Phloem Sucrose Loading via Proton Gradient

Steps:

• Proton pumps actively export H+ from companion cells into the apoplast, creating an electrochemical gradient.
• This gradient powers sucrose-H+ co-transporters to import sucrose from the apoplast into companion cells against its concentration gradient.
• Sucrose then diffuses into adjacent sieve tube elements for phloem transport.
• Mesophyll cells produce sucrose but do not directly load it into phloem.

Why C is correct:

• It accurately describes proton export from companion cells to establish a proton motive force, driving active sucrose uptake via co-transport per chemiosmotic principles.

Why the others are wrong:

• A: Proton pumps export H+, not import to raise pH; co-transporters import sucrose, not export it.
• B: Mesophyll cells do not load sucrose or raise pH; they supply sucrose to apoplast.
• D: Loading occurs in companion cells, not mesophyll; co-transporters move high apoplast sucrose into cells, not low sucrose out.

Final answer: C