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Sourav PanNovember 9, 2024

How do plants transport organic compounds from sources to sinks?

How do plants transport organic compounds from sources to sinks?

Sourav Pan
Sourav PanNovember 9, 2024

Answered

Plants transport organic compounds from sources to sinks through a process known as translocation, primarily facilitated by the phloem tissue. Here’s an overview of how this process works:

Key Concepts

1. Sources and Sinks

  • Sources: These are parts of the plant that produce organic compounds, primarily through photosynthesis. Common sources include mature leaves and green stems.
  • Sinks: These are areas where organic compounds are utilized or stored, such as growing roots, developing fruits, and young leaves.

2. Phloem Structure

  • The phloem is composed of living cells called sieve tube elements and companion cells. Sieve tube elements lack nuclei and are connected end-to-end to form long tubes that facilitate the flow of nutrients.
  • Companion cells support sieve tube elements by performing metabolic functions and loading/unloading sugars.

Mechanism of Transport

1. Loading at the Source

  • Active Transport: Organic compounds, mainly sucrose, are actively loaded into the phloem from source tissues. This process involves:
    • Proton Pumping: Companion cells use ATP to pump protons (H+) out of the cell, creating a proton gradient.
    • Co-Transport: Protons return to the companion cell through a co-transport protein, bringing sucrose along with them against its concentration gradient (this is known as phloem loading).
    • Sugars can also move through plasmodesmata (symplastic route) from mesophyll cells to companion cells.

2. Transport Through the Phloem

  • Once loaded into the phloem, the organic compounds are transported through the sieve tubes. The movement occurs due to:
    • Pressure Flow Mechanism: The high concentration of sugars in the phloem at the source creates a high osmotic pressure, drawing water into the phloem from surrounding xylem vessels. This influx of water generates positive pressure that pushes the sap containing sugars toward sinks.
    • The flow can occur in multiple directions depending on where sources and sinks are located in the plant.

3. Unloading at the Sink

  • At sink tissues, sucrose is actively unloaded from the phloem:
    • Active Transport or Diffusion: Sucrose can be transported into sink cells either by active transport mechanisms or by facilitated diffusion through specific transport proteins.
    • Once inside sink cells, sucrose may be converted into starch or other forms for storage or used immediately for energy.

4. Water Recycling

  • After unloading, water that was drawn into the phloem returns to the xylem, maintaining a continuous flow system and reinforcing the pressure gradient necessary for effective translocation.

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