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How is Sugar Transported in Phloem?

Published in Plant Transport 2 mins read

Sugar, primarily in the form of sucrose, RFOs (Raffinose family oligosaccharides), and polyols, is transported within the phloem through specialized cells called sieve tubes.

Here's a breakdown of the process:

  • Loading into Sieve Tubes: At the source, such as leaves, sugars are actively loaded into the sieve tube elements. This process often requires energy, as sugars are moved against a concentration gradient.
  • Hydrostatic Pressure Generation: The active transport of sugars into the sieve tubes increases the solute concentration. This causes water to move from the xylem into the sieve tubes by osmosis. The influx of water generates a high hydrostatic pressure within the sieve tubes.
  • Bulk Flow: The high pressure at the source end of the phloem drives the movement of the phloem sap, which contains the dissolved sugars, towards areas of lower pressure, known as sinks (e.g., roots, fruits, growing tissues). This is referred to as bulk flow or pressure flow.
  • Unloading at Sink: When phloem sap reaches sink tissue, sucrose and other sugars are actively unloaded by these tissues.
  • Recycling of Water: Water within phloem sap returns to the xylem as sugars are being unloaded.
Process Description
Sugar Loading Active transport of sugars into sieve tube elements at the source.
Water Influx Water moves from xylem to sieve tubes due to increased solute concentration.
Hydrostatic Pressure High pressure builds up in the sieve tubes.
Bulk Flow/Pressure Flow Phloem sap moves from high-pressure source to low-pressure sink due to the pressure.
Sugar Unloading Sugars are unloaded at the sink for utilization or storage.
Water Recycling Water returns to the xylem after sugars are unloaded.

This process is essential for the plant to distribute the products of photosynthesis (sugars) from the leaves to other parts where they are needed for growth and storage. The movement of sucrose, RFOs and polyols is transported in the sieve tubes to the sink organs.