Ringing Experiments: Removing a ring of bark (which contains the phloem but leaves the xylem intact) results in a bulge forming above the ring. This bulge contains a high concentration of sugars, demonstrating that the phloem is the primary route for downward sugar transport and that its removal blocks the flow.
Tracer Studies: Using radioactive isotopes like allows scientists to track the movement of sugars produced during photosynthesis. These tracers show that sugars move from the leaves to other parts of the plant, following the predicted source-to-sink pathways regardless of gravity.
Differential Solute Speeds: Observations have shown that different solutes, such as amino acids and sucrose, move at different rates within the same sieve tube. According to the mass flow hypothesis, all substances dissolved in the sap should move at the same velocity as part of a single bulk flow.
Bidirectional Movement: Experiments have detected different substances moving in opposite directions within the same sieve tube simultaneously. Mass flow implies a unidirectional movement driven by a single pressure gradient, making these observations difficult to reconcile with the theory.
The Role of Sieve Plates: Sieve tubes contain sieve plates that appear to act as barriers, creating resistance to flow. If the goal of the system is efficient bulk flow, the evolutionary presence of these restrictive structures is counter-intuitive and remains a point of contention.
| Feature | Mass Flow | Diffusion |
|---|---|---|
| Driving Force | Hydrostatic pressure gradient | Concentration gradient |
| Speed | Relatively fast (bulk movement) | Extremely slow over long distances |
| Energy | Indirectly active (ATP for loading) | Entirely passive |
| Mechanism | All solutes move together | Solutes move independently |
Evaluate Command Word: When asked to 'evaluate' the mass flow hypothesis, you must provide both supporting evidence (e.g., ringing, pressure) and contradictory evidence (e.g., solute speeds, bidirectional flow).
Check the Mechanism: Always specify that ATP is required for active loading at the source, not for the bulk flow itself. The flow is a physical response to the pressure gradient created by osmosis.
Identify Source and Sink: In data-based questions, identify which tissue is producing sugar (source) and which is consuming it (sink) to predict the direction of flow. Remember that a tissue can switch roles depending on the season (e.g., a tuber in spring vs. autumn).
