Membrane Transport Mechanisms · USMLE Step 1

Which scenario would most likely require primary active transport rather than passive mechanisms for ion movement across the cell membrane?

Diffusion of CO₂ out of a metabolically active cell
Uptake of glucose into a red blood cell down its concentration gradient
Keeping intracellular K⁺ high and Na⁺ low against their gradients
Movement of water molecules through aquaporin channels

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Correct answer: Keeping intracellular K⁺ high and Na⁺ low against their gradients

Maintaining high intracellular K⁺ and low Na⁺ concentrations requires the Na⁺/K⁺-ATPase pump, which uses ATP to transport ions against their gradients — a primary active transport process essential for cell homeostasis.

Difficulty: Medium Question 17 of 24

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Which scenario would most likely require primary active transport rather than passive mechanisms for ion movement across the cell membrane?

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