Understanding Plant Transport Systems: Xylem and Phloem

 
Plant transport:
Xylem and Phloem
Chapter 4 pp. 47 -54
 
Lecture Outline
:
Evolution of vascular tissue
The xylem transports water and nutrients
The phloem transports sugars and other organic compounds
Xylem and phloem structure and function
How do roots take up water and nutrients from the soil?
Cohesion-tension mechanism and transpiration relate to the physical
properties of water
The function of stomata in transpiration
Sugar transport in the phloem
The pressure-flow hypothesis describes transport of sugar in plants
 
http://maple.dnr.cornell.edu
 
Sugar maple, 
Acer saccharum
 
Plant classification and vascular tissue
 
nonvascular
plants
 
seedless
vascular
plants
 
gymnosperms
(cone-bearing
seed plants)
 
angiosperms
(flowering
 seed plants)
 
H
2
O
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m
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a
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H
2
O
 
H
2
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m
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a
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O
2
 
C
O
2
 
C
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O
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H
2
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H
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m
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O
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C
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C
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O
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H
2
O
 
L
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t
 
S
u
g
a
r
Long-distance transport in the 
phloem
 and 
xylem
Transpiration
 is
the evaporative
loss of water
from the leaves
of a plant.
But first, 
water and minerals 
must
travel from the soil through the dermal
and ground tissue into the 
vascular
cylinder 
(xylem/phloem) of a root…
 
eudicot root cross section
 
100 µm
 
Epidermis
 
Cortex
 
Endodermis
 
Vascular
cylinder
 
Pericycle
 
Xylem
 
Phloem
 
Dermal
 
Ground
 
Vascular
 
Key
to labels
 
Cross section of a eudicot root
 
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Transport of water and mineral nutrients
from 
root hairs 
to the xylem
Transpiration is the movement of
water against gravity, from the soil
to the leaves, 
without using any
energy… 
HOW do plants do this?
 
http://photo.accuweather.com
Transpiration is the movement of
water against gravity, from the soil
to the leaves, 
without using any
energy… 
HOW do plants do this?
Transpiration depends on:
1.
the evaporation of H
2
O
from the leaves pulls
water upwards from
the roots (
tension
)
2.
the physical properties
of water (
cohesion
)
 
http://photo.accuweather.com
 
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The ascent
of 
xylem sap
(water and
minerals)
 
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r
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The ascent
of 
xylem sap
(water and
minerals)
Water has 
cohesive
 behavior
 
 
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h
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(
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H
2
O = water
 
The ascent of xylem sap
depends on 
hydrogen
bonds
 between water
molecules
 
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The ascent
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(water and
minerals)
 
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s
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p
 
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r
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The ascent
of 
xylem sap
(water and
minerals)
 
K
 
H
2
O
 
H
2
O
 
H
2
O
 
H
2
O
 
H
2
O
 
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2
O
 
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O
 
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Osmosis
 of water causes stomata (leaf
pores) to open and close
 
Water flows into the cells,
stoma opens
 
Water flows out of the cells,
stoma closes
 
H
2
O
a
n
d
 
m
i
n
e
r
a
l
s
 
O
2
 
C
O
2
 
C
O
2
 
O
2
 
H
2
O
 
L
i
g
h
t
 
S
u
g
a
r
The 
Pressure-flow
hypothesis 
describes
how sugars are
transported from
sources
 to 
sinks
 via
the 
phloem
(this 
does
 require
energy!)
Source to sink
 
Daucus carota 
roots are sources in
the springtime, and shoot is the
sink
 
Daucus carota 
leaves are sources in
the summer time, and flowers/fruits
and developing organs are sinks
Starch - 
how plants store sugar
 
C
h
l
o
r
o
p
l
a
s
t
 
S
t
a
r
c
h
 
starch molecules 
are simply
glucose molecules linked
together (similar to cellulose)
Structure of the phloem
4
 
Fig. 36-20
3
2
1
1
2
3
 
Vessel
(xylem)
 
Sieve tube
(phloem)
 
Source cell
(leaf)
 
Load sugar into the phloem at
the 
source
 (requires ENERGY!)
 
Water from the xylem flows
into the phloem (OSMOSIS)
 
Unload sugar at the 
sink
 
Water from the phloem flows
back into the xylem (OSMOSIS) –
water is recycled, creating a
constant circuit!
 
Sink cell
(storage
root)
 
Sugar
 
H
2
O
 
H
2
O
 
 
Bulk flow
 
H
2
O
 
Sugar
 
            Bulk flow
 
Pressure-flow hypothesis
Lecture Review, Chap 4
Do any plants lack vascular tissue?  If so, give an example of a
kind of plant that lacks this particular tissue type.
Relate structure to function in sieve-tube cells, vessel cells, and
tracheid cells.
How are water and mineral nutrients from the soil transferred
into the vascular cylinder of a root?
Define transpiration.
Describe the cohesion-tension mechanism and relate it to the
function of the stomata and the properties of water.
Why do water molecules stick together?  How does this relate to
why plants can move water against the force of gravity without
using any energy?
Trace the path of sugar in the phloem from source to sink.  What
is the name of the hypothesis to describe this flow?
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Explore the intricate system of plant transport through xylem and phloem, from how roots absorb water to the pressure-flow hypothesis in sugar transport. Learn about the evolution, structure, and function of vascular tissues and the critical role of transpiration. Dive into the classification of plants based on vascular tissue and the mechanisms governing long-distance transport.


