Mass Transport

    Cards (6)

    • 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