The mechanism is driven by osmotic potential changes. When sucrose is actively loaded into the phloem at the source, it significantly lowers the water potential () inside the sieve tube.
This low water potential causes water to move from the adjacent xylem into the phloem by osmosis. Because the phloem cells have rigid walls, the influx of water creates a high hydrostatic pressure () at the source end.
At the sink, sucrose is removed from the phloem for use or storage, which increases the water potential. Water then leaves the phloem and returns to the xylem, resulting in a low hydrostatic pressure at the sink end.
The resulting pressure gradient () forces the entire mass of phloem sap to move through the sieve tubes toward the sink.
| Feature | Phloem (Mass Flow) | Xylem (Cohesion-Tension) |
|---|---|---|
| Driving Force | Hydrostatic pressure gradient (Active/Passive) | Transpiration pull/Negative pressure (Passive) |
| Direction | Source to Sink (Bidirectional overall) | Roots to Leaves (Unidirectional) |
| Cell State | Living cells (Sieve tubes) | Dead cells (Vessels/Tracheids) |
| Energy Requirement | Requires ATP for loading/unloading | Primarily solar energy (evaporation) |
Identify the Source and Sink: In exam questions, the 'source' isn't always the leaf. For example, in early spring, a storage tuber (like a potato) acts as the source, and the growing buds act as the sink.
Explain the Water Potential: Always link the movement of water to the change in solute concentration. Use the phrase 'lowers water potential' to explain why water moves from xylem to phloem.
Pressure vs. Concentration: Remember that while concentration gradients initiate the process, it is the hydrostatic pressure gradient that actually moves the sap over long distances.
Check for Metabolic Inhibitors: If a question mentions that a metabolic poison (like cyanide) stops translocation, it is because the active loading of sucrose requires ATP from respiration.
Misconception: Thinking that the sap moves by diffusion. Diffusion is far too slow to account for the observed speeds of translocation in large plants; mass flow is the only viable physical explanation.
Pitfall: Forgetting the role of the xylem. The mass flow hypothesis cannot function without the xylem providing a reservoir of water to create the necessary pressure at the source.
Misconception: Assuming phloem cells are dead like xylem. Sieve tube elements must be living to maintain the semi-permeable membranes required for osmosis and to support the active transport performed by companion cells.
