translocation

• translocation: the movement of sugars and amino acids from the source to the sink
• translocation 1

  active transport of sucrose into companion cells which increases the concentration of sucrose
• Translocation 2

  solute diffuses from the companion cells into sieve tube elements
• translocation 3

  the increased solute concentration results in an increased hydrostatic pressure
• translocation 4

  water is drawn from the xylem into the phloem
• translocation 5

  solutes are removed from sink cells by active transport which decreases solute concentration
• translocation 6

  water is taken back by the xylem
• translocation 7

  the difference in hydrostatic pressure causes mass flow
• sink cell = a cell which uses and stores photosynthesis products
• source cell = a cell which makes photosynthesis products
• companion cell: performs metabolic functions which maintain the sieve tube
• transfer cells (next to phloem): collect photosynthesis products from source cell
• order of translocation: active transport > diffusion > pressure > water > active transport > water > mass flow = A D P W A W M
• Simple steps of translocation

  1. active transport of sucrose into companion cells
  2. diffusion of sucrose from companion cells into sieve tube elements
  3. water potential in phloem decreases
  4. water diffuses into phloem by osmosis
  5. sucrose diffuses out of the phloem and into the sink cells
  6. water potential in sink cells decreases
  7. water diffuses into sink cells by osmosis