[Campbell Biology P.849] Which statement accurately describes the active transport of sucros... | Practice Question

Which statement accurately describes the active transport of sucrose into companion cells and sieve-tube elements, as depicted in Figure 36.16b?

  • A: Sucrose moves directly from mesophyll cells into sieve-tube elements primarily via plasmodesmata without apoplast involvement.
  • B: Proton pumps hydrolyze ATP to create a high H+ concentration gradient outside the cell, which then drives sucrose accumulation via a cotransporter.
  • C: Sucrose transport into sieve-tube elements is always passive, driven by its concentration gradient.
  • D: The cotransport protein simultaneously moves sucrose out of the cell and H+ into the cell.

Explanation

Figure 36.16b and the accompanying text explain that proton pumps hydrolyze ATP to generate an H+ gradient across the membrane high H+ concentration outside . This gradient then powers the H+/sucrose cotransporter, which moves sucrose into the companion cell/sieve-tube element along with the H+ diffusing back into the cell, enabling active accumulation of sucrose.