Proton pumps drive phloem loading for mass flow

Steps:

• Identify leaf vein structures: xylem for water, phloem for sugar transport via mass flow.
• Recall mass flow mechanism: pressure-flow hypothesis relies on active loading in phloem.
• Link proton pumps: H+-ATPases in companion cells create gradient for sucrose uptake.
• Match to micrograph: structure with proton pumps is companion cell in phloem.

Why C is correct:

• Companion cells use proton pumps (H+-ATPases) to acidify cytoplasm, enabling sucrose-H+ symport for phloem loading per pressure-flow model.

Why the others are wrong:

• A (xylem vessel): Passive water conduction, no proton pumps involved.
• B (sieve tube): Lacks organelles; relies on companion cells for loading, not direct proton pumping.
• D (mesophyll cell): Photosynthesis site, contributes sugars but no role in mass flow via pumps.

Final answer: C