Phloem Sucrose Loading Mechanism

Steps:

• Proton pumps actively export H+ from companion cells, creating an electrochemical gradient.
• Sucrose enters companion cells via H+/sucrose symporters, moving against its concentration gradient.
• Inside companion cells, sucrose concentration rises above that in mesophyll, enabling mass flow.
• Sucrose diffuses symplastically into sieve tubes via plasmodesmata.

Why C is correct:

• Sucrose uptake into companion cells occurs against its concentration gradient via secondary active transport, not down it.

Why the others are wrong:

• A: Proton pumps use ATP to actively transport H+ out, correctly described.
• B: Sucrose entry follows the H+ gradient (facilitated diffusion for protons), but the statement aligns with symport mechanics.
• D: Companion cells connect to sieve tubes via plasmodesmata for symplastic sucrose transfer.

Final answer: C