ATP Drives Proton Pumping for Sucrose Loading

Steps:

• Companion cells actively load sucrose into phloem using energy from ATP.
• ATP powers H+-ATPase pumps to export protons from companion cell cytoplasm.
• Proton gradient lowers pH in apoplast, driving sucrose-H+ symporter uptake.
• Sucrose enters sieve tube via diffusion or facilitated transport.

Why C is correct:

• ATP hydrolysis by H+-ATPase actively removes protons from companion cell cytoplasm, creating electrochemical gradient for sucrose symport (per active transport definition).

Why the others are wrong:

• A: ATP indirectly enables sucrose movement via proton gradient, not direct transport.
• B: Protons are pumped from companion cell, not sieve tube element.
• D: Sucrose uptake into companion cell uses proton gradient, not direct ATP energy.

Final answer: C