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Chapter 36
TRANSPORT IN PLANTS
Plants absorb water and minerals through their roots and transport them to the leaves and
stems for metabolic use, e. g. photosynthesis.
OVERVIEW OF TRANSPORT MECHANISMS
Transport at the cellular level depends on the selective permeability of membranes.
Biological membranes are usually permeable to small molecules and lipid-soluble substance.
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Water, gases (O2, N2, CO2, CO), small polar molecules (glycerol), larger non-polar
molecules (hydrophobic substances like hydrocarbons).
Biological membranes are impermeable to and use proteins to transport the following types of
molecules,
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Ions, amino acids and sugars, e. g. glucose.
Transport methods (review chapter 8):
1.
2.
3.
4.
5.
6.
Simple diffusion: along concentration gradient.
Facilitated diffusion: protein channels, along concentration gradient.
Carrier-mediated transport: pumps, require ATP.
Cotransport
Endocytosis
Exocytosis
Proton pumps play an important role in transport across the membrane.
The energy stored in a proton gradient is used to transport solutes across the membrane.
In chemiosmosis, the proton gradient is used to synthesize ATP.
The proton gradient across the membrane creates membrane potential that can be harnessed
to perform cellular work.
Osmosis
Osmosis is the passive transport of water across a membrane.
Water will move across a cell membrane in the hypotonic (low solute concentration) →
hypertonic direction (high solute concentration).
In plants, the presence of the cell wall that limits the expansion of the cell adds another factor
that affects osmosis.
The combine effect of solute concentration and pressure makes what is called the water
potential, represented by the letter psi, ψ.
Water moves across the membrane from the region of high water potential to that of low water
potential.
Water potential is measured in megapascals, MPa.

1 MPa = 10 atmospheres or 14.5 pounds/inch2.
Water potential for pure water in an open container is standardized a 0 MPa.
Adding solutes lowers the water potential because the water molecules surrounding the solute
have less freedom of movement due to intermolecular attractions.
Any solution at atmospheric pressure has a negative water potential.
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There is an inverse relationship of ψ to solute concentration (osmotic potential).
Increasing the pressure on water increases the ψ.
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There is direct relationship of ψ to physical pressure.
It is possible to create a negative pressure or tension.
Water potential equals the combine pressure and solute concentration (osmotic potential).
ψ = ψp + ψs
A cell placed in a hypertonic solution will become plasmolyzed.
A plant cell placed in pure water will absorb water and become turgid.
Turgor pressure will increased and the cell will push against the rigid wall. The partially elastic
wall will push back increasing the ψp until it becomes great enough to offset the tendency of
water to enter.
There are water channels that allow the flux of water in and out of the cell. These transport
proteins are called aquaporins.
Aquaporins affect only the rate at which the water flows. They do not affect the concentration
gradient or the direction of the water flow.
Vacuolated cells have three major compartments: cell wall, cytosol and central vacuole.
The membrane that bounds the vacuole is called tonoplast and regulates the traffic between
the vacuole and the cytosol.
In most plant tissues, the cell wall and the cytosol is continuous from cell to cell.
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Plasmodesmata connect the cytosol forming the symplast.
The cell walls of adjacent cells are in contact and form the apoplast.
Lateral transport in plant tissues can occur via the symplast, the apoplast or through the cell
membranes between cells.
ABSORPTION OF WATER AND MINERALS THROUGH THE ROOTS.
Water and minerals from the soil enter the epidermis of roots, cross the root cortex, pass into
the vascular bundle and into the xylem vessels where they move up the plant body.
Root hairs are the most important avenue of absorption near root tips.
The hyphae involved in mycorrhizae are important in the absorption of water and minerals.
The mycelium of the fungus provides an enormous surface area for absorption.
Endodermis, the innermost layer of the cortex, controls mineral uptake into the xylem.
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Cells have a Casparian strip around the radial and transverse walls, that is
impermeable to water and minerals. The Casparian strip is made of suberin.

