Proton Gradient Drives Sucrose Loading in Phloem

Steps:

• Companion cell cytoplasm (X) actively pumps H+ into apoplast (Y) using ATP, establishing an electrochemical gradient.
• Sucrose in apoplast (Y) binds with H+ via symporter protein.
• H+ moves from Y to sieve tube/companion cell (Z) down its gradient, co-transporting sucrose.
• Loaded sucrose enables mass flow in phloem.

Why B is correct:

• Protons move from apoplast (Y) to cell interior (Z) by facilitated diffusion down the electrochemical gradient created by active pumping, as per the chemiosmotic principle.

Why the others are wrong:

• A: Sucrose moves from Y to Z by secondary active transport via symport, not primary active transport.
• C: Protons move from X to Y by active transport using proton ATPase, not diffusion.
• D: Sucrose moves from Y to Z (not Z to X) by secondary active transport, not primary active.

Final answer: B