The figure below shows two transporters typically found in the membrane of neurons. Which feature do these transporters share?

Question

BlackTom AI · Accepted Answer

Both transporters use the inward sodium electrochemical gradient as their driving force, so Na+ moves down its concentration/electrical gradient into the neuron.

1) Relevant knowledge points and concepts
- Cells maintain a large Na+ gradient (high outside, low inside) and a negative inside membrane potential via the Na+/K+ ATPase (primary active transport).  
- Secondary active transporters (symporters/antiporters) harness that Na+ electrochemical gradient to move other solutes against their own gradients.  
- The electrochemical free energy for Na+ movement is ΔG = RT ln([Na+]in/[Na+]out) + zFΔψ; because [Na+]in < [Na+]out and Δψ is negative inside, ΔG for inward Na+ is strongly negative (favorable).

2) Step-by-step reasoning
- The two transporters shown (typical neuronal transporters) are Na+-dependent cotransporters rather than ATPases or voltage-gated channels.  
- They allow Na+ to enter the cell down its electrochemical gradient; that favorable Na+ influx provides the energy to carry other molecules (e.g., neurotransmitters, glucose) into the cell.  
- Therefore the common feature is that both transporters transport Na+ down its concentration/electrochemical gradient.

3) Intermediate/quantitative step
- Using ΔG expression: because [Na+]in/[Na+]out < 1, ln(...) is negative; z for Na+ = +1 and Δψ (inside minus outside) is typically about −70 mV, so zFΔψ is negative. Sum is negative → inward Na+ movement is energetically favorable.

4) Why the correct answer is correct
- Answer (e) correctly states that both transporters permit Na+ to move down its gradient; that is the hallmark of Na+-coupled secondary transporters in neurons.

5) Why the other options are incorrect
- (a) Transporting Na+ up its gradient describes primary active transporters (e.g., Na+/K+ ATPase), not these cotransporters.  
- (b) Hydrolyzing ATP is a feature of pumps (ATPases), not of Na+-driven secondary transporters.  
- (c) Voltage-dependence describes channels or some transporters, but the common defining feature here is Na+-driven transport, not voltage gating.  
- (d) Co-transporting both ions up their gradients would require external energy; cotransporters use Na+ flowing down its gradient to drive other substrates up, not both up.

6) Conclusion
The correct answer is e) They transport Na+ ions down their concentration gradient, which matches the provided answer.

The figure below shows two transporters typically found in the membrane of neurons. Which feature do these transporters share? 单项选择题

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