Phloem and Mass flow hypothesis/Translocation

Question 1

What does the phloem transport?

Answer 1

assimilates - e.g glucose and amino acids

Question 2

What is the direction of the phloem?

Answer 2

up and down

Question 3

What types of cells make up the phloem?

Answer 3

a) sieve tube cells

b) companion cells

Question 4

What do companion cells do?

Answer 4

• support the sieve tube cell

- provide ATP

Question 5

What do companion cells contain a lot of?

Answer 5

mitochondria

Question 6

Describe a sieve tube cell

Answer 6

• living
• have little cytoplasm to make space for transporting sap
• have few organelles - no nucleus
• hollow

Question 7

what are sieve plates?

Answer 7

thin pores found between sieve tube cells

Question 8

describe the tracing experiment

Answer 8

• plants are grown in an atmosphere rich in carbon-14
• this radioactive label is incorporated into sugars during photosynthesis
• the carbon-14 can be traced using xrays as it moves down the trunk/stem

Question 9

describe a ringing experiment

Answer 9

• ring of bark is removed from a tree trunk including the phloem
• so only xylem is left
• solutes cannot move up or down
• so above the ring a swelling of fluid forms which can be tested

Question 10

what is the result of a ringing experiment? how is it evidence for mass flow?

Answer 10

• more solutes (e.g sugars) will be present above the ring than below
• evidence solutes are moving down

Question 11

what is the result of a tracer experiment?

Answer 11

• xrays taken of stems show blackened areas where there is C14
• other tissues don’t blacken the film
• so we can see the phloem is where sugars have been carried

Question 12

what is plasmodesmata?

Answer 12

• connects sieve tube elements to companion cells

- and allows the cytoplasm to be shared

Question 13

what is the source?

Answer 13

an area where the concentration of solute is HIGH

Question 14

what happens at the source?

Answer 14

1. solute actively transported using ATP from companion cell into sieve tube cel
2. water potential of phloem decreases
3. so water enters by osmosis from xylem and companion cells
4. creating a high hydrostatic pressue in the phloem

Question 15

what happens at the sink?

Answer 15

1. solute converted/used
2. creating a low concentration of solute
3. so the water potential in phloem increases and water moves out by osmosis
4. hydrostatic pressure decreases