Bidirectional Phloem Transport via Separate Sieve Tubes

Steps:

• Phloem transports nutrients like sucrose and amino acids from sources (e.g., leaves) to sinks (e.g., roots or fruits) via mass flow.
• Mass flow follows pressure gradients created by active loading at sources and unloading at sinks.
• Plants have multiple sources and sinks in different directions, requiring separate pathways.
• Different sieve tube elements carry mass flow in opposite directions simultaneously to serve various sinks.

Why D is correct:

• Mass flow is unidirectional within each sieve tube but occurs in different directions in separate tubes, allowing simultaneous bidirectional transport as defined by the pressure-flow hypothesis.

Why the others are wrong:

• A: Active transport loads/unloads solutes but does not drive bulk movement; mass flow occurs in all sieve tubes.
• B: Mass flow is unidirectional in each individual tube due to pressure gradients, not bidirectional simultaneously.
• C: Active transport occurs at source/sink interfaces, while mass flow handles bulk directional transport in each tube.

Final answer: D