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35
Transport in Plants
O2 (product of photosynthesis)
CO2 (reactant of
photosynthesis)
H2O (transpiration)
H2O,
carbohydrates,
etc.
CO2 enters and O2
and H2O exit the
leaves via the stomata
(see Figure 35.8).
H2O and dissolved
minerals
35.1 The Pathways of Water and Solutes in a Plant (Page 727)
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Figure 35.01 Date 07-13-12
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© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
3
(A)
In this tube, the solute
potentials on the two
sides of the membrane
differ, but the pressure
potentials are the
same.
Pure water
y = 0 MPa
The right side of the
tube has a lower water
potential, so there is a
net movement of
water to the right.
Membrane
In this tube, a piston is
used to increase the
pressure potential of
the right side.
Solution
yp = 0 MPa
ys = –1.0 MPa
Pure water
y = 0 MPa
y = –1.0 MPa
The water potentials
of the two sides are
equal, so there is no
net movement of
water.
Solution
yp = +1.0 MPa
ys = –1.0 MPa
y = 0 MPa
(B)
The inside of the cell
has a lower solute
potential than the
surrounding water.
The cell has a
pressure potential
of zero.
Pure water
y = 0 MPa
The cell has a lower
water potential than
the water outside, so
there is net movement
of water into the cell.
The cell has a
negative solute
potential, but has
a positive pressure
potential.
Flaccid cell
yp = 0 MPa
ys = –1.0 MPa
The pressure potential
of the cell balances its
solute potential, so the
cell’s water potential is
zero. There is no net
movement of water.
Pure water
y = 0 MPa
y = –1.0 MPa
Turgid cell
yp = +1.0 MPa
ys = –1.0 MPa
y = 0 MPa
35.2 Water Potential, Solute Potential, and Pressure Potential (Page 728)
The cells of this
plant have low
turgor pressure and
the plant is wilted.
LIFE The Science of Biology 10E Sadava
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Morales Studio
Figure 35.02 Date 07-13-12
The water potential of cells
of this plant is zero because
the negative solute potential
is balanced by an equally
positive pressure potential.
The plant is upright because
its cells are turgid.
35.3 A Wilted Plant (Page 728)
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
1 A proton pump generates
differences in H+ concentration and electric charge
across the membrane.
Outside
of cell
Plasma
membrane
H+
4
2 The difference in
electric charge causes
cations such as K+ to
enter the cell.
H+
H+
H+
H+
K+
Proton
pump
ATP
Inside
of cell
H+
ADP +
fusion of H+ to the transport
(against an electrochemical
gradient) of anions such as
Cl– into the cell.
H+
H+
H+
3 Symport couples the dif-
H+
Cl–
H+
Symport
protein
Potassium
channel
K+
K+
Pi
K+
+
K
+
K
H+
Cl–
Cl–
Cl–
35.4 The Proton Pump in Transport of K+ and Cl– (Page 729)
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Figure 35.04 Date 07-13-12
Cell membrane
Cytoplasm
Plasmodesmata
Cell wall
Water and solutes can move
in the symplast by crossing a
cell membrane and passing
through plasmodesmata.
Water and solutes can
move through the
apoplast without passing
through cell membrane.
35.5 Apoplast and Symplast (Page 729)
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
5
The Casparian strip prevents water and
solutes in the apoplast from passing
between the endodermal cells into the stele.
Epidermis
Root hair
Endodermis
Cortex
Cortex
Endodermis
Pericycle
Pericycle
Xylem
Stele
Casparian strip
Soil
solution
Apoplast
Symplast
Plasmodesmata
Water and solutes travel through
the symplast or apoplast until
they reach the endodermis.
At the Casparian strip,
water and solutes in
the apoplast must
enter the symplast to
cross the endodermis.
Inside the stele,
solutes are actively
transported into
the apoplast and
water follows
passively, forming
the xylem sap.
35.6 Pathways to the Root Xylem (Page 730)
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Figure 35.06 Date 09-10-12
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
6
3 Tension pulls water
from the veins into the
apoplast of the
mesophyll cells...
Leaf
Vein
4 ...then pulls the water
column through the
veins in the leaves...
2 Water evaporates
from mesophyll
cell walls.
Mesophyll
cell
Stem
H2O
5 ...and then upward in
the xylem of the root
and stem.
1 During transpiration
water vapor diffuses
out of the stomata.
Xylem
H
H
H O
O
H
H
O H
H
H O
Root
6 Water molecules
form a cohesive
water column
from the roots to
the leaves.
