Proton-driven active loading of sucrose into phloem

Steps:

• H+-ATPase pumps protons out of companion cell cytoplasm into the apoplast (cell wall), matching statement 1.
• This establishes a proton gradient with higher concentration in the apoplast, not symplast, so statement 2 is inaccurate.
• The plasma membrane is largely impermeable to protons, preventing easy back-diffusion, but statement 3 overstates as "unable" ignores minor leaks.
• Sucrose enters companion cells via proton-sucrose symporter using the gradient, confirming statement 4.

Why B is correct:

• Statements 1 and 4 describe the core mechanism: proton pumping creates the electrochemical gradient for symport-driven sucrose uptake, per the chemiosmotic theory in plant transport.

Why the others are wrong:

• A includes 2, which misplaces high proton concentration in symplast instead of apoplast.
• C includes 2 (incorrect) and 3 (incomplete, as proton impermeability supports but does not fully define loading).
• D includes 3, which is not a primary process in sucrose loading.

Final answer: B