To maintain a gradient for a solute with higher outside the cell, which mechanism most directly uses energy?

• A. Degrade gradient
• B. Open channels
• C. Use transporter pump
• D. Convert to other molecule

Answer: (C)

Maintaining a gradient for a solute that’s more concentrated outside the cell requires moving that solute against its concentration gradient, and that needs energy. This is done through active transport, typically by transporter pumps such as the Na+/K+-ATPase, which use ATP to drive the movement of ions against their gradient. Open channels would let ions flow passively down the gradient, not require energy, so they can’t maintain the outside-high condition. Degrading the gradient or converting the solute to another molecule doesn’t directly establish or sustain the gradient through energy use. So the mechanism that directly uses energy to keep the gradient is the transporter pump performing active transport.