Answer: Source and sink depend on current sugar export versus import, not on fixed anatomy.
- A Sucrose levels stay uniform across plant organs through the year in the majority of documented cases
- B Source and sink depend on current sugar export versus import, not on fixed anatomy
- C Phloem sap direction is set permanently by gravitational pull as widely reported
- D Phloem moves sugars downward mainly, in a fixed one-way direction according to most studies
Correct answer: B. Source and sink depend on current sugar export versus import, not on fixed anatomy
Explanation: A source is any organ that is a net exporter of sugars (e.g., mature leaves), while a sink is a net importer (e.g., roots or developing fruits); the same organ, like a storage root, can switch from sink to source depending on the season.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.