The movement is governed by the relationship between solute potential () and pressure potential ().
Active transport of sucrose into the phloem requires ATP, making the loading process an active physiological event, even if the subsequent flow is physical.
The sieve plates within the phloem act as structural supports but also provide resistance to flow, which is a point of contention in the hypothesis.
The hypothesis assumes that the concentration of sucrose is significantly higher in the source than in the sink, maintaining the osmotic gradient.
Sieve Plate Resistance: Calculations suggest that the resistance provided by sieve plates would require a much higher pressure gradient than is actually measured in plants.
Solute Velocity: Different solutes (e.g., amino acids vs. sugars) have been observed moving at different speeds, which contradicts the idea of a uniform mass flow.
Bidirectional Movement: In some experiments, solutes appear to move in opposite directions within the same sieve tube simultaneously, which a single pressure gradient cannot explain.
Function of Sieve Plates: If mass flow were the only mechanism, sieve plates would seem to be an evolutionary disadvantage; their presence suggests they might play a role in maintaining pressure or preventing sap loss during injury.
| Feature | Mass Flow (Phloem) | Transpiration (Xylem) |
|---|---|---|
| Driving Force | Hydrostatic pressure (Push) | Tension/Negative pressure (Pull) |
| Energy Requirement | Active loading (ATP required) | Passive (Solar energy driven) |
| Direction | Source to Sink (Variable) | Roots to Leaves (Unidirectional) |
| Substances | Organic solutes (Sucrose) | Water and Minerals |
Identify the Source and Sink: Always determine which part of the plant is producing/releasing sugar (source) and which is using/storing it (sink) before applying the hypothesis.
Link Osmosis to Pressure: When explaining the mechanism, you must explicitly state that the entry of water via osmosis is what increases the hydrostatic pressure.
Evaluate, Don't Just Describe: If a question asks to 'evaluate', you must provide both supporting evidence (e.g., aphid studies) and contradictory evidence (e.g., different solute speeds).
Common Mistake: Do not confuse water potential with hydrostatic pressure. Water potential drives the entry of water, while hydrostatic pressure is the result of that entry within a confined space.
