Mass Transport in Plants - Phloem Flashcards

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1
Q

3 parts of phloem tissue

A
  • companion cell
  • sieve tube element
  • fibre and parenchyma
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2
Q

name given to the perforated end walls in sieve tubes

A

sieve plates

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3
Q

structure of sieve tube element

A
  • elongated, stacked to form a continuous tube
  • end walls start to diintegrate
  • no nucleus or other organelles
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4
Q

function of the sieve tube

A

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

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5
Q

function of the perforated end walls in sieve tubes

A

allow cytoplasmic connections

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6
Q

how are companion cells connected to the sieve tube?

A

by plasmodesmata

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7
Q

functions of a companion cell

A
  • synthesise ATP and proteins for sieve tube
  • load and unload sucrose to and from sieve tube
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8
Q

when can the sink become the source?

A

when stored products are used

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9
Q

directionof transport in phloem

A

from source to sink

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10
Q

what is the source in translocation?

A

where sucrose and other organic solutes are made

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11
Q

what is the sink in translocation?

A

where sucrose is stored or used

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12
Q

is translocation unidirectional or bidirectional?

A

bidirectional

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13
Q

3 suggested theories of translocation

A
  1. diffusion through phloem
  2. cytoplasmic streaming
  3. munch’s mass flow hypothesis
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14
Q

criticisms of diffusioin through phloem and cytoplasmic streaming theories

A
  1. only effective across sort distances, too slow across long distances
  2. cytosol of cytoplasm moves in currents, carrying moleculs through cells
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15
Q

2 assumptions of munch’s mass flow hypothesis

A
  1. passive flow of sucrose
  2. sucrose flowing down hydrostatic pressure gradient
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16
Q

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

A
  • no
  • A and B sugar solutions reach equilibrium so no movemnt of liquids anywhere
17
Q

definition of semi-permeable

A

permeable to water but not solutes

18
Q

3 criticisms of munch’s mass flow hypothesis

A
  1. translocation not passive
    1. companion cells contain many mitochondria for respiration for ATP
    2. phloem tissue has very high O2 consumption so must have high RS rate so synthesise much ATP
    3. 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
19
Q

7 steps of mass flow from source to sink

A
  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
20
Q

4 steps of sucrose loading in the phloem

A
  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
21
Q

outline the aphid experiment

A
  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.
22
Q

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

A

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

23
Q

outlie the ringing experiment

A
  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
24
Q

outline the tracer experiment

A
  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
25
Q

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

A

kills it and stops all compounds in their plac and state

26
Q

what does the tracer experiment evidence

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