Phloem Loading via Proton Gradient

Steps:

• Companion cells hydrolyze ATP to activate H+-ATPase pumps.
• These pumps export H+ ions from companion cells into the apoplast, creating a proton gradient.
• The gradient powers sucrose-H+ symporters to load sucrose into companion cells from the apoplast.
• Sucrose then moves from companion cells to sieve tube elements via plasmodesmata.

Why B is correct:

• ATP fuels H+-ATPase in companion cell membranes to pump H+ out, generating the electrochemical gradient essential for active sucrose uptake per the chemiosmotic theory.

Why the others are wrong:

• A: H+ ions are exported to the apoplast, not imported into the sieve tube element.
• C: Sucrose enters via secondary active transport with H+, not direct ATP pumping.
• D: Sucrose diffuses passively from companion cell to sieve tube, not via active pumping.

Final answer: B