Phloem Sucrose Loading by Companion Cells

Steps:

• Companion cells actively uptake sucrose from the apoplast using proton-sucrose cotransporter proteins, powered by a proton gradient.
• This active transport concentrates sucrose inside companion cells against its gradient.
• Sucrose then moves passively from companion cells into sieve tube elements through connecting plasmodesmata.
• The process establishes osmotic pressure for phloem sap flow.

Why A is correct:

• Active loading via cotransporter proteins uses H+-sucrose symporters to harness electrochemical gradients, while plasmodesmata permit passive diffusion, defining the symplast-apoplast pathway in plant physiology.

Why the others are wrong:

• B: Plasmodesmata enable passive symplastic movement, not active transport.
• C: Cotransporter proteins drive active transport, not passive.
• D: Plasmodesmata support passive flow only, without active mechanisms.

Final answer: A