3.3.9 Translocation

Question 1

Source examples

Answer 1

• Green leaves and green stem
• Storage organs (tubers and tap roots)
• Food stores in seeds (germinating)

Question 2

Sink examples

Answer 2

• Meristem
• Growing roots or roots that are actively transporting mineral ions
• Storage organs (fruits, seeds, etc)

Question 3

Why is sucrose used as the transport molecule?

Answer 3

• Less reactive than glucose
• Compact, good for energy storage

Question 4

Translocation hypothesis

Answer 4

1. H+ ions actively pumped out of companion cell into cell wall using ATP
2. High concentration of H+ ions in cell wall causes facilitated diffusion down a concentration gradient of H+ ions AND SUCROSE, which is cotransported back into companion cell by transport proteins
3. High concentration of sucrose in companion cell, so diffuses through plasmodesmata into sieve tube element. Actively done as well
4. Water in xylem diffuses into sieve tube element by osmosis down the concentration gradient due to the increased concentration of sucrose in sieve tube, increasing hydrostatic pressure at the source
5. Sap moves down sieve tube from a higher hydrostatic pressure at source to a lower hydrostatic pressure at sink
6. Sucrose is actively transported out of plasmodesmata at the sink, increasing water potential so water moves out of sieve tube, decreasing hydrostatic pressure
7. To maintain concentration gradient from sieve tube and sink, sucrose is converted to other molecules.