Phloem primarily pushes fluids, not pulls them.
How Phloem Transport Works
Phloem is the vascular tissue in plants responsible for transporting sugars, produced during photosynthesis, from source (leaves) to sink (other parts of the plant, such as roots, stems, and fruits). Instead of relying on a pulling mechanism like xylem which transports water, phloem utilizes a process called pressure flow or mass flow.
Pressure Flow Mechanism
- Loading at the Source: Sugars, primarily sucrose, are actively transported into the phloem sieve tubes at the source (e.g., leaves). This increases the sugar concentration and reduces the water potential inside the sieve tube.
- Water Entry: Due to the reduced water potential, water from the adjacent xylem flows into the sieve tubes via osmosis. This increases the turgor pressure (pressure exerted by the fluid on the cell wall).
- Pressure Gradient: The high turgor pressure at the source end of the phloem creates a pressure gradient. This means there's a higher pressure at the source compared to the sink.
- Bulk Flow (Pushing): Driven by the pressure gradient, the phloem sap (sugar-rich fluid) is pushed down (and up) the phloem towards the areas of lower pressure – the sinks.
- Unloading at the Sink: At the sink (e.g., roots), sugars are actively transported out of the phloem sieve tubes, into the cortex cells, where they are then consumed or converted to starch. This reduces the sugar concentration and water potential within the sieve tube at the sink.
- Water Movement: Water leaves the phloem at the sink due to the increased water potential, often returning to the xylem.
- Reference Note: As the fluid is pushed down (and up) the phloem, sugars are removed by the cortex cells of both stem and root (the "sinks") and consumed or converted into starch.15-Dec-2021
Examples of Pressure Flow in Action
- Sugar Transport to Roots: In the fall, plants transport sugars from the leaves down to the roots for storage over winter. The phloem sap is pushed towards the roots.
- Fruit Development: During fruit development, sugars are transported from the leaves to the developing fruits. The phloem is actively pushing the sap towards the developing fruit as a sink.
- Stem Growth: When a plant is growing, sugars are pushed to the stem and other developing parts of the plant.
Table: Comparison of Xylem and Phloem Transport
| Feature | Xylem | Phloem |
|---|---|---|
| Substance | Water and minerals | Sugars and other organic compounds |
| Direction | Upward only | Both upward and downward |
| Mechanism | Pulling (transpirational pull) | Pushing (pressure flow) |
| Living Cells | Primarily dead cells, no cytoplasm | Living cells (sieve tube elements, companion cells) |
| Energy Usage | Passive (mostly) | Active (at loading and unloading points) |
In conclusion, phloem uses the pressure flow mechanism to push fluids from source to sink.
