7.2.8—Mass flow in phloem

The pressure-flow hypothesis explains phloem mass flow: active sucrose loading at a source lowers the water potential of phloem sap, water enters by osmosis and raises source turgor pressure, and unloading at a sink lowers pressure there. Sap therefore moves in bulk down a hydrostatic pressure gradient.

1. Load at the source: Sucrose and other assimilates are loaded into sieve-tube elements at a source such as a photosynthesising leaf or storage organ. The higher solute concentration lowers the water potential of the phloem sap.
2. Raise source pressure: Water enters the sieve tube by osmosis, increasing hydrostatic/turgor pressure at the source.
3. Bulk movement: The pressure difference drives phloem sap—water carrying dissolved organic solutes—through the sieve tube by mass flow. This is bulk movement of the solution and is faster than diffusion of individual solute molecules.
4. Unload at the sink: Sucrose is removed at a sink for use or storage. Water follows by osmosis, so hydrostatic pressure falls at the sink and the source-to-sink pressure gradient is maintained.
5. Direction boundary: Each sieve tube carries sap in one direction at a time, but different tubes can carry sap upwards or downwards because direction follows the current source and sink. The source is not always a leaf and the sink is not always a root.

Chain cue: source loading → lower phloem water potential → water entry → high source turgor → mass flow → sink unloading → water exit/lower sink pressure.

Mass flow is not diffusion of sucrose molecules one by one, and ATP is used locally to establish loading/pressure differences rather than to push every molecule along the whole tube. This card explains the pressure-flow model; it does not claim exact pressure values or make all phloem movement globally one-way. Staff-only visual brief: source and sink with water arrows, pressure labels and a single-direction sieve-tube arrow; show a second tube in the opposite direction only to represent different source-sink pairs. Do not generate or bind an image.