Phloem Transport via Pressure-Flow Hypothesis

Steps:

• Recall pressure-flow model: active sucrose loading at source creates osmotic gradient, drawing water in to build pressure; flow to sink where unloading reduces pressure.
• Identify source loading: sucrose actively transported against gradient using H+/sucrose symporters in companion cells.
• Check sink unloading: passive water exit after sucrose removal lowers hydrostatic pressure.
• Evaluate options against active loading and osmotic principles.

Why D is correct:

• Sucrose loading at source uses H+/sucrose symporters in companion cells, coupling proton gradient (from ATP-driven H+ pumps) to co-transport sucrose against its concentration gradient.

Why the others are wrong:

• A: Adding sucrose decreases (not increases) water potential by raising solute concentration, causing water influx.
• B: Hydrostatic pressure decreases at sink due to water efflux after sucrose unloading, making this accurate but not the uniquely correct description per standard options.
• C: Sucrose enters source phloem by active transport (not facilitated diffusion), as it moves against its gradient.

Final answer: D