Phloem Sieve Tube Connectivity

Steps:

• Sieve tube elements form phloem conduits for sucrose transport, lacking nuclei and relying on adjacent cells.
• They connect end-to-end via sieve plates with pores derived from plasmodesmata for symplastic flow.
• Companion cells link to sieve tubes through plasmodesmata, enabling solute exchange.
• Evaluate options against phloem's non-lignified, living structure for mass flow.

Why D is correct:

• Plasmodesmata are cytoplasmic channels that connect adjacent plant cells, allowing symplastic movement of water and solutes between sieve tube elements and companion cells as defined in plant cell communication.

Why the others are wrong:

• A: Companion cells supply metabolic support and loading, not structural support, which is provided by xylem.
• B: Phloem sieve tubes have non-lignified walls to facilitate flexible transport; mass flow occurs for sugars and minerals via pressure gradients.
• C: Sieve tube diameter remains constant; narrowing does not occur during sucrose movement.

Final answer: D