted. If we were to let sodium freely permeate across the cell membrane it would settle at its equilibrium potential where the two forces equally oppose each other. This value is +66mV, sodium would still have a favorable chemical gradient, but an unfavorable electrical gradient. At this point net inward movement down the chemical gradient would equally oppose the net outward movement down sodium's electrical gradient. Answer the questions below based on the above information:

ANSWER CHOICES: chemical gradient, electrical gradient, chemical and electrical gradient, equilibrium point

1. if a cell has a membrane potential of -90mV and sodium is allowed to permeate across the cell membrane, which gradients would sodium be moving down?  

2. if a cell has a membrane potential of -10mV and sodium is allowed to permeate across the cell membrane, which gradients would sodium be moving down?

3. If a cell has a membrane potential of 0 mV and sodium can permeate the cell membrane, which gradients would sodium be moving down?

4. If a cell has a membrane potential of +66 mV and sodium is allowed to permeate across the cell membrane, which gradients would sodium be moving down?

5. If a cell has a membrane potential at +90mV and sodium is allowed to permeate across the cell membrane, which gradients would sodium be moving down?

6. Out of all the above scenerios, which membrane potential would yield the highest net rate of Na+ transport? The lowest?

Answer 1

1. If a cell has a membrane potential of -90mV and sodium is allowed to permeate across the cell membrane, then sodium would move down by both chemical and electrical gradient. Both the gradients allow sodium to down the gradient.

2. If a cell has a membrane potential of -10mV and sodium is allowed to permeate across the cell membrane, then sodium would move down by both chemical and electrical gradient. Both the gradients allow sodium to down the gradient.

3. 3. If a cell has a membrane potential of 0 mV and sodium can permeate the cell membrane, then sodium would move down by chemical gradient. Due to zero membrane potential, sodium would flow due to chemical gradient only.

4. If a cell has a membrane potential of +66 mV and sodium is allowed to permeate across the cell membrane, sodium would be at equilibrium point. At this point, sodium would still have a favorable chemical gradient, but an unfavorable electrical gradient and these two gradients oppose each other and resultant flow would be at equilibrium.