Mass Transport

• Phloem Tissue Structure:

  • Sieve tube - living cells, no nucleus, few organelles
  • Companion cells - provide ATP for active transport
• Source-Sink Explanation:

  • Source = photosynthesising cell, sucrose via active transport to sieve tube lowers water potential, water enters via osmosis increasing the hydrostatic pressure
  • Solutes move down the pressure gradient as higher pressure at source than sink
  • Sink = respiring cell, uses the sucrose, making the water potential more positive, water leaves via osmosis lowering hydrostatic pressure
  • Pressure changes causes solution to go to sink cell via phloem to maintain the gradient
• Translocation Process:

  • Sucrose moves down concentration gradient via facilitated diffusion into companion cell
  • Active transport of H+ ions into cell wall from companion cell using ATP
  • Creates concentration gradient, H+ and Sucrose co-transport into sieve tube
  • Sucrose creates negative water potential, water moves in via osmosis from xylem, increasing hydrostatic pressure, causing movement to sink
  • Sucrose enters sink via active transport, making negative water potential, water moves in via osmosis, reducing hydrostatic pressure in sieve tube, maintaining gradient
• Sucrose in the sink is either used for respiration or for storage as insoluble starch
• Tracers like radioactively labelling carbon can be absorbed to make sugars, showing existance of phloem as thin slices can be placed on x-ray film and exposed to radioactivity
• Ringing trees of their bark and phloem, causing swelling above the ring showing it contains sugars as water is moving into the phloem