Transport Across a Cell Membrane

[bobeanie95]

Q: How do potassium ions travel as they move into the cell?

A: Potassium moves up its concentration gradient, but down the membrane potential when it enters a cell.

I'm confused as to what they mean by "down the membrane potential."

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[aldol16]

The cell membrane is polarized such that the interior is negative relative to the exterior. Therefore, when potassium moves into the cell, it moves down a membrane potential gradient because it's a positive charge rolling downhill towards a negative charge - in other words, this is favorable.

 

[bobeanie95]

Cool, thank you!

 

[NextStepTutor_2]

Q: How do potassium ions travel as they move into the cell?

A: Potassium moves up its concentration gradient, but down the membrane potential when it enters a cell.

I'm confused as to what they mean by "down the membrane potential."

Hi @bobeanie95 !


This "membrane potential" is the entire basis for propagation of action potentials along your nerves. The resting membrane potential is established by several mechanisms, including the Na/K pump, which pumps 3 Na out of the cell for every 2K pumped into the cell. This results in an accumulation of K+ ions inside the cell and Na+ ions outside the cell.

This activity can also be seen in standard intra/extra cellular concentration of common ions you will learn in med school:


Ion [Intracellular] mM [Extracellular] mM

K+ 140 5

Na+ 5-15 145

Cl- 4 110

Ca++ 1x10^-4 3-5


This movement of ions (3+ out, 2+ in) results in the interior of the membrane having a potential that is relatively negative compared to the outside.

This is why when the K+ moves inside the cell, it is said to moving down the membrane potential (going where it is electrically favored to go) but up its concentration gradient (going against where rules of solute transport would dictate). The AAMC could even bring in the Nernst equation if they wanted to ask you to determine concentrations of a given ion or to predict membrane potential.

Hope this helps, good luck!

 

[bobeanie95]

@NextStepTutor_2 Thank you for your thorough explanation. However, I was under the impression that the membrane potential is created predominantly by potassium leak channels and the Na/K pump tends to just maintain that potential. The main reason I'm confused is that if the potassium leak channels are leaking out potassium to create a negatively charged cytoplasm then isn't the pump somewhat counteracting this effect by re-pumping potassium back inside? In other words, how is the resting membrane potential (around -70mV) maintained if you have potassium leaking out and some entering. Does potassium leaving/entering occur at different rates so that the voltage isn't zero?

Source: https://www.dartmouth.edu/~rswenson/NeuroSci/chapter_2.html

 

[aalamruad]

@NextStepTutor_2 Thank you for your thorough explanation. However, I was under the impression that the membrane potential is created predominantly by potassium leak channels and the Na/K pump tends to just maintain that potential. The main reason I'm confused is that if the potassium leak channels are leaking out potassium to create a negatively charged cytoplasm then isn't the pump somewhat counteracting this effect by re-pumping potassium back inside? In other words, how is the resting membrane potential (around -70mV) maintained if you have potassium leaking out and some entering. Does potassium leaving/entering occur at different rates so that the voltage isn't zero?

Source: https://www.dartmouth.edu/~rswenson/NeuroSci/chapter_2.html

No, the Na+/K+ pump actually helps keep the resting membrane potential negative, rather than counteracting that effect as you suggested. What you're forgetting is that K+ isn't the only ion that contributes to the resting potential. Remember, it's not called the "potassium pump," it's called the "sodium potassium pump." It pumps 3 sodium ions out of the cell for every 2 potassium ions that it pumps in, leading to a net decrease in intracellular potential. Then, those potassium ions can be slowly leaked back out of the cell, further contributing to the resting membrane potential.

 

[bobeanie95]

@walloobi thanks so much!