Imagine that you were able to experimentally make the Vm of the two-cell chamber of the previous question more negative than -58 mV (e.g. -75 mV). What would you predict to happen to K+ ions?

Question

BlackTom AI · Accepted Answer

The Nernst equilibrium for K+ is -58 mV, so at Vm = -58 mV there is no net K+ flux (electrical and chemical forces balance). Making Vm more negative than Ek (for example Vm = -75 mV) increases the electrical driving force that attracts positive ions into the inside compartment.

Relevant concepts:
- Nernst potential (Ek) is the membrane voltage where chemical and electrical gradients for an ion balance and net flux = 0.
- Driving force for a cation = Vm − Eion. If this quantity is negative (Vm < Eion), the electrical force drives positive ions inward (from outside → inside).

Step-by-step reasoning and calculation:
1. Given Ek (K+) = −58 mV and experimental Vm = −75 mV.
2. Compute the driving force: Vm − Ek = (−75 mV) − (−58 mV) = −17 mV.
3. A negative driving force of −17 mV for K+ (a positively charged ion) means a net inward-directed force: K+ will move from the outside compartment into the inside compartment until the electrochemical gradients re-equilibrate.

Why the correct answer is correct:
- At Vm more negative than Ek, the electrical attraction of K+ into the inside compartment exceeds the chemical tendency for K+ to move out, so net flux is inward (outside → inside). This matches choice b.

Why the other options are incorrect:
- a) predicts outward flow; that would happen only if Vm were more positive than Ek (Vm > −58 mV), not here.
- c) no flow occurs only at Vm = Ek (−58 mV), not when Vm = −75 mV.
- d) equal bidirectional flow implies equilibrium; again only at Vm = Ek.
- e) is not applicable.

Conclusion: The net movement of K+ will be from the outside compartment to the inside compartment, confirming answer b).

Imagine that you were able to experimentally make the Vm of the two-cell chamber of the previous question more negative than -58 mV (e.g. -75 mV). What would you predict to happen to K+ ions? 单项选择题

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