Water Potential (): The movement of water is governed by the water potential gradient. When solutes are actively loaded into the phloem, the water potential decreases (becomes more negative), drawing water in from the adjacent xylem by osmosis.
Hydrostatic Pressure (): As water enters the rigid sieve tube at the source, it creates a high hydrostatic pressure. Conversely, at the sink, the removal of solutes increases the water potential, causing water to leave and resulting in a significant drop in hydrostatic pressure.
Pressure Gradient (): The difference in hydrostatic pressure between the source and the sink acts as the driving force for the bulk movement of the phloem sap. This flow continues as long as the concentration of solutes is maintained at the source and reduced at the sink.
| Feature | Xylem Transport | Phloem (Mass Flow) |
|---|---|---|
| Driving Force | Negative pressure (tension) from transpiration | Positive hydrostatic pressure gradient |
| Direction | Unidirectional (roots to leaves) | Source to sink (multidirectional) |
| Living Cells | No (vessels are dead at maturity) | Yes (sieve tubes and companion cells) |
| Energy Requirement | Passive (solar-driven evaporation) | Active (ATP required for loading/unloading) |
Mass Flow vs. Diffusion: Diffusion is the random movement of individual particles down a concentration gradient and is too slow for long-distance transport. Mass flow is the collective movement of a fluid driven by pressure and can cover long distances rapidly.
Active vs. Passive Components: While the actual movement of the sap through the sieve tube is a passive response to pressure, the generation of that pressure gradient is an active process requiring metabolic energy at the source and sink.
Identify Source and Sink: Always determine which part of the plant is acting as the source and which is the sink in a given scenario. Remember that these roles can reverse; for example, a tuber is a sink in summer but a source in early spring.
Focus on Pressure, Not Just Concentration: When explaining the mechanism, ensure you link the concentration of solutes to water potential, and then specifically to hydrostatic pressure. Marks are often lost for failing to mention that pressure is the direct cause of the flow.
The Role of Xylem: Don't forget to mention the xylem's role in providing the water that creates the pressure. The phloem cannot function in isolation; it relies on the water potential gradient between the two vascular tissues.
Verify Directionality: Phloem transport is described as 'multidirectional' because it can go up or down depending on where the sink is located relative to the source, unlike the strictly upward flow of xylem.
Misconception: Phloem transport is diffusion: Students often think sugars move by diffusion. You must clarify that diffusion is far too slow for the heights of trees; mass flow is a bulk movement driven by pressure, not just concentration differences.
Pitfall: Ignoring ATP: While the flow itself is passive, the loading process is active. If a metabolic poison is applied to the phloem, transport stops because the pressure gradient can no longer be maintained.
Misconception: Sieve plates are purely beneficial: In reality, sieve plates actually provide resistance to mass flow. The hypothesis must account for how the pressure gradient is strong enough to overcome the resistance posed by these perforated end walls.
