If sodium (\(\mathrm{Na^+}\)) does not move down its concentration gradient in a co-transport pump (such as the sodium-glucose symporter), what would be the likely outcome?

• A. The sodium-potassium pump would reverse its direction.
• B. ATP production in the cell would immediately increase.
• C. Potassium ions would be transported more efficiently.
• D. Glucose transport into the cell would decrease or stop.

Answer: (D)

Verified step by step guidance

1. Understand the role of the sodium-glucose symporter: This is a type of co-transport pump that uses the energy from sodium ions (\(\mathrm{Na^+}\)) moving down their concentration gradient to transport glucose into the cell against its concentration gradient.
2. Recognize the dependency on the sodium gradient: The sodium-glucose symporter relies on a steep sodium gradient, which is maintained by the sodium-potassium pump. If sodium does not move down its gradient, the symporter cannot function effectively.
3. Analyze the impact on glucose transport: Without the movement of sodium ions into the cell, the symporter cannot provide the energy needed to transport glucose into the cell. This would result in a decrease or cessation of glucose transport.
4. Eliminate incorrect options: The sodium-potassium pump would not reverse its direction because it is an active transport mechanism powered by ATP. ATP production would not immediately increase because this process is not directly linked to ATP synthesis. Potassium ion transport would not become more efficient because it depends on the sodium-potassium pump, which is unaffected by the sodium-glucose symporter.
5. Conclude the correct outcome: Since the sodium-glucose symporter cannot function without sodium moving down its gradient, glucose transport into the cell would decrease or stop.