Answer: Flow direction depends on relative source and sink position, not fixed anatomy.
- A Flow direction depends on relative source and sink position, not fixed anatomy
- B Phloem transports mainly water, for which direction is irrelevant in most reference accounts
- C Sucrose can move mainly from root to leaf, rarely the reverse under normal conditions
- D Phloem sap flow is fixed in one direction by gravity as frequently documented
Correct answer: A. Flow direction depends on relative source and sink position, not fixed anatomy
Explanation: Unlike the unidirectional upward flow in xylem, phloem transport is bidirectional, moving sugars from whichever organ is currently a source to whichever organ is currently a sink, regardless of position.
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.