Plasmodesmata as membrane-lined channels in plant cells

Steps:

• Recall plasmodesmata connect cytoplasm of adjacent plant cells through cell walls.
• Identify their structure: narrow channels lined by plasma membrane extensions.
• Evaluate options against definition: channels for symplastic transport, not apoplastic or lignified.
• Select matching description: A aligns with membrane lining.

Why A is correct:

• Plasmodesmata are cytoplasmic channels through plant cell walls, continuous with and lined by the plasma membrane, allowing symplastic transport of substances.

Why the others are wrong:

• B: Plasmodesmata form from membrane extensions, not primarily proteins.
• C: Apoplast pathway involves cell walls and extracellular spaces; plasmodesmata enable symplast movement, not water via apoplast.
• D: Lignified pits occur in xylem vessel walls for water conduction; plasmodesmata are not lignified.

Final answer: A