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Chapter 25 Flowering Plants: Nutrition

and TransportLecture Outline

BiologySylvia S. Mader

Michael Windelspecht

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25.3 Transport Mechanisms in Plants

• 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.

2

3

4

stoma

sugar

sugar

Stem

phloem

phloem

phloem

xylem

xylem

xylem

Root

Leaf

H2O

H2O

H2O

H2OCO2

O2

H2OCO2

O2

intercellular spaces

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Transport Mechanisms in Plants

• 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

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The Concept of Water Potential

• 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.

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Water Potential and Turgor Pressure

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low

erh

igh

er

Extracellular fluid:

a. Plant cells need water.

Inside the cell:

H2Oentersthe cell

b. Plant cells are turgid.

Pressure potentialincreases untilthe cell is turgid

Equal waterpotential inside andoutside the cell

cell wallcentral vacuole Wilted Turgid

cell wallcentral vacuole

water potentialpressure potentialosmotic potential

water potentialpressure potentialosmotic potential

Transport Mechanisms in Plants

• 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

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Root Pressure and Guttation

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Transport Mechanisms in Plants

• 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.

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Transport Mechanisms in Plants• 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.

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Cohesion-Tension Model of Xylem Transport

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H2O

H2O

H2O

stoma

xylem

cell wall

water molecule

xylem

water molecule

root hair

Leaves

Roots

Stem

mesophyllcells

intercellularspace

xylem inleaf vein

• Transpiration creates tension.

• Tension pulls the water column upward from the roots to the leaves.

cohesion by hydrogen bondingbetween water molecules

adhesion due topolarity of watermolecules

• Cohesion makes water continuous.

• Adhesion keeps water column in place.

• Water enters xylem at root.• Water column extends from leaves to the root.

H2O

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Transport Mechanisms in Plants• 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.

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Opening and Closing of Stomata

15© Jeremy Burgess/SPL/Science Source

Closed stoma

K+

a.

b. 25 µm

25 µm

Open Stoma

H2O H2O

H+

K+vacuole

guard cell

H2O

K+ exits guard cells, and water follows.

H2O

K+ enters guard cells, and water follows.

stoma

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Transport Mechanisms in Plants• 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.

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17

Acquiring Phloem Sap

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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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Transport Mechanisms in Plants

• 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.

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Pressure-Flow Model of Phloem Transport

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mesophyll cell of leaf

Leaf

phloemxylemwater

sugar

phloemxylem

Root

• Sugar is stored in the sink.• Cells can use it for cellular respiration.• Water exits by osmosis and returns to the xylem.

• Phloem contents flow from a source to a sink.• Xylem flows from the roots to the leaves.

Roots

Stems

Leaves• Leaves are the main source of sugar production.• Sugar (pink) is actively transported into sieve tubes.• Water (blue) follows by osmosis.

cortex cellof root

xylemphloem