Transcript Chapt 36 Plant Transport

Transport in
Plants
AP Biology
2006-2007
Transport in plants
 H2O & minerals


transport in xylem
transpiration
 evaporation, adhesion & cohesion
 negative pressure
 Sugars


transport in phloem
bulk flow
 Calvin cycle in leaves loads sucrose into phloem
 positive pressure
 Gas exchange

photosynthesis
 CO2 in; O2 out
 stomates

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respiration
 O2 in; CO2 out
 roots exchange gases within air spaces in soil
Why does
over-watering
kill a plant?
Ascent of xylem fluid
Transpiration pull generated by leaf
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Control of Stomates
Epidermal cell
-non photosynthetic
Guard cell
Nucleus
Chloroplasts
-photosynthetic
 Uptake of K+ ions
by guard cells



proton pumps
water enters by
osmosis
guard cells
become turgid
 Loss of K+ ions
H+
H+
H2O
H2O
K+
H+
K+
K+
H2O
K+
H2O
K+
H2O
K+
H+
H2O
K+
H2O
K+
H2O
H+
Thickened inner
cell wall (rigid)
by guard cells


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water leaves by
K+
K+
osmosis
H2O
H2O
+
H+
H
guard cells
Stoma open
become flaccid
water moves
into guard cells
H2O
K+
H2O
K+
Stoma closed
water moves out
of guard cells
The guard cells control the opening
and closing of the stomata
Guard cells flaccid
Guard cells turgid
Thin outer wall
Thick inner wall
Stoma closed
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Stoma open
Regulating Stomatal Opening:-the
potassium ion pump hypothesis
Guard cells flaccid
K+
K+ ions have the same
concentration in guard cells
and epidermal cells
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
Stoma closed
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Light activates or decreased
CO2 activates proton pumps
moving H+ ion our of Guard
Cells and allowing K+ to
transport from the
epidermal cells into the
guard cells
Regulating Stomatal Opening
H2O
H2O
K+
H2O
K+
K+
K+
H2O
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K+
K+
Increased concentration
of K+ in guard cells
K+
K+
H2O
K+
K+
K+
K+
Lowers the  in the
guard cells
Water moves in by
osmosis, down  gradient
Guard cells
turgid
H2O
K+
K+
H2O
K+
K+
H2O
K+
K+
H2O
K+
K+
H2O
K+
K+
H2O
K+
Stoma open
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Increased concentration
of K+ in guard cells
K+
Lowers the  in the
guard cells
Water moves in by
osmosis, down  gradient
Water & mineral absorption
 Water absorption from soil
osmosis
 aquaporins

 Mineral absorption
active transport
 proton pumps

 active transport of
H+
aquaporin
root hair
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proton
pumps
H2O
Mineral absorption
 Proton pumps

active transport of H+ ions out of cell
 chemiosmosis
 H+ gradient

creates membrane
potential
 difference in charge
 drives cation uptake

creates gradient
 cotransport of other
solutes against their
gradient
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Fig. 36-11
Cell wall
Cytosol
Vacuole
Plasmodesma
Vacuolar membrane
Plasma membrane
(a) Cell compartments
Key
Transmembrane route
Apoplast
Symplast
Apoplast
Symplast
Symplastic route
(b) Transport routes between cells
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Apoplastic route
Absorption of Water and Minerals by Root Cells
• Most water and mineral absorption occurs
near root tips, where the epidermis is
permeable to water and root hairs are located
• Root hairs account for much of the surface
area of roots
• After soil solution enters the roots, the
extensive surface area of cortical cell
membranes enhances uptake of water and
selected minerals
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Water flow through root
 Porous cell wall
water can flow through cell wall route &
not enter cells
 plant needs to force water into cells

Casparian strip
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Controlling the route of water in root
 Endodermis

cell layer surrounding
vascular cylinder of root
• The waxy Casparian
strip of the endodermal
wall blocks apoplastic
transfer of minerals
from the cortex to the
vascular cylinder
• forces fluid through
selective cell membrane
 filtered & forced into
xylem cells
Animation: Transport in Roots
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Root anatomy
Phloem
dicot
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Phloem
Casparian strip
Xylem
Xylem
monocot
Mycorrhizae increase absorption
 Symbiotic relationship between fungi & plant



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symbiotic fungi greatly increases surface area for
absorption of water & minerals
increases volume of soil reached by plant
increases transport to host plant
Mycorrhizae
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Endomycorrhizae
Cell Wall
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From: http://shachar-hill.plantbiology.msu.edu/?page_id=44
Cohesion and Adhesion in the Ascent of
Xylem Sap
• The transpirational pull on xylem sap is
•
transmitted all the way from the leaves to the
root tips and even into the soil solution
Transpirational pull is facilitated by cohesion
of water molecules to each other and adhesion
of water molecules to cell walls
Animation: Transpiration
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Fig. 36-15
Xylem
sap
Outside air ψ
= −100.0 Mpa
Mesophyll
cells
Stoma
Leaf ψ (air spaces)
= −7.0 Mpa
Water
molecule
Transpiration
Leaf ψ (cell walls)
= −1.0 Mpa
Trunk xylem ψ
= −0.8 Mpa
Water potential gradient
Xylem
cells
Atmosphere
Adhesion
by hydrogen
bonding
Cell
wall
Cohesion
Cohesion and by hydrogen
adhesion in
bonding
the xylem
Water
molecule
Root
hair
Trunk xylem ψ
= −0.6 Mpa
Soil
particle
Soil ψ
= −0.3 Mpa
Water
Water uptake
from soil
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Transport of sugars in phloem
 Loading of sucrose into phloem
flow through cells via plasmodesmata
 proton pumps

 cotransport of sucrose into cells down
proton gradient
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Pressure flow in phloem
 Mass flow hypothesis

“source to sink” flow
 direction of transport in phloem is
dependent on plant’s needs

phloem loading
 active transport of sucrose
into phloem
 increased sucrose concentration
decreases H2O potential

water flows in from xylem cells
 increase in pressure due to
increase in H2O causes flow
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On a plant…
What’s a source…What’s a sink?
can flow
1m/hr
Bulk Flow by Positive Pressure: The
Mechanism of Translocation in
Angiosperms
 In studying angiosperms, researchers have
concluded that sap moves through a sieve
tube by bulk flow driven by positive pressure
Animation: Translocation of Phloem Sap in Summer
Animation: Translocation of Phloem Sap in Spring
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Experimentation
 Testing pressure flow
hypothesis

using aphids to measure sap
flow & sugar concentration
along plant stem
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Maple
sugaring
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AP Biology
2006-2007