Mechanisms of Phloem Loading for Assimilates

Steps:

• Assimilates like sucrose are produced in source mesophyll cells and must enter phloem sieve tubes against a concentration gradient.
• In apoplastic loading, sucrose exits mesophyll cells into the apoplast via simple diffusion through membrane channels.
• Sucrose then enters companion cells via facilitated diffusion coupled with active proton pumping (H+-sucrose symporters).
• From companion cells, sucrose moves to sieve tubes via plasmodesmata, involving facilitated diffusion, while active transport maintains the gradient.

Why A is correct:

• Phloem loading follows the mass flow hypothesis, requiring active transport to build osmotic pressure, facilitated diffusion for sucrose-proton cotransport, and simple diffusion for apoplastic release, as defined in plant transport physiology.

Why the others are wrong:

• B: Exocytosis is not involved in assimilate transfer; it's mainly for protein secretion.
• C: Omits facilitated diffusion essential for symport across companion cell membranes.
• D: Ignores diffusion processes needed for sucrose movement between cells and apoplast.

Final answer: A