7.8 transport of organic substances in the phloem

Question 1

what is the definition of translocation?

Answer 1

movement of organic molecules & mineral ions

Question 2

where does translocation occur from and to?

Answer 2

source to sink

Question 3

what does the phloem transport?

Answer 3

sucrose, amino acid, K+, Cl-, Mg 2+, PO4 3-

Question 4

explain the phloem loading stage of the mass flow theory. (1-3)

Answer 4

1) sucrose diffuse from photosynthesising cells to companion cells, by facilitated diffusion
2) H+ ions are actively transported from companion cells to space within cell walls using ATP
3) sucrose molecules are transported by co-transport

Question 5

explain the mass flow through sieve tube elements in the mass flow theory. (4-5)

Answer 5

• entry of sucrose into sieve tube elements lowers the water potential, therefore the xylem has a higher water potential than the sieve tubes

4) water moves from xylem -> sieve tubes by osmosis
- creates a higher hydrostatic pressure than sinks
5) mass transport of sucrose down hydrostatic gradient from source -> sink

Question 6

why is there a lower hydrostatic pressure at sinks?

Answer 6

• glucose is used up for respiration/storage
• sucrose actively transported into sinks
• water moves in by osmosis
  -> lower hydrostatic pressure

Question 7

explain the final phase of the mass flow theory.

Answer 7

sucrose actively transported by companion cells out of sieve tubes into sink cells