pH gradient drives active sucrose loading Steps:

• Proton pumps (H+-ATPase) in companion cell plasma membranes export H+ to the apoplast (cell wall), acidifying it.
• This establishes a pH gradient: lower pH (higher [H+]) in wall than in cytoplasm.
• Sucrose-H+ symporters use the H+ influx down this gradient to actively transport sucrose against its concentration gradient into companion cells.
• The correct statement reflects this wall-cytoplasm pH difference to support symport mechanism.

Why C is correct:

• It states pH_wall < pH_cytoplasm, creating the proton motive force for sucrose-H+ symport as defined in chemiosmotic transport theory.

Why the others are wrong:

• A: Reverses the gradient (pH_cytoplasm < pH_wall), preventing H+ influx needed for symport.
• B: Falsely lowers pH in both cell types versus wall and wrongly includes sieve elements, irrelevant to companion cell loading.
• D: Compares pH between sieve tubes and companion cells, not addressing the apoplast-cytoplasm gradient for loading.

Final answer: C