Translocation 9.4

Question 1

What is translocation

Answer 1

The process of the plant transporting organic compounds in the phloem from sources to sinks (the tissue that needs them)

Question 2

What type of transporation is translocation

Answer 2

active in most plants

Question 3

what are assimilates

Answer 3

The products of photosynthesis that are transported

Question 4

what is the main assimilate transported around the plant

Answer 4

Sucrose

Question 5

what are the main sources of assimilates in plant

sources as in scientific term

Answer 5

• green leaves/stems
• storages organs
• food stores in seeds when they germinate

Question 6

What are the main sinks in a plant

Answer 6

• roots that are growing and/or actively absorbing mineral ions
• meristems that are actively dividing
• parts of plants developing seeds, fruits and storage organs

Question 7

what are the 2 main ways that phloem loading can occur

Answer 7

the symplast route and the apoplast route

Question 8

explain the symplast route of phloem loading

Answer 8

• sucrose moves from source through cytoplasm of mesophyll cells and into sieve tubes by diffusion through plasmodesmata
  ^largely passive
• water follows by osmosis
• hydrostatic pressure moves substances in phloe,

Question 9

explain the apoplast route of phloem loading

Answer 9

sucrose from the source travels through the cell walls and inter-cellular spaces to the companion cells and sieve elements by diffusion down a concentration gradient

Question 10

How is the concentration gradient needed for the apoplast route maintained

Answer 10

It is maintained by the removal of sucrose into the phloem vessels

Question 11

How does sucrose move in respect to companion cells during the apoplast route

Answer 11

In the companion cells sucrose is moved into the cytoplasm across the cell membrane in an active process

Question 12

How is sucrose moved into companion cells during unloading

Answer 12

H+ pumped out of companaion cell
H+ then return to companion cells down conc gradient using a co-transport protein
sucrose moves in with hydrogen ions thanks to co-transport protein

Question 13

How have companion cells evolved to be better exchange surfaces for sucrose

Answer 13

Have large surface area
cell membrane contain many proton pump proteins
have many mitochondria to make ATP needed for active transport

Question 14

what occurs as a result of the build up of sucrose in the companion cells and sieve elements and what does this cause

Answer 14

water moves into the companion cells and sieve elements by osmosis, leads to high turgor pressure
water carrying the sucrose moves into the tubes of the sieve elements
then water moves from areas of high pressure to low pressure which are sinks in mass flwo

Question 15

what is the main mechanism of phloem unloading and how does it work

Answer 15

• diffusion of sucrose from phloem into surrounding cells
• sucrose rapidly moves into other cells by diffusion or is converted into glucose/starch
  ^maintain concentration gradient of sucrose

Question 16

what happens to the water in the phloem when the source is unloaded

Answer 16

The loss of the solutes from the phloem leads to a rise in the water potential of the phloem.
water moves out into the surrounding cells by osmosis.
Some of the water that carried the solute to the sink is drawn into the transpiration stream in the xylem

Question 17

what evidence is there to support the main principles of translocation

Answer 17

• Advances in microscopy allow us to see the adaptations of the companion cells for active transport.
• If the mitochondria of the companion cells are poisoned, translocation stops.
• The flow of sugars in the phloem is about 10,000times faster than it would be by diffusion alone, suggesting an active process is driving the mass flow.
  -aphids can be used to show the translocation of organic solutes in the phloem.
  It has been shown that there is a positive pressure in the phloem that forces the sap out through the stylet.
  The pressure and therefore the flow rate in the phloem is lower closer to the sink than it is near the source.
  The concentration of sucrose in the phloem sap is also higher near to the source than near the sink