Mass transport

• Photosynthesis

  Carbon dioxide + water creates glucose and oxygen
• Organic substances

  Substances containing carbon, required by plant cells for respiration
• Phloem

  Tissue responsible for transporting organic substances made in the leaf during photosynthesis up and down the plant
• Sieve tube elements

  • End walls are perforated to allow continuous flow of sugar solution
  • Living cells without nucleus and few organelles, to maximise flow
• Companion cells

  Provide the ATP required for active transport of sucrose into the sieve tube elements
• Translocation in plants

  1. Photosynthesis in source (leaf) cells creates organic substances

  2. Organic substances actively transported into sieve tube elements by companion cells

  3. Dissolved organic substances lower water potential in sieve tubes

  4. Water moves into sieve tubes by osmosis, increasing hydrostatic pressure

  5. High pressure forces organic substances towards sink (respiring) cells

  6. Organic substances actively transported into sink cells, decreasing water potential

  7. Water moves out of sieve tubes by osmosis, lowering hydrostatic pressure
• Photosynthesis in source (leaf) cells

  Creates organic substances like sucrose
• Organic substances in sieve tube elements

  Lowers water potential, increasing hydrostatic pressure
• High hydrostatic pressure in sieve tubes near source

  Forces organic substances towards sink (respiring) cells
• Organic substances transported into sink (respiring) cells

  Decreases water potential, causing water to move out by osmosis and lowering hydrostatic pressure
• Investigations to identify phloem transport

  • Tracing radioactively labelled carbon
  • Ringing experiments
• Mass transport of organic substances in plants is called translocation