Botany · Ch 1 — Transport in Plants
The Pressure Flow or Mass Flow Hypothesis
The Pressure Flow or Mass Flow Hypothesis
The Pressure Flow (Mass Flow) Hypothesis
The generally accepted explanation for how sugars travel from source to sink through the phloem is the pressure flow hypothesis (also called the mass flow hypothesis). It proceeds in a clear cycle:
- At the source: glucose made in photosynthesis is converted into sucrose, which is then moved — first into companion cells, then into the living sieve-tube cells — by active transport.
- This active loading makes the phloem sap at the source hypertonic, so water moves in from the adjacent xylem by osmosis, raising the local hydrostatic (osmotic) pressure.
- This pressure difference pushes phloem sap to flow toward regions of lower pressure elsewhere in the plant.
- At the sink: sucrose is actively removed from the phloem sap into the surrounding cells, which either use it for energy, store it as starch, or build it into cellulose. As sugar is removed, the local osmotic pressure falls, and water correspondingly moves back out of the phloem — largely returning to the xylem.
In short: active loading of sugar at the source sets up the water-potential gradient that drives the whole bulk (mass) flow of sap toward the sink. Structurally, phloem is built from sieve-tube cells joined end to end through perforated sieve plates, with cytoplasmic strands threading through the small pores to link the cells into a continuous conducting column (Figure 11.10). …
What this figure shows. A stylised teal-green plant silhouette (a Y-shaped stem with a leaf at upper right, a leaf-tip/bud at upper left, and roots at the bottom) with a pale tube (the phloem, leader-labelled 'Phloem' at the centre fork) running through the stem from the leaf down to the root, drawn as a sequence of small rectangular sieve-tube segments each containing small red, yellow and black dots (sugar/solute particles) with density shown by shading intensity (a small swatch key '=High turgor pressure' at upper right). Leader-labelled captions around the figure read: 'Sugars enter sieve tubes; water follows by osmosis' (at the leaf, upper right, where dot-density is highest), 'Sugars leave sieve tubes; water follows by osmosis' (at the tip of stem, upper left, and again at the root, lower centre, both leader-lines pointing to lower dot-density regions), and 'Sugar solution flows to regions of low turgor pressure' with a vertical double-headed arrow drawn beside the main stem segment between the leaf/source and the tip-of-stem and root sinks — showing sugars loaded at the lea …