Phloem loading via proton-sucrose cotransport

Steps:

• Active transport pumps protons out of companion cells, creating an electrochemical gradient.
• Sucrose enters companion cells via cotransport with protons down the gradient.
• Sucrose diffuses from companion cells into sieve tubes.
• This sequence enables sucrose loading against its concentration gradient.

Why D is correct:

• Matches the proton gradient model, where H+-ATPase pumps protons out, driving symport of sucrose-H+ into companion cells, followed by diffusion into sieve tubes.

Why the others are wrong:

• A: Incorrect proton direction (into, not out) and omits cotransport.
• B: Reverses proton transport (out but cotransport misplaced) and garbles diffusion.
• C: Starts with wrong active transport of sucrose and confuses diffusion with cotransport.

Final answer: D