Translocation is the process by which plants transport soluble organic compounds, especially sugars, from source tissues (like leaves) where they are produced by photosynthesis to sink tissues (like roots, stems, and developing fruits) where they are used or stored.
Here's a breakdown of how translocation works:
1. Photosynthesis and Sugar Production
- The process begins in the leaves, where photosynthesis takes place.
- During photosynthesis, plants convert carbon dioxide and water into glucose (a simple sugar) using light energy.
- This glucose is then often converted into sucrose, a more stable and transportable sugar.
2. Loading into the Phloem
- Sucrose needs to be actively transported into the phloem sieve tubes. This requires energy (ATP).
- The phloem is the vascular tissue responsible for transporting sugars throughout the plant. It consists of sieve tube elements (long, specialized cells) and companion cells (which support the sieve tube elements).
- The pressure flow hypothesis is the most widely accepted model for translocation. It suggests that sugars are actively loaded into the phloem, creating a high solute concentration.
- This high solute concentration lowers the water potential inside the phloem. Water then moves into the phloem from the adjacent xylem (the water-conducting tissue) by osmosis.
3. Pressure Gradient and Bulk Flow
- The influx of water increases the pressure inside the phloem at the source (leaves).
- At the sink tissues (roots, fruits, etc.), sugars are actively unloaded from the phloem, reducing the solute concentration.
- This increases the water potential, and water moves out of the phloem back into the xylem by osmosis.
- The removal of water decreases the pressure inside the phloem at the sink.
- The difference in pressure between the source (high pressure) and the sink (low pressure) drives the bulk flow of phloem sap (the sugar-rich solution) from source to sink.
4. Unloading at the Sink
- At the sink tissues, sucrose is actively transported out of the phloem.
- Once unloaded, the sugars are used for growth, respiration, or storage as starch or other complex carbohydrates.
Summary
Translocation is driven by a pressure gradient established through active loading and unloading of sugars at source and sink tissues, respectively. This pressure gradient drives the bulk flow of phloem sap, transporting sugars throughout the plant to where they are needed.
| Process | Location | Description | Energy Required? |
|---|---|---|---|
| Photosynthesis | Leaves | Production of sugars (glucose, then sucrose) | Yes |
| Phloem Loading | Source Tissues | Active transport of sucrose into phloem, lowering water potential. | Yes |
| Water Influx | Source Tissues | Water moves from xylem to phloem due to osmosis, increasing pressure. | No |
| Bulk Flow | Phloem | Movement of phloem sap from source to sink due to pressure gradient. | No |
| Phloem Unloading | Sink Tissues | Active transport of sucrose out of phloem, increasing water potential. | Yes |
| Water Efflux | Sink Tissues | Water moves from phloem to xylem due to osmosis, decreasing pressure. | No |
| Sugar Utilization/Storage | Sink Tissues | Sugars used for growth, respiration, or converted to storage compounds. | Yes |
