Phloem Sucrose Loading Mechanism

Steps:

• Recall that ATP powers H+-ATPase pumps to create a proton gradient across the companion cell membrane.
• Sucrose enters companion cells against its gradient via sucrose-H+ symporters (co-transporters) using the proton gradient, not direct ATP energy.
• Proton pumps transport H+ ions out, not sucrose directly.
• Evaluate statements: 1 incorrectly attributes direct ATP use to sucrose movement; 2 correctly identifies co-transporter in companion cell membrane; 3 wrongly states proton pumps move sucrose.

Why D is correct:

• Sucrose loading uses secondary active transport via co-transporters driven by the proton gradient (definition of symport mechanism).

Why the others are wrong:

• A includes incorrect statements 1 and 3.
• B includes incorrect statement 1.
• C includes both incorrect statements 1 and 3.

Final answer: D