Mitochondria in Companion Cells Power Phloem Loading

Steps:

• Sieve tubes lack nuclei and organelles, relying on companion cells for metabolic support.
• Cotransport of sucrose into sieve tubes requires ATP for proton pumps.
• Companion cells generate ATP via mitochondria to drive this active loading process.
• Diagram positions confirm mitochondria in companion cell region.

Why B is correct:

• Companion cells house mitochondria that produce ATP for H+/sucrose cotransport, enabling phloem loading per the companion cell-sieve tube functional unit definition.

Why the others are wrong:

• A: Sieve tube interior lacks mitochondria, as it contains only cytoplasm for mass flow.
• C: Sieve plate area has no organelles, serving only for translocation.
• D: Cell wall or exterior has no cellular components for energy production.

Final answer: B