Sieve tube structure in phloem transport

Steps:

• Recall sieve tubes are living phloem cells for sugar transport, connected end-to-end.
• Note they lack nuclei, have thin cytoplasm lining walls, and feature sieve plates with pores.
• Identify cell walls as primary cellulose, not lignified.
• Eliminate options with incorrect features like nuclei or vacuoles.

Why D is correct:

• Sieve tube elements have a thin cytoplasm layer, cellulose walls, and sieve plates, as defined in plant vascular tissue anatomy.

Why the others are wrong:

• A: Lignified walls and empty lumen describe xylem vessels, not living sieve tubes.
• B: Nuclei are absent in mature sieve tubes; dense cytoplasm is inaccurate.
• C: Tonoplast and large vacuoles are not present; sieve tubes maintain thin cytoplasm without central vacuoles.

Final answer: D