Sucrose loading via H+-ATPase proton pump

Steps:

• Companion cells actively pump H+ out using ATP-driven H+-ATPase, creating a proton gradient.
• This gradient drives sucrose uptake into companion cells via a proton-sucrose symporter.
• Sucrose enters passively as H+ flows back in down its electrochemical gradient.
• The active step is H+ extrusion, enabling phloem loading.

Why B is correct:

• H+ movement out of the cytoplasm is active transport via H+-ATPase, establishing the proton motive force for sucrose symport (per chemiosmotic theory).

Why the others are wrong:

• A: H+ influx into cytoplasm is passive via symporter, not active.
• C: Sucrose is uncharged (not ions) and loads into, not out of, companion cells.
• D: Sucrose moves into companion cells for loading, not out.

Final answer: B