TRANSPORT IN THE PHLOEM

Question 1

What is translocation? (1)

Answer 1

the name given to the transport of manufactured solutes, such as sucrose and amino acids in the phloem

Question 2

what is mass flow? (1)

Answer 2

a method of transport in which pressure differences are used to move a fluid to carry substances in one direction.

Question 3

what is the mass flow hypothesis? (1)

Answer 3

this model suggests that pressure differences to drive the fluid movement are generated in different parts of the plant.

Question 4

what is the source area? (2)

Answer 4

sugars dissolve in the cytoplasm, causing a lowered water potential, this causes water to enter the cells by osmosis and build up a high hydrostatic pressure.

Question 5

what is the sink area? (2)

Answer 5

sugars used up in respiration raise the water potential and therefore water tends to flow out of the cell forming a region of lower hydrostatic pressure.

Question 6

what are the strengths of this model? (3)

Answer 6

-its possible to measure the gradients suggested and show they are present
-when pierced by insect mouthparts the contents of the sieve tube flow out showing them under pressure which model predicts.
-model links the phloem and xylem systems in a plausible way

Question 7

what ae the weaknesses of this model? (3)

Answer 7

-starch grains ae found in many plant cells, not just sinks.
-organic solutes move around the plant in different directions, not just to the lowest pressure sinks.
-sieve tubes and companion cells are living tissue and do not work if killed off, model does not explain why.