[Campbell Biology P.187] According to Figure 7.19 and the accompanying text on cotransport, ... | Practice Question

According to Figure 7.19 and the accompanying text on cotransport, which statement accurately describes the energy coupling in the H+/sucrose cotransporter system in a plant cell?

  • A: The cotransporter directly uses ATP hydrolysis to move both H+ and sucrose into the cell.
  • B: The "downhill" diffusion of sucrose provides the energy to pump H+ out of the cell against its gradient.
  • C: The proton pump uses ATP to establish an H+ gradient, and the potential energy stored in this gradient is then used by the cotransporter to move sucrose into the cell against its concentration gradient.
  • D: Both H+ and sucrose move down their respective electrochemical gradients into the cell, with their transport coupled.

Explanation

Figure 7.19 and the text explain that the ATP-driven proton pump first establishes an H+ gradient by pumping H+ out of the cell. This creates a potential energy source. The H+/sucrose cotransporter then utilizes the 'downhill' diffusion of H+ back into the cell driven by its electrochemical gradient to simultaneously transport sucrose 'uphill' into the cell against its own concentration gradient. Therefore, ATP indirectly provides the energy for sucrose transport.