H2O
7 Water moves
H2O
into the xylem
by osmosis.
8 Water enters the root from
Xylem
the soil by osmosis.
35.7 The Transpiration–Cohesion–Tension Mechanism (Page 731)
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Figure 35.06 Date 07-13-12
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
7
RESEARCHTOOLS
35.8 Measuring the Pressure of Xylem Sap with a Pressure Chamber
Without pressure
1 By applying just
enough pressure…
With pressure
2 …so that xylem sap is
pushed back to the cut
surface of a plant sample,…
3 …a scientist can determine
the tension on the sap in
the living plant.
Gas pressure
Pressure
gauge
Pressure release valve
(Page 732)
Life10e_35.07
Sinauer
7-11-12
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
8
(A)
Guard
cells
10 µm
Stoma
(B)
Cl–
K+
1 In the light, guard cells
actively pump protons
out, thus facilitating the
entry of K+ and Cl–.
H+
2 Higher internal K+ and
Guard
cells
Cl– concentrations give
guard cells a more
negative water potential,
causing them to take up
water, increase in
pressure, and stretch,
opening the stoma.
H2O
Stoma
Cellulose
microfibrils
K+
H2O
Cl–
35.9 Stomata (Page 733)
3 In the absence of light,
K+ and Cl– diffuse
passively out of the
guard cells, and water
follows by osmosis. The
guard cells shrink and
the stoma closes.
Remove a ring of
bark to girdle the tree.
Organic solutes accumulate
in the phloem above the
girdle, causing swelling.
Time
LIFE The Science of Biology 10E Sadava
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Figure 35.08 Date 07-13-12
Bark
Wood
In-Text Art (Page 734)
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
9
Pores of sieve plate
Sieve plate
Mature sieve tube elements do not have
nuclei and have lost most of their organelles.
Sieve tube element
Phloem sap
The companion cell is a fully
functional cell with a nucleus.
Companion cell
Pores
Sieve plate
Dr. R. Kessel & Dr. G. Shih/Visuals Unlimited.
35.10 Sieve Tubes (Page 735)
Sieve tube
element
LIFE The Science of Biology 10E Sadava
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Figure 35.09
Date 07-13-12
The aphid’s stylet
has successfully
penetrated the
sieve tube.
Sap droplet
Longistigma caryae
(aphid)
Stylet
In-Text Art (Page 735)
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
10
Phloem
sieve
tube
Xylem
1 Transpiration
pulls water up
xylem vessels.
Source
cell
2 Source cells load sucrose
into phloem sieve tubes,
reducing their water
potential…
H2O
H2O
Sucrose
3 …so water is taken up
from xylem vessels
by osmosis, raising the
pressure potential in
the sieve tubes.
H2O
Sink cell
4 Internal pressure
differences drive the
sap along the sieve
tube to sink cells.
Sucrose
H2O
5 Sucrose is unloaded
into sink cells, increasing
the water potential in the
sieve tube…
6 …and water moves
back to xylem vessels.
35.11 The Pressure Flow Model (Page 736)
LIFE The Science of Biology 10E Sadava
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Figure 35.10
Date 07-13-12
TablE35.1
Mechanisms of Sap Flow in Plant Vascular Tissues
Xylem
Phloem
Driving force for
bulk flow
Transpiration from
leaves
Active transport of
sucrose at source
and sink
Site of bulk flow
Nonliving vessel
Living sieve tube
elements and tracheids elements
Pressure
Negative (pull from top; Positive (push from
potential in sap tension)
source; pressure)
(Page 736)
© 2014 Sinauer Associates, Inc.
Chapter 35 | Transport in Plants
11
INVESTIGATINGLIFE
35.12 Manipulating Sucrose Transport from the Phloem
HYPOTHESIS Reducing the sucrose concentration in a sink
organ will increase the transport and unloading of sucrose from
the phloem.
Method
Plants were transformed with a gene for invertase, an
enzyme that hydrolyzes sucrose.
The potato plant has
underground tubers,
modified stems that
store starch.
Results
Wild-type plants
Transgenic plants
Phloem
Tuber
sink cell
Sucrose
The wild-type plants had a high
level of sucrose in developing
tubers. The tubers were normal
in size and number.
The genetically modified plants
had a low level of sucrose in
developing tubers and produced
fewer but much larger potatoes.
CONCLUSION Increasing sink strength increases sucrose
transport into developing tissues.
Go to BioPortal for discussion and relevant links for all
INVESTIGATINGLIFE figures.
(Page 737)
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© 2014 Sinauer Associates, Inc.