Phloem Mass Flow Driven by Pressure Gradient

Steps:

• Identify key components: Phloem transports sucrose from sources (production sites) to sinks (storage/use sites) via sieve tubes.
• Recall mechanism: Sucrose loading at source lowers water potential, drawing water in to create high hydrostatic pressure.
• Understand flow: Pressure pushes sap through sieve tubes toward sinks, where unloading raises water potential and lowers pressure.
• Evaluate options: Match descriptions to pressure-flow hypothesis for correct statement.

Why D is correct:

• In the pressure-flow hypothesis, sucrose solution moves via mass flow down a hydrostatic pressure gradient from high-pressure sources to low-pressure sinks.

Why the others are wrong:

• A: Sources load sucrose; sinks unload it.
• B: Loading sucrose lowers water potential, drawing water in to increase turgor pressure.
• C: Mass flow is bulk transport driven by pressure, faster than diffusion.

Final answer: D