Phloem Mass Flow Mechanism

Steps:

• Mass flow in phloem follows the pressure-flow hypothesis, where sugars load at sources, drawing in water to create high hydrostatic pressure.
• This pressure pushes sap through sieve tubes toward sinks, where sugars unload and water exits, lowering pressure.
• The driving force is the hydrostatic pressure gradient between source and sink.
• Direction of sap movement is always from source (high pressure) to sink (low pressure).

Why B is correct:

• The pressure-flow hypothesis defines mass flow as driven by a hydrostatic pressure gradient, with sap translocating from sources to sinks.

Why the others are wrong:

• A: Direction reversed; sap flows source to sink, not sink to source.
• C: Water potential gradient incorrect cause; hydrostatic pressure drives bulk flow.
• D: Water potential gradient wrong; mass flow relies on hydrostatic pressure, not water potential.

Final answer: B