7.8 transport of organic molecules in the phloem (DO EVERYDAY

Question 1

what are sources?

Answer 1

photosynthesising cells

Question 2

what are sinks?

Answer 2

respiring cells

Question 3

how to transfer sucrose into sieve elements from photosynthesising elements

Answer 3

• sucrose is diffused by facilitated diffused into companion cells from photosynthesising cells
• H+ are actively transported from companion cells to cell wall
• H+ diffuses by facilitated diffusion into sieve tube elements
• sucrose moves with H+ ions in co- transport

Question 4

what is mass flow?

Answer 4

bulk movement of a substance through a given channel or area in a specified time

Question 5

what is the phloem made up of?

Answer 5

sieve tube elements

Question 6

what do the end walls of sieve tube elements form?

Answer 6

sieve plates

Question 7

mass flow hypothesis

Answer 7

1. In source sugars actively transported into sieve tubes
2. By companion cells;
3. Lowers water potential of sieve cell tube and water enters by osmosis;
4. Increase in hydrostatic pressure
5. respiring cells use the sucrose for respiration/ storage so hydrostatic pressure is lower
6. mass movement goes down hydrostatic gradient to sink

Question 8

evidence supporting the mass flow hypothesis

Answer 8

• pressure within sieve tubes => shown by sap when being released
• concentration of sucrose is higher is source than sink
• downwards flow in the phloem occurs in daylight, by ceases at night
• increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem a little later
• metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem
• companion cells possess many mitochondria and readily produce ATP

Question 9

evidence against the mass flow hypothesis

Answer 9

• the function of the sieve plates is unclear, as they would seem to hinder mass flow
• not all solutes move at the same speed- they should do so if movement is by mass flow
• sucrose is delivered at more or less the same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration, with the mass flow theory would suggest

Question 10

transfer of sucrose from the sieve tube elements into storage or other sink cells

Answer 10

sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells