How would you expect an H⁺ ion to move out of the cell if [H⁺] inside the cell is lower than extracellular fluid?

• A. Simple diffusion
• B. Facilitated diffusion
• C. Active transport
• D. None of these

Answer: (C)

Verified step by step guidance

1. Understand the concentration gradient: The problem states that the concentration of H+ ions inside the cell is lower than outside. This means there is a concentration gradient where H+ ions naturally want to move into the cell, not out.
2. Identify the type of transport: Since H+ ions need to move against their concentration gradient (from lower to higher concentration), this process cannot occur through simple diffusion or facilitated diffusion, which both move substances down their concentration gradient.
3. Define active transport: Active transport is a process that moves ions or molecules against their concentration gradient, which requires energy, often in the form of ATP.
4. Consider the role of transport proteins: Active transport typically involves specific transport proteins, such as pumps, that use energy to move ions like H+ across the cell membrane against the gradient.
5. Conclude the mechanism: Given the need to move H+ ions from a lower to a higher concentration, active transport is the correct mechanism for this process, as it can move ions against the gradient using energy.