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  1. Sugar maple, Acer saccharum Plant transport: Xylem and Phloem Chapter 4 pp. 47 -54 Lecture Outline: Evolution of vascular tissue The xylem transports water and nutrients The phloem transports sugars and other organic compounds Xylem and phloem structure and function How do roots take up water and nutrients from the soil? Cohesion-tension mechanism and transpiration relate to the physical properties of water The function of stomata in transpiration Sugar transport in the phloem The pressure-flow hypothesis describes transport of sugar in plants http://maple.dnr.cornell.edu

  2. Plant classification and vascular tissue seedless vascular plants nonvascular plants gymnosperms (cone-bearing seed plants) angiosperms (flowering seed plants)

  3. H2O H2O and minerals

  4. O2 CO2 H2O O2 H2O and minerals CO2

  5. O2 CO2 Light Sugar H2O O2 H2O and minerals CO2

  6. Long-distance transport in the phloem and xylem Phloem Xylem Vessel Tracheids 100 m Sieve-tube element (left) and companion cell: cross section Pits Vessel element Sieve-tube elements: longitudinal view Sieve plate with pores Tracheid Tracheids

  7. Transpiration is the evaporative loss of water from the leaves of a plant. H2O H2O and minerals

  8. But first, water and minerals must travel from the soil through the dermal and ground tissue into the vascular cylinder (xylem/phloem) of a root eudicot root cross section

  9. Cross section of a eudicot root Epidermis Key to labels Cortex Dermal Ground Vascular Endodermis Vascular cylinder Pericycle Xylem 100 m Phloem

  10. Casparian strip Transport of water and mineral nutrients from root hairs to the xylem Endodermal cell Pathway along apoplast Pathway through symplast Casparian strip Plasma membrane Apoplastic route Xylem Symplastic route Root hair Epidermis Endodermis Vascular cylinder Cortex

  11. Transpiration is the movement of water against gravity, from the soil to the leaves, without using any energy HOW do plants do this? http://photo.accuweather.com

  12. Transpiration is the movement of water against gravity, from the soil to the leaves, without using any energy HOW do plants do this? Transpiration depends on: 1. the evaporation of H2O from the leaves pulls water upwards from the roots (tension) 2. the physical properties of water (cohesion) http://photo.accuweather.com

  13. Xylem sap The ascent of xylem sap (water and minerals) Mesophyll cells Stoma Stoma Water molecule Atmosphere Transpiration Adhesion by hydrogen bondingCell Xylem cells wall Water potential gradient Cohesion by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Soil particle Water uptake from soil Water

  14. Xylem sap The ascent of xylem sap (water and minerals) Mesophyll cells Stoma Stoma Water molecule Atmosphere Transpiration Adhesion by hydrogen bonding Xylem cells Cell wall Water potential gradient Cohesion by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Soil particle Water Water uptake from soil

  15. Water has cohesive behavior

  16. The ascent of xylem sap depends on hydrogen bonds between water molecules The oxygen (O) is partially The hydrogen (H) is partially + H H2O = water O + H + +

  17. Xylem sap The ascent of xylem sap (water and minerals) Mesophyll cells Stoma Stoma Water molecule Atmosphere Transpiration Adhesion by hydrogen bonding Xylem cells Cell wall Water potential gradient Cohesion by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Soil particle Water Water uptake from soil

  18. Xylem sap The ascent of xylem sap (water and minerals) Stomata function in transpiration Water molecule Atmosphere Transpiration Adhesion by hydrogen bondingCell Xylem cells wall Water potential gradient Cohesion by hydrogen bonding Cohesion and adhesion in the xylem Water molecule Root hair Soil particle Water Water uptake from soil

  19. Osmosis of water causes stomata (leaf pores) to open and close Guard cells turgid Guard cells flaccid H2O H2O H2O H2O H2O K+ + H2O H2O H2O H2O H2O Water flows out of the cells, stoma closes Water flows into the cells, stoma opens

  20. O2 CO2 Light Sugar H2O O2 H2O and minerals CO2

  21. O2 CO2 Light Sugar H2O The Pressure-flow hypothesis describes how sugars are transported from sources to sinks via the phloem (this does require energy!) O2 H2O and minerals CO2

  22. Source to sink Daucus carota leaves are sources in the summer time, and flowers/fruits and developing organs are sinks Daucus carota roots are sources in the springtime, and shoot is the sink

  23. Starch - how plants store sugar Chloroplast Starch starch molecules are simply glucose molecules linked together (similar to cellulose)

  24. Structure of the phloem Sieve-tube element Sieve plate 10 m Nucleus of companion cells Sieve-tube elements: longitudinal view Sieve plate with pores (SEM)

  25. Pressure-flow hypothesis Source cell (leaf) Vessel (xylem) Sieve tube (phloem) 1 Load sugar into the phloem at the source (requires ENERGY!) H2O Sugar 1 H2O 2 Water from the xylem flows into the phloem (OSMOSIS) 2 Bulk flow Bulk flow 3 Unload sugar at the sink Sink cell (storage root) Water from the phloem flows back into the xylem (OSMOSIS) water is recycled, creating a constant circuit! 4 3 Sugar H2O Fig. 36-20

  26. Lecture Review, Chap 4 Do any plants lack vascular tissue? If so, give an example of a kind of plant that lacks this particular tissue type. Relate structure to function in sieve-tube cells, vessel cells, and tracheid cells. How are water and mineral nutrients from the soil transferred into the vascular cylinder of a root? Define transpiration. Describe the cohesion-tension mechanism and relate it to the function of the stomata and the properties of water. Why do water molecules stick together? How does this relate to why plants can move water against the force of gravity without using any energy? Trace the path of sugar in the phloem from source to sink. What is the name of the hypothesis to describe this flow?

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