Phloem transport, also known as translocation, works by moving sugars (primarily sucrose), amino acids, and other organic molecules from sources (areas of production, like leaves) to sinks (areas of storage or use, like roots, developing fruits, and growing stems) throughout the plant. This process relies on a pressure-flow mechanism.
Here's a breakdown of the process:
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Sugar Loading at the Source:
- Active Transport: Sugars, mainly sucrose, are actively transported from source cells (e.g., photosynthetic cells in leaves) into the companion cells. This often involves a proton (H+) pump that creates a concentration gradient, which is then used to co-transport sucrose via a sucrose-H+ symporter.
- Movement into Sieve Elements: From the companion cells, the sucrose moves into the sieve tube elements through plasmodesmata (small channels connecting plant cells).
- Increased Solute Concentration: The high concentration of sugar in the sieve tube elements lowers the water potential inside the sieve tubes.
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Water Uptake and Pressure Gradient:
- Osmosis: Due to the lower water potential in the sieve tube elements, water enters from the adjacent xylem vessels via osmosis.
- Turgor Pressure Increase: The influx of water increases the turgor pressure (hydrostatic pressure) inside the sieve tube elements at the source.
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Pressure-Flow Mechanism:
- Bulk Flow: The high turgor pressure at the source drives the phloem sap (sugar-rich solution) through the sieve tubes towards the sink. This movement is a mass flow driven by a pressure gradient.
- Sieve Plates: Sieve plates, which are perforated end walls between sieve tube elements, allow the flow of sap while still providing some structural support.
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Sugar Unloading at the Sink:
- Active or Passive Transport: Sugars are unloaded at the sink cells (e.g., root cells, developing fruits). Unloading can occur through active transport, where energy is required to move sugars against a concentration gradient, or passive transport, where sugars move down the concentration gradient.
- Water Potential Increase: As sugars are removed from the sieve tube elements at the sink, the water potential inside increases.
- Water Exits: Water then moves out of the sieve tube elements and back into the xylem via osmosis, reducing the turgor pressure at the sink.
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Water Recirculation: The water that exits the phloem near the sink re-enters the xylem and is transported back to the leaves, maintaining a continuous flow of water throughout the plant.
In essence, phloem transport is a continuous cycle of sugar loading at the source, water uptake, pressure-driven bulk flow, sugar unloading at the sink, and water recirculation. This efficient system ensures that all parts of the plant receive the necessary nutrients for growth, development, and survival.
