Phloem transport via pressure-flow hypothesis

Steps:

• Sucrose loading at source raises solute concentration, lowering water potential.
• Water enters sieve tubes osmotically from xylem, building hydrostatic pressure.
• At sink, sucrose unloading reduces solute concentration, allowing water exit and pressure drop.
• Hydrostatic pressure difference propels bulk sap flow from high to low pressure.

Why B is correct:

• Mass flow in phloem follows the pressure-flow hypothesis, where hydrostatic pressure gradient (ΔP) drives bulk movement, as P = osmotic pressure + turgor pressure.

Why the others are wrong:

• A: Diffusion gradients enable passive molecular spread, not bulk mass flow in sieve tubes.
• C: Water potential gradient facilitates water influx but does not directly drive sap movement; pressure does.
• D: Concentration gradient of sucrose triggers osmosis indirectly but is not the force for mass flow.

Final answer: B