Phloem source and sink roles in transport

Steps:

• Phloem transports sugars from sources (production/loading sites) to sinks (storage/consumption sites) via sieve tubes and companion cells.
• Source cells actively load sucrose into phloem for translocation.
• Sink cells unload sucrose for use or storage.
• Diagrams typically label source (e.g., leaf mesophyll) and sink (e.g., root) relative to sieve elements.

Why B is correct:

• Source cells load sugars into phloem and can include non-photosynthetic tissues like storage organs releasing reserves, per mass flow theory.

Why the others are wrong:

• A: Sinks unload sugars from phloem; cell Y likely represents a companion cell or sieve element, not a sink.
• C: Sources are often photosynthetic but can be storage sites mobilizing sugars, so not required.
• D: Glucose moves directly from source cells to sieve tubes via plasmodesmata; companion cells assist but do not mediate via Y.

Final answer: B