CHAPTER 36
TRANSPORT IN
PLANTS
1
Overview of Transport in a
Vascular Plant
2
TERMS TO BE REVIEWED
1.
2.
3.
4.
5.
6.
7.
8.
9.
passive transport
transport proteins
active transport
cotransport
water potential
osmosis
solute concentration
aquaporins
tonoplast
3
10.
11.
12.
13.
14.
chemiosmosis
proton pump
membrane potential
turgor pressure
plasmolysis
I. INTRODUCTION
1. Most water and mineral absorption occurs in the cells at the tips of
the roots (Cellular Level).
2. Root hairs are modified epidermal cells that are specialized for
water absorption.
They absorb soil solution which consists of water molecules and
dissolved mineral ions that are not bound tightly to soil particles.
3. Soil solution flows through the hydrophilic walls of epidermal
cells and travels along the cell walls and the intercellular spaces
into the root cortex
4. Movement of soil solution into the cell involves: osmosis, diffusion,
active transport, proton pumps, aquaporins, water potential,
cotransport, transport proteins
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II. THREE MAJOR PATHWAYS OF
TRANSPORT IN PLANTS
A. Transport is also regulated by the compartmental structure of
plant cells
B. The plasma membrane directly controls the traffic of molecules
into and out of the protoplast
The plasma membrane is a barrier between two major
compartments, the cell wall and the cytosol
C. The third major compartment in most mature plant cells is the
vacuole, a large organelle that occupies as much as 90% or
more of the protoplast’s volume
 The vacuolar membrane regulates transport between the
cytosol and the vacuole
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Cell Compartments:
 Cell wall
 Cytosol
 Vacuole
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D. In most plant tissues, the cell wall and cytosol are continuous
from cell to cell
 The cytoplasmic continuum is called the symplast
 The cytoplasm of neighboring cells is connected by channels
called plasmodesmata
 The apoplast is the continuum of cell walls and extracellular
spaces
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E. Water and minerals can travel through a plant root by three
routes: (Short-Distance Transport)
 Transmembrane route: out of one cell, across a cell wall,
and into another cell
 Symplastic route: via the continuum of cytosol
 Apoplastic route: via the cell walls and extracellular spaces
F. Long-Distance Transport at the Whole Plant Level
 Involves movement along the vertical axis (up and down)
 Involves bulk flow which is the movement of fluid in xylem
and phloem driven by pressure differences at opposite
ends of xylem vessels and sieve tubes
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Transport Routes Between Cells
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III. ABSORPTION OF WATER AND
MINERALS BY ROOTS
1. Pathway:
Epidermis cortex endodermis stele (xylem)
2. Mineral ions enter epidermal cells by diffusion and active
transport using carrier proteins
3. Movement usually a combination of apoplastic and symplastic
routes
4. Only minerals and water using the symplastic route move
directly into xylem
5. Minerals and water using apoplastic route are blocked at the
endodermis by the Casparian strip and must enter an
endodermal cell to move into xylem
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Lateral Transport
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6. Mycorrhiza hyphae absorb water and selected minerals and
can enable older regions of the roots to supply water and
minerals to the plants.
Mycorrhiza
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IV. TRANSPORT OF XYLEM SAP
A. Ascent of xylem sap depends on:
1. transpiration-the loss of water vapor from leaves and other
aerial parts of the plant
2. physical properties of water-cohesion and adhesion
B. Xylem sap (composed of water and minerals) flows upward
against gravity
C. Xylem vessels are close to each leaf cell
D. Water must move upward to replace that lost by transpiration
E. Pushing Xylem Sap: Root Pressure
1. Usually occurs at night when transpiration is low
2. Root pressure (upward push of xylem sap) is generated by
accumulation of minerals in stele which lowers the water
potential and forces fluid up the xylem
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3. More water entering leaves than is transpired can result in
guttation (discharge of water droplets at the leaf margin)
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F. Pulling Xylem Sap: The Transpiration-Cohesion-Tension
Mechanism
1. Water vapor in the airspaces of a leaf diffuses down its water
potential gradient and exits the leaf via stomata by transpiration.