Minerals most pass through carrier proteins in the plasma membrane of the
endodermal walls. It requires ATP.
Parenchyma cells within the stele discharge minerals into the xylem vessels and tracheids.
Vessels and tracheids lack protoplast and consist of tubes made mostly of cellulose. Their cell
wall and lumen form part of the apoplast.
TRANSPORT OF XYLEM SAP
Water and minerals that have been transferred to the xylem are transported upwards in the
xylem sap.
Plants lose a large amount of water by transpiration, the loss of water vapor from leaves and
other aerial parts of the plant.

An average maple tree loses an average of 200 liter/hour in the summer time or
about 53 gallons/hour.
Roots have many solutes dissolved in their cells, which lowers their water potential in relation to
the soil in which they grow.

Water moves in from the soil into the roots by osmosis.
When soil is very dry, its water potential is very low.
Unless the soil is extremely dry, roots have a lower water potential (very negative) than the soil
and water tends to move by osmosis from the soil into the roots.
Root pressure
Cells in the root pump ions into the root stele.
The endodermis prevents these ions from leaking back into the cortex.
The water potential in the stele is lowered and water flows in from the root cortex generating a
positive pressure that forces fluid up the xylem. This pressure is called root pressure.
Root pressure pushes water from the root up the stem.
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Not strong enough to push the up tall plants.
It is very low or non-existent during the summer months.
Movement of water is greatest in the summer months when root pressure is the
lowest.
Guttation is the release of water droplets through small openings on leaves of plants.
Guttation is the result of root pressure.
Root pressure can force the water up a few meters only. It is not the main mechanism that
brings water to the top of the plant but it contributes.
Many plants do not generate root pressure at all.
Cohesion-Tension Theory
Also known as the Transpiration-Cohesion Theory.
Water is constantly being lost through the stomata. This water is replaced with water vapor from
the mesophyll cells.
1. As water evaporates, a meniscus is formed by the remaining water in the cell wall spaces,
attracted by adhesion to the hydrophilic wall.
2. Cohesive forces also operate on the surface of the water film.
3. These two forces, adhesion and cohesion, create the meniscus that has a negative force.
4. This negative pressure draws water out of the xylem through the mesophyll, and toward the
cells and surface film bordering the air spaces near stomata.
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There is a gradient in water potential from the atmosphere down to the soil.
The atmosphere has very negative water potential.
Leaves have higher water potential than the atmosphere and lose water to it.
Stems have higher water potential than the leaves; the roots higher than the stem; and the
soil higher than the roots.
The gradient creates a pull of the column of water in the xylem due to the hydrogen bonds that
exist between the water molecules (cohesion).
Adhesion of the water molecules to the xylem walls maintains an unbroken column of water.
The walls of the vessels and tracheids are hydrophilic and increase the adhesion of water
molecules.
The transpirational pull is transmitted from the leaves to the root tips and even into the soil
solution.
The plant does not spend any of its energy in bringing the water up to the top.
Solar energy drives transpiration by causing water to evaporate from the moist walls of
mesophyll cells and by maintaining a high humidity in the air spaces with a leaf.
CONTROL OF TRANSPIRATION
Photosynthesis consumes CO2 and produces O2. Both gases diffuse in and out of the leaf
respectively through the stomata.
The spongy mesophyll of the leaf increases the surface area exposed to CO2 but also increases
the surface area of evaporation.
A plant loses 90% of the water through the open stomata.
Water loss is a trade-off for allowing CO2 to enter the leaf.
The waxy cuticle covering most of the leaf surface prevents evaporation.
The transpiration-to-photosynthesis ratio evaluates how efficiently a plant uses water.
It is the amount of water lost per gram of CO2 assimilated into organic material by
photosynthesis.

For many plants species this ratio is 600:1 or 600 g of water are lost for each gram of
CO2 incorporated into carbohydrