How do plants transport organic compounds from sources to sinks?

Sourav PanNovember 9, 2024

Answered step-by-step

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.