translocation

• what is translocation?

  Movement of sugars and other organic molecules in plants from one part of the plant to another
• where does translocation happen?

  phloem
• what are the two key cells in the phloem?

  1. sieve tubes elements
  2. companion cells
• what are features of sieve tube elements?

  1. living cells
  2. no nucleus
  3. few organelles
  4. end walls are perforated
• what are companion cells?

  they provide ATP for the co-transport of organic substances
• what is a source?


  the site of production
• what is a sink?

  where organic substances are used for respiration
• give a brief explanation on how the mass flow hypothesis works?

  • at the source cell, sucrose dissolves and lowers the water potential so water moves in by osmosis
  • at the sink cell, sucrose is being used so has a positive water potential so water moves out by osmosis
  • due to a higher hydrostatic pressure at the source cell, and a lower hydrostatic pressure at the sink cell, the solution is pushed from the sink cell to the source cell
• how does translocation occur?

  1. H+ moves into source cells by active transport
  2. H+ ions and sucrose moves into the companion cell by co-transport (facilitated diffusion)
  3. sucrose diffuses into the phloem moving down the concentration gradient which lowers the water potential
  4. water moves into the phloem increasing hydrostatic pressure
  5. water moves from higher hydrostatic pressure from source cells to sink cells where theres lower hydrostatic pressure
  6. sucrose is removed from the phloem via diffusion or active transport and increases water potential so water goes back to the xylem via osmosis
• what are two methods to investigate translocation?

  1. tracers
  2. ringing experiment
• how does tracing work?

  1. radioactively label carbon and supply to a plant
  2. all the carbons in the sugars produced by photosynthesis will be radioactive
  3. sliced form the stem will be placed on x-ray films that will turn black
• how does the ringing experiment work?

  1. slice out a ring of bark and phloem
  2. the trunk above will swell
  3. taking a sample of the liquid causing the swelling will be full of sugars