Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
The
tensioncohesion
model
Fig. 33-11, p. 644
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Ψp = -2T/r
T= surface
Tension of water
7.28 X 10-8 MPa
r = radius of
curvature of
Air-water
interface
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
r (μm) ψMPa
0.5
-0.3
0.05
-3
0.01
-15
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
The
tensioncohesion
model
Fig. 33-11, p. 644
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Evidences for Cohesion-Tension
Theory
• 1. What force is necessary to
overcome gravity?
• 0.01 MPa per meter
• There requires 1MPa to draw
water up a 100 meter tree.
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CHAPTER 33 Stems and Plant Transport
What force is required to over come
friction within the xylem vessels?
• Friction exerts 0.02 MPa per
meter in a xylem vessel (40 μm)
or tracheid.
• Requires 2 MPa to overcome
friction in a 100 meter tree.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
So, it only requires 3 MPa to pull water up a
100 meter tree to overcome gravity and
fiction.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Tensile strength – strength required to
break a column of pure water (degassed)
is sufficient to withstand such tension.
Tensile strength of pure water = -30MPa
-30MPa
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
If dissolved air and mineral nutrients are
present, the tensile strength decreases.
• So, if the column of water in the
plant is under tension, you
would expect the column to
break if air gets into the xylem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Cavitation refers to the process where a
xylem vessel will fill with gas (water vapor
and air) as air is sucked into the vessel from
the surrounding cell walls of the xylem.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Cavitation refers to the process where a
xylem vessel will fill with gas (water vapor
and air) as air is sucked into the vessel from
the surrounding cell walls of the xylem.
gas
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
• Foresters in Harvard forests
detected cavitation using
ultrasonic detectors
during the middle of the day.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
If the water is under tension and being
sucked up, you would expect dye to be
drawn into the stems and leaves of plants, if
you cut the plants at the base of the stem
and place it into a color dye.
This is true.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
If the column of water is under tension, you
would expect the stems to shrink during the
day and contract at night.
• True for plants in Harvard
forests. Stems shrink by about
500 μm during the day relative
night
to night. day
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Positive pressure
Root pressure
nutrients
and sugars
concentrated here
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Root pressure pumps sugars and water up
maple trees giving rise to maple syrup
Freeze thaw cycles cause sugars to move
from xylem parenchyma cells into vessels
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Positive pressure
Root pressure
Water moves in
nutrients
and sugars
concentrated here
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Transport of sugars in the phloem
sucrose
Sucrose made in the leaves gets loaded into the phloem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Other substances transported in phloem
Amino acids, organic acids,
proteins, hormones and some
mineral elements are
transported in phloem.
Movement is bidirectional
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Sugars move by bulk flow from source to
sink along a turgor pressure gradient in
the phloem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
At SOURCE (Leaf cell)
Fig. 33-12, p. 646
1. Compantion cells use
ATP to Pump H+
out of cell.
The proton gradient
drives the uptake of
sucrose with H+ on a
protein carrier.
Companion cell
Sieve tube member
Direction of
water movement
Direction of
sucrose movement
Water then moves into
the sieve tube cells
increasing the
turgor pressure
in those cells.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
At SOURCE (Leaf cell)
1. At the sink, sugar is
unloaded actively
(uses ATP) out of the
sieve tube cells.
Fig. 33-12, p. 646
2. Water moves into
the adjacent cells
causing a decrease in
turgor
pressure in
the sieve tube cells.
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Biology, Seventh Edition
XYLEM
CHAPTER 33 Stems and Plant Transport
PHLOEM
At SOURCE (Leaf cell)
Fig. 33-12, p. 646
Figure 33-12
Page 646
Companion cell
Sieve tube element
Direction of
water movement
Direction of
sucrose movement
Sucrose actively
loaded into sieve tube
elements (requires
ATP).
Water diffuses
from xylem as a result
of decreased (more
negative) water
potential in sieve tube.
At SINK (Root cell)
Sucrose actively
unloaded into sink cell,
such as parenchyma
cell in the root cortex
(requires ATP).
Water diffuses
from phloem to xylem
as a result of increased
(less negative) water
Sieve tube
Vessel
running through running through
length of plant
length of plant
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Aphids used to study translocation in plants
Fig. 33-13, p. 647
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Phloem has a positive hydrostatic pressure
because sap flows out of
decapitated stylets of aphids that are stuck
in the phloem tissue
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Main differences between movement of
water and sugars in xylem and phloem
Trait
Xylem
Phloem
Pressure
potential
Cells
Negative
Positive
Dead at
maturity
100 m h-1
Living at
maturity
1 m h-1
Unidirectional
Bidirectional
Rate of
movement
Direction of
movement
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CHAPTER 33 Stems and Plant Transport
What are woody plants doing at this time of
year? How do they deal with the cold?
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CHAPTER 33 Stems and Plant Transport
Three ways that plants survive the
cold temperatures.
• Lipid membrane changes
Saturated lipids
Unsaturated lipids
during winter
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Two: Increase Sugar concentrations to
lower the freezing point of water inside
the cell (2-3 degree change)
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Three: Prevent ice formation inside
cells
Ice crystals
ruptures cells
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CHAPTER 33 Stems and Plant Transport
Antifreeze proteins
Antifreeze
proteins
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Ice
forms
outside of cell
Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Quiz 6 on Content in Chapter 33
Question 1.
• The inner bark of a woody dicot
stem is composed of
• A. secondary xylem tissue
• B. cork cambium and cork
parenchyma tissues
• C. secondary phloem tissue
• D. vascular cambium
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CHAPTER 33 Stems and Plant Transport
Question 2.
• Wood consists of
• A. secondary and primary xylem
tissues
• B. secondary xylem and phloem
tissues.
• C. only secondary xylem tissue
• D. vascular cambium, primary and
secondary xylem tissues
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 3.
• The fine speckles that one sees
on the woody twigs of trees are
called
• A. lentils
• B. stomata
• C. lenticels
• D. hydathodes
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 4.
•
•
•
•
•
•
Softwood lacks
A. vessels
B. tracheids
C. fibres
D. A and C
E. B and C
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 5
The pathway of least resistance
to water movement in plants
is via the
A. Symplast
B. Apoplast
C. Xylem
D. Phloem
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 6.
• Which is not true about xylem?
• A. It consists mainly of dead cells
• B. It is found in all organs of the
plant
• C. The hydrostatic pressure (turgor
pressure) is negative in the xylem.
• D. The hydrostatic pressure (turgor
pressure) is positive in the xylem.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 7.
• The cohesion-tension theory for water movement in
plants states that
• A. water is pushed up the plant due to positive
pressure that builds up in the roots.
• B. water is pulled up the plant as a continuous
column due to a tension that is generated on the
column of water as water evaporates from the
mesophyll cells into the air spaces of the leaf.
• C. water moves by osmosis from one xylem vessel to
the next until it reaches the leaf cells.
• D. water moves as a cohesive body from the leaves
to the roots.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 8.
• Cavitation refers to
• A. cavities forming in the leaves as
transpiration occurs.
• B. vessels and tracheids filling with air
and vapor.
• C. cavities forming in the leaves as the
xylem cells rupture under tension.
• D. sieve tube elements filling with air
and vapor as the xylem cells rupture.
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 9.
•
•
•
•
•
•
Sugars move along a
A. turgor pressure gradient
B. tension gradient
C. concentration gradient
D. A and C
E. B and C
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Biology, Seventh Edition
CHAPTER 33 Stems and Plant Transport
Question 10.
• The movement of water in the
xylem and phloem is
• A. fastest in the phloem
• B. fastest in the xylem
• C. at the same speed
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