Sucrose transport in phloem follows the pressure-flow mechanism

Steps:

• Recall that phloem transports sugars like sucrose from source (leaves) to sink (roots, fruits).
• Understand sieve tube elements form continuous tubes for bulk flow under pressure.
• Link high sucrose concentration at source to water influx, creating turgor pressure.
• Conclude this drives mass flow of sucrose solution through sieve tubes to sinks.

Why D is correct:

• Mass flow describes bulk movement of fluid driven by pressure gradients, as in the pressure-flow hypothesis where sucrose solution moves passively along phloem.

Why the others are wrong:

• A: Active transport requires energy for uphill movement, but sucrose flows down a pressure gradient without ATP.
• B: Facilitated diffusion uses carriers for passive movement, but sucrose travels in bulk solution, not individually.
• C: Simple diffusion is too slow for long-distance transport over meters in plants.

Final answer: D