Phloem Transport Mechanism

Steps:

• Identify that sucrose and amino acids move in phloem tissue, not xylem.
• Recall phloem transport uses pressure-flow hypothesis, creating mass flow from high-pressure sources (e.g., leaves) to low-pressure sinks (e.g., roots).
• Note direction is always source to sink, typically one-way per pathway but adaptable to plant needs.
• Eliminate options with wrong tissue (xylem uses cohesion-tension) or reversed direction.

Why C is correct:

• Mass flow drives solutes from source to sink via turgor pressure gradients, as defined in the pressure-flow model.

Why the others are wrong:

• A: Cohesion-tension applies to xylem water transport, not phloem solutes; directions vary but mechanism is incorrect.
• B: Flow is source to sink, not reverse; mass flow direction is wrong.
• D: Cohesion-tension is xylem-specific, not for phloem; ignores mass flow.

Final answer: C