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Extracellular Na+ concentration is higher than intracellular Na+ concentrations. If channels are open up in the membrane of the cell allowing Na+ to cross the membrane (and only Na+), eventually the movement of Na+ into the cell will slow down and may even stop. This is due to the charge (voltage) inside the cell opposing the movement of Na+.

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READ THIS QUESTION/SCENARIO CAREFULLY:

Extracellular Na+ concentration is higher than intracellular Na+ concentrations.  If channels are open up in the membrane of the cell allowing Na+ to cross the membrane (and only Na+), eventually the movement of Na+ into the cell will slow down and may even stop.  This is due to the charge (voltage) inside the cell opposing the movement of Na+.

BlackTom AI · Accepted Answer

Question restatement: The scenario describes extracellular Na+ concentration being higher than intracellular Na+ concentration, and Na+ channels opening to allow Na+ to cross the membrane. It claims that the influx of Na+ will slow down or stop because the inside becomes positively charged, opposing further Na+ entry.

First, consider the driving forces for Na+ movement. Na+ tends to move into the cell because its concentration is higher outside than inside, creating a chemical (concentration) gradient. At the same time, as Na+ ions enter, the inside of the cell becomes more positive, creating an electrical (voltage) gradient that opposes further positive charge entry.

As Na+ channels stay open, the net driving force decreases over time as two opposing forces—the decreasing concentration gradient (as Na+ accumulates inside) and the increasing positive membrane potential (which repels additional Na+ ions)—work against continued influx.

This convergence toward a balance point is described by concepts such as the electrochemical gradient and the Nernst/Goldman calculations for Na+. The system will move toward an equilibrium where the electrical and chemical gradients cancel out, at which point net Na+ influx ceases or becomes negligible.

Why this reinforces the statement: The key idea is that initial favorable movement of Na+ into the cell is progressively opposed by the build-up of positive charge inside, which reduces the driving force and can halt net movement if channels remain open long enough.

Why this statement is not incorrect or misleading: It does not claim that Na+ never moves or that there is no driving force; it acknowledges that the driving force weakens due to the opposing voltage inside, which is exactly what happens in real cellular membranes when ions move and charge separation occurs.

In summary, the assertion that inward Na+ flux slows or stops as the inside becomes more positive (opposing further Na+ entry) aligns with fundamental principles of membrane physiology and electrochemical gradients, making the statement true.

BIO 112-Intro. Human Physiology-861, 862, 863 Quiz 2

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