Phloem transport via pressure-flow hypothesis

Steps:

• Sucrose is actively loaded into sieve tubes at the source, decreasing water potential.
• Water enters sieve tubes from xylem, generating high hydrostatic pressure at the source.
• At the sink, sucrose is unloaded, increasing water potential and causing water to exit.
• The resulting hydrostatic pressure gradient drives mass flow of sucrose solution through phloem.

Why C is correct:

• The pressure-flow hypothesis states that phloem sap moves from high to low hydrostatic pressure, enabling bulk transport of assimilates.

Why the others are wrong:

• A: Water potential gradients facilitate water entry/exit but do not directly drive sap flow through sieve tubes.
• B: ATP powers active sucrose loading at the source but not the bulk transport in phloem.
• D: Not enough information.

Final answer: C