Phloem Loading

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•  Active transport is used to load organic compounds into phloem sieve tubes at the source

Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells

  • Materials can pass into the sieve tube via interconnecting plasmodesmata (symplastic loading)
  • Alternatively, materials can be pumped across the intervening cell wall by membrane proteins (apoplastic loading)

Apoplastic loading of sucrose into the phloem sieve tubes is an active transport process that requires ATP expenditure

  • Hydrogen ions (H+) are actively transported out of phloem cells by proton pumps (involves the hydrolysis of ATP)
  • The concentration of hydrogen ions consequently builds up outside of the cell, creating a proton gradient
  • Hydrogen ions passively diffuse back into the phloem cell via a co-transport protein, which requires sucrose movement
  • This results in a build up of sucrose within the phloem sieve tube for subsequent transport from the source

Phloem Loading

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