Phloem unloading and evidence

Question 1

Where is the sucrose unloaded?

Answer 1

sinks are at any point where the cell needs it

Question 2

Whhat is the mechanism for phloem unloading?

Answer 2

diffusion of the sucrose from the phloem into the surrounding cells

Question 3

What is sucrose converted in?

Answer 3

glucose for respiration

or

starch for storage

Question 4

Why is it converted?

Answer 4

so a concentration gradient of sucrose is maintained between the contents of the phloem and the surrounding cells

Question 5

What happens to the remaining water in the phloem?

Answer 5

loss of solutes leads to an increase in water potential, water moves into the surrounding cells by osmosis

some ofthe water that carried the cell is drawin to the transpiration stream

Question 6

What is the evidence for active transport occuring in loading?

Answer 6

advances in microscopy allows us to see the adaptation of the companion cells for active transport

if the mitochondria of the companion cells are poisoned, translocation stops

the flow of sugar in the phloem is 10,000x faster than it would be by diffusion alone

Question 7

What is the evidence for the pressure difference?

Answer 7

it has been shown that in aphids there is a positive pressure in thephloem 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

Question 8

What are the uncertainties?

Answer 8

not all solutes in the phloem move at the same rate

sucrose always seems to move at the same rate regardless of the concentration at the sink

no one is sure of the role of the sieve plates