1. Biology Sylvia S. Mader Michael Windelspecht
Chapter 25
Flowering Plants: Nutrition and Transport
Lecture Outline
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2. 25.3 Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
25.3 Transport Mechanisms in Plants
Plant Nutrition
Vascular tissues transport water and nutrients.
Xylem transports water and minerals.
Two types of conducting cells
Tracheids
Vessel elements
Water flows passively from an area of higher water potential to an area of lower water potential.
Phloem transports organic materials.
Conducting cells are sieve-tube members.
They have companion cells to provide proteins.
End walls are sieve plates.
Plasmodesmata extend through sieve plates.

4. Plant Transport and Water Potential
Biology, 9th ed, Sylvia Mader
Chapter 26
Plant Nutrition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Leaf
intercellular
spaces
xylem
phloem
stoma O2
CO2
H2O O2
CO2
H2O
sugar
H2O
Stem
xylem
phloem
sugar
Root
H2O
H2O
xylem
phloem

5. Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
Transport Mechanisms in Plants
Plant Nutrition
Potential energy is stored energy.
Water potential is the energy of water.
Water moves passively from a region of higher potential to a region of lower potential.
In terms of cells, two factors usually determine water potential:
Water pressure across a membrane
Solute concentration across a membrane

6. The Concept of Water Potential
Biology, 9th ed, Sylvia Mader
Chapter 26
The Concept of Water Potential
Plant Nutrition
Pressure potential is the effect that pressure has on water potential.
Water moves across a membrane from the area of higher pressure to the area of lower pressure.
The higher the water pressure, the higher the water potential.
Pressure potential that increases due to osmosis is called turgor pressure.
Osmotic potential takes into account the presence of solutes.
Water tends to move from the area of lower solute concentration to the area of higher solute concentration.
The lower the concentration of solutes (osmotic potential), the higher the water potential.

7. Water Potential and Turgor Pressure
Biology, 9th ed, Sylvia Mader
Chapter 26
Water Potential and Turgor Pressure
Plant Nutrition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
central vacuole
Wilted
central vacuole
Turgid
cell wall
cell wall
H2O
enters
the cell
Extracellular fluid:
water potential
pressure potential
osmotic potential
Equal water
potential inside and
outside the cell
higher
Inside the cell:
water potential
pressure potential
osmotic potential
Pressure potential
increases until
the cell is turgid
lower
a. Plant cells need water.
b. Plant cells are turgid.

8. Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
Transport Mechanisms in Plants
Plant Nutrition
Water Transport
Xylem vessels form an open pipeline.
The vessel elements are separated by perforated plates.
Water moves into and out of tracheids through pits.
Water entering roots creates a positive pressure (root pressure).
It pushes xylem sap upward.
May be responsible for guttation
Water forced out vein endings along edges of leaves

9. Root Pressure and Guttation
Biology, 9th ed, Sylvia Mader
Chapter 26
Root Pressure and Guttation
Plant Nutrition

10. Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
Transport Mechanisms in Plants
Plant Nutrition
Cohesion-tension model of xylem transport suggests a passive xylem transport.
Cohesion is the tendency of water molecules to cling together.
Adhesion is the ability of the polar water molecules to interact with molecules of vessel walls.
A continuous water column moves passively upward due to transpiration.

11. Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
Transport Mechanisms in Plants
Plant Nutrition
Leaves
Transpiration causes water loss through stomata.
Water molecules that evaporate are replaced by water molecules from leaf veins.
Due to cohesion, transpiration exerts a pulling force (tension) drawing water through the xylem to the leaf cells.
Waxy cuticle prevents water loss when stomata are closed.
Stem
Tension in xylem pulls the water column upward.
Roots
Water enters xylem passively by osmosis and is pulled upward due to tension in xylem.

12. Cohesion-Tension Model of Xylem Transport
Biology, 9th ed, Sylvia Mader
Cohesion-Tension Model of Xylem Transport
Chapter 26
Plant Nutrition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
mesophyll
cells
xylem in
leaf vein
Leaves
• Transpiration
creates tension.
stoma
• Tension pulls the water
column upward from
the roots to the leaves.
intercellular
space
H2O
cohesion by hydrogen bonding
between water molecules
adhesion due to
polarity of water
molecules
H2O
cell wall
water molecule
Stem
• Cohesion makes
water continuous.
• Adhesion keeps water
column in place.
xylem
H2O
water molecule
root hair
H2O
Roots
• Water enters xylem at
root.
• Water column extends
from leaves to the root.
xylem

13. Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
Transport Mechanisms in Plants
Plant Nutrition
Opening and Closing of Stomata:
Each stoma in a leaf epidermis is bordered by guard cells.
Increased turgor pressure in guard cells opens stoma.
Active transport of K+ into guard cells causes water to enter by osmosis and stomata to open.
H+ ions accumulate outside guard cells as K+ moves in.
Opening and closing of stomata is regulated by light.
ABA (abscisic acid) can also cause stomata to close.

15. Opening and Closing of Stomata
Biology, 9th ed, Sylvia Mader
Opening and Closing of Stomata
Chapter 26
Plant Nutrition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Open Stoma
H2O
H2O
vacuole K+
guard cell
stoma H+
K+ enters guard cells, and water follows. a.
25 µm
Closed stoma
H2O
H2O K+
K+ exits guard cells, and water follows.
25 µm b.
© Jeremy Burgess/SPL/Science Source

16. Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
Transport Mechanisms in Plants
Plant Nutrition
Organic Nutrient Transport:
Role of phloem
Phloem transports sugar.
Phloem sap doesn’t only move upward or downward as xylem does.
Travels from source (sugar’s origin) to sink (sugars are unloaded)
Girdling of tree below the level of leaves causes bark to swell just above the cut.
Sugar accumulates in the swollen tissue.
Radioactive tracer studies confirm that phloem transports organic nutrients.
Phloem sap can be collected using aphids.

18. Biology, 9th ed, Sylvia Mader
Acquiring Phloem Sap
Chapter 26
Plant Nutrition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
a. An aphid feeding on a plant stem
b. Aphid stylet in place
25.19a: © M. H. Zimmermann/Harvard Forest, Harvard University; 25.19b: © Steven P. Lynch

19. Transport Mechanisms in Plants
Biology, 9th ed, Sylvia Mader
Chapter 26
Transport Mechanisms in Plants
Plant Nutrition
Pressure-Flow Model of Phloem Transport
Sieve tubes form a continuous pathway for organic nutrient transport.
Sieve-tube members are aligned end to end.
Strands of plasmodesmata extend through sieve plates between sieve-tube members.
Positive pressure drives the movement of sap in sieve tubes.
Sucrose is actively transported into phloem at the leaves.
Water follows by osmosis, creating positive pressure.
The increase in pressure causes flow that moves water and sucrose from the source to the sink.

21. Pressure-Flow Model of Phloem Transport
Biology, 9th ed, Sylvia Mader
Chapter 26
Plant Nutrition
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
mesophyll cell of leaf
Leaf
phloem
water
xylem
sugar
Leaves
• Leaves are the main
source of sugar
production.
• Sugar (pink) is actively
transported into sieve
tubes.
• Water (blue) follows
by osmosis.
xylem
phloem
Stems
• Phloem contents flow
from a source to a sink.
• Xylem flows from the
roots to the leaves.
Roots
• Sugar is stored in
the sink.
• Cells can use it for
cellular respiration.
• Water exits by
osmosis and returns
to the xylem.
cortex cell
of root
xylem
phloem
Root