2. Transpiration produces negative pressure in the leaf, which
exerts a pulling force on water in the xylem, pulling water into
the leaf
3. Involves:
 Cohesion
 Adhesion
 Tension (Negative Pressure)
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TRANSPIRATION
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4. Cavitation—formation of water vapor pockets in xylem that
breaks the chain of water molecules and the pull is stopped
Once the water chain is broken the xylem vessels is no
longer functional
Can occur during drought stress or freezing
5. Ascent of xylem sap is ultimately solar powered.
.
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Ascent of Xylem Sap
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V. CONTROL OF TRANSPIRATION
A. Introduction
1. About 95% of water taken in is lost by transpiration through
the stomata
2. Amount of water lost by a leaf depends on the number of
stomata and the average size of the opening
3. Guard cells, by controlling the size of stomata, help conserve
water
4. Benefits of transpiration:
a. Assists in mineral transfer from roots to shoots
b. Reduces leaf temperatures
5. If transpiration exceeds delivery of water by xylem, plant wilts.
6. Rate of transpiration is greatest on a sunny, warm, dry, and
windy day
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7. Stomata are more concentrated on bottom of leaf away
from the sun to reduce evaporation
8. Waxy cuticle also prevents water loss
B. How Stomata Open and Close
1. Guard Cells control stomatal diameter by changing shape.
Turgid
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Flaccid
Turgid Guard Cells
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Flaccid Guard Cells
2. When guard cells take in water, they become turgid and the
gap between cells increases.
3. When guard cells lose water, they become flaccid and the
gap between cells decreases.
4. Changes in turgor pressure results primarily from the
reversible uptake and loss of K+ by guard cells
 Stomata open—guard cells accumulate K+ and gain water
 Stomata closed—guard cells lose K+ and lose water
5. Generally, stomata are opened during the day and closed at
night.
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Cells Turgid/Stoma Open
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Cells Flaccid/Stoma Closed
Cells Turgid/Stoma Open
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Cells Flaccid/Stoma Closed
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6. Stoma open at dawn because:
 Light stimulates guard cells to accumulate K+ and become
turgid
Decrease of CO2 because of PS
 Internal clock of guard cells (circadian rhythms—cycles that
have intervals of approximately 24 hours)
7. Stoma close during daytime because of:
 Water deficiency (environmental stress)
 Production of abscisic acid
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C. Adaptations to Reduce Transpiration
1. Xerophytes—plants adapted to arid climates
2. Modifications of xerophytes:
 small, thick leaves
 thick cuticle
 store water in fleshy stems during rainy season
 stomata concentrated on lower leaf surface
 stomata may be located in depressions
 CAM pathway for PS
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VI. TRANSPORT IN PHLOEM
A. Introduction
1. Translocation—transport of organic products of PS by
phloem throughout the plant
-in angiosperms it involves sieve-tube members
-direction varies
2. Phloem sap—aqueous solution that is mostly sucrose
3. Sugar source—an organ that is a net producer of sugar
such as mature leaves
4. Sugar sink—an organ that is a net consumer or storer of
sugar, such as a tuber or bulb
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B. Movement of phloem sap
1. Phloem sap moves from source to sink
2. Direction of flow depends on location of sugar source
and sugar sink which depends on the season
3. Sugar is first loaded into sieve-tube members before
moving to the sink.
4. Can involve:
 Symplastic and/or apoplastic routes
 Transfer cells—modified companion cells with
structures which increase cells’ surface area and
enhance transfer between apoplast and symplast
 Active transport and cotransport
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Loading of Sucrose into Phloem
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5. Sugar will diffuse from phloem to the sink.
6. Water follows by osmosis.
C. Pressure Flow (Bulk Flow) of Phloem Sap in Angiosperms
1. Phloem sap moves by bulk flow driven by positive
pressure (pressure flow)
2. Higher levels of sugar at the source lowers the water
potential and causes water to flow into the tube
3. Xylem recycles the water from sink to source.
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Pressure Flow in a Sieve Tube
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