Active phloem loading via concentration gradient

Steps:

• Sucrose forms in mesophyll cells at lower concentration.
• Active transport loads it into companion cells, raising concentration for osmotic pull.
• High companion concentration drives passive diffusion to sieve tubes.
• Sieve concentration stays high but lower than companion to maintain flow.

Why B is correct:

• B establishes an increasing gradient from mesophyll (5) to companion (15) for active loading, per pressure-flow hypothesis.

Why the others are wrong:

• A: Sieve (15) exceeds companion (10), requiring active transport where passive diffusion occurs.
• C: Mesophyll (15) exceeds companion (5), reversing gradient and blocking loading.
• D: Mesophyll (15) exceeds companion (10), causing sucrose leakage from phloem.

Final answer: B