Answer: Sugar transport in phloem (source to sink via turgor pressure gradient).
- A Upward water movement in xylem driven by transpirational pull
- B Sugar transport in phloem (source to sink via turgor pressure gradient)
- C Root pressure generated by active ion accumulation in root xylem at night
- D Stomatal opening triggered by guard cell turgor changes at dawn
Correct answer: B. Sugar transport in phloem (source to sink via turgor pressure gradient)
Explanation: Pressure flow (Munch hypothesis): sugars loaded at source increase osmotic pressure, water enters by osmosis, creating pressure that pushes sap through phloem to sink.
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.