Translocation

• Sucrose - transport of sugar in plants

  • starch is insoluble - hard to transport
  • glucose is too metabolically active and would increase the oncotic pressure of phloem vessels
• Sources


  • where it is made (leaves)
  • where it is stored (roots)
  • higher concentration of sugar compared to sinks
• Sinks


  • where it is stored (roots)
  • where it is used (meristem tissue or areas of growth, so anywhere)
  • lower concentration of sugar compared to sources
• Sources and sinks


  • Generally travels from sources to sinks
  • enzymes maintain gradients - conversion of sucrose from starch would increase conc. of sucrose
  • INVERTASE - breaks down sucrose into glucose and fructose, lowering conc. of sucrose
  • in spring - source is often the roots, sink is leaves
• Mass flow in translocation

  1. active transport into phloem at source, lowering water potential at source, water enters phloem at source by osmosis, high pressure at source end
  2. solutes are removed from sink end of phloem, water potential of phloem at sink increases, water leaves phloem by osmosis, low pressure at sink end
  3. pressure gradient from source to sink end, gradient pushes solutes along sieve tubes to where they're needed
• Active loading (step one of translocation)

  • movement of substances into companion cells and then into sieve tubes, against a concentration gradient
  • involves, ATP, H+ and cotransport proteins

  1. H+ ions are moved into surrounding tissue using ATP (active transport), creating a concentration gradient
  2. H+ and sucrose binds to co-transport protein in the companion cell membrane to reenter the cell (facilitated diffusion), sucrose is moving against the concentration gradient

  • same process is used to get them out of the phloem