Phloem

Structure:

• The phloem consists of long tubes made of living cells called sieve tube elements.

• The ends of these cells form sieve plates, which allow cytoplasm and solutes to pass through.

• Sieve tube elements rely on companion cells for functions like respiration and waste removal.

Function:

• The primary role of the phloem is to transport organic molecules (e.g., sucrose) both up and down the plant.

• In addition to sucrose, the phloem transports:

  • Amino acids

  • Hormones

  • Some minerals

Translocation

Phloem tissue moves solutes from source cells (e.g., leaves, where substances are produced) to sink cells (parts of the plant where substances are needed or stored). This process is known as translocation.

How Translocation Works:

1. Loading solutes into sieve tubes:

   • Solutes are actively transported from source cells into the phloem, with the help of companion cells.

   • This lowers the water potential inside the sieve tubes.

2. Water movement:

   • Water enters the sieve tubes by osmosis, creating high pressure near the source cells.

3. Unloading solutes at sink cells:

   • Solutes are actively transported out of the phloem into the sink cells.

   • This increases the water potential inside the sieve tubes.

4. Water movement:

   • Water leaves the sieve tubes by osmosis, creating low pressure near the sink cells.

5. Flow of water and solutes:

   • Water and solutes flow down the pressure gradient in the phloem from the source to the sink.

The process of translocation in phloem

Evidence for translocation

Ringing Experiments:

• Method: A ring of phloem is removed from the circumference of a woody stem.

• Observation: A swelling forms above the ring.

  • This indicates that solutes are transported down the stem and accumulate above the removed phloem.

Tracer Experiments:

• Method: Radioactive isotopes (e.g., carbon-14) are used to label carbon-containing compounds.

• Findings:

  • Sugars are observed moving both upwards (e.g., to flowers) and downwards (e.g., to roots), demonstrating bidirectional transport in the phloem.

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