STAGES

• WHAT ARE THE STAGES OF TRANSLOCATION?

  1. Sucrose from source to sieve
  2. Sucrose within sieve tube element
  3. Sucrose to sink
• WHAT HAPPENS IN THE SUCROSE FROM SOURCE TO SIEVE?

  1. Photosynthesis in chloroplasts creates organic substances (e.g. sucrose)
  2. Creates high concentration gradient of sucrose at production site
  3. Sucrose diffuses down concentration gradient into companion cell (facilitated diffusion)
  4. Active transport of H+ occurs from companion cell into spaces within cell walls using energy
  5. Creates a concentration gradient
  6. H+ move down their concentration gradient via carrier proteins in sieve tube elements
  7. Co-transport of sucrose with H+ ions occurs via protein co-transporters to transport sucrose into STE
• WHAT HAPPENS IN SUCROSE WITHIN SIEVE TUBE ELEMENT?

  1. Increase of sucrose in STE lowers water potential
  2. Water enters STE from surrounding xylem by osmosis
  3. Increase in water volume in STE causes increase in hydrostatic pressure
  4. Liquid forced towards sink
• WHAT HAPPENS IN SUCROSE TO SINK?

  1. Sucrose used in respiration at sink or stored as insoluble starch
  2. More sucrose actively transported into sink cell
  3. Water potential decreases
  4. Causing osmosis of water from STE into sink cell (some returns to xylem)
  5. Removal of water decreases volume in STE
  6. Hydrostatic pressure decreases
  7. Movement of soluble organic substances is due to difference in hydrostatic pressure between source and sink end of STE