Mass Transport in Plants - Phloem

Question 1

3 parts of phloem tissue

Answer 1

• companion cell
• sieve tube element
• fibre and parenchyma

Question 2

name given to the perforated end walls in sieve tubes

Answer 2

sieve plates

Question 3

structure of sieve tube element

Answer 3

• elongated, stacked to form a continuous tube
• end walls start to diintegrate
• no nucleus or other organelles

Question 4

function of the sieve tube

Answer 4

transport organic solutes (products of PS): sucrose and a.a. as sap

Question 5

function of the perforated end walls in sieve tubes

Answer 5

allow cytoplasmic connections

Question 6

how are companion cells connected to the sieve tube?

Answer 6

by plasmodesmata

Question 7

functions of a companion cell

Answer 7

• synthesise ATP and proteins for sieve tube
• load and unload sucrose to and from sieve tube

Question 8

when can the sink become the source?

Answer 8

when stored products are used

Question 9

directionof transport in phloem

Answer 9

from source to sink

Question 10

what is the source in translocation?

Answer 10

where sucrose and other organic solutes are made

Question 11

what is the sink in translocation?

Answer 11

where sucrose is stored or used

Question 12

is translocation unidirectional or bidirectional?

Answer 12

bidirectional

Question 13

3 suggested theories of translocation

Answer 13

1. diffusion through phloem
2. cytoplasmic streaming
3. munch’s mass flow hypothesis

Question 14

criticisms of diffusioin through phloem and cytoplasmic streaming theories

Answer 14

1. only effective across sort distances, too slow across long distances
2. cytosol of cytoplasm moves in currents, carrying moleculs through cells

Question 15

2 assumptions of munch’s mass flow hypothesis

Answer 15

1. passive flow of sucrose
2. sucrose flowing down hydrostatic pressure gradient

Question 16

will mass flow continue indefinitely in munch’s model? why or why not?

Answer 16

• no
• A and B sugar solutions reach equilibrium so no movemnt of liquids anywhere

Question 17

definition of semi-permeable

Answer 17

permeable to water but not solutes

Question 18

3 criticisms of munch’s mass flow hypothesis

Answer 18

1. translocation not passive
   
   2. companion cells contain many mitochondria for respiration for ATP
   3. phloem tissue has very high O2 consumption so must have high RS rate so synthesise much ATP
   4. adding RS inhibitors to phloem tissue causes translocation rates to decrease so must be active
2. sucrose and a.a seen to translocate at different rates in same phloem tissue
3. sucrose and a.a. seen to translocate in different directions in same phloem tissue

Question 19

7 steps of mass flow from source to sink

Answer 19

1. sucrose actively loaded into sieve tube element from cmpanion cell, decreasing WP in sieve tube below xylem
2. water moves across from xylem into sieve tube by osmosis, down WP gradient, increasing hydrostatic pressure in sieve tube
3. sap moves down a hydrostatic pressure gradient form source to sink
4. sucrose diffuses into sink cell, increasing WP in sieve tube above that in xylem
5. water and mineral ions enter xylem, decreasing WP in xylem below sieve tube
6. water moves by osmosis down WP gradient form sieve tube to xylem
7. water and mineral ions move up xylem by cohesion tension

Question 20

4 steps of sucrose loading in the phloem

Answer 20

1. H ions aactively transported out companion cell into source tissue, increasing concentration above the companion cell
2. H ions move passively back into companion cell via contransport, bringing sucrose with, which moves actively
3. sucrose concentration above that in sieve tube
4. sucrose passively enters the sieve tube via facilitated diffusion

Question 21

outline the aphid experiment

Answer 21

1. aphids placed on different parts of the stem
2. stylets pierce the stem to reach phloem and aphids feed on it
3. anaethetise aphids while feeding and cut off from stylet - sap flows continuously
4. collect sap
5. analyse sucrose conc. and compare from different parts of stem to see different conc.

Question 22

why does the sap flow continuously from the aphid’s stylet?

Answer 22

aphids release enzyme that prevents coaagulation which is still in the stylet so it doesn’t clog

Question 23

outlie the ringing experiment

Answer 23

1. remove ring of bark and phloem so xylem remains
2. leave for a week
3. bulge forms above ring - evidence of sap flowing downwards
4. diameter smaller below cut - reduced growth as can’t respire for energy for growth

Question 24

outline the tracer experiment

Answer 24

1. cover leaf with bottle containing radioactive carbon so leaf exposed to carbon
2. leaf PS’s, using 14CO2, producing radioactive glucose which becomes radioactive sucrose
3. @ varying time intervals, freeze plant w/ liquid nitrogen
4. autoradiograph of plant can now be produced - black areas show 14C and compounds containing it

Question 25

why is the plant frozen with liquid nitrogen in the tracer experiment?

Answer 25

kills it and stops all compounds in their plac and state

Question 26

what does the tracer experiment evidence

Answer 26

• eveidence of traslocation
• evidence translocation is bidirectional as 14C above and below exposed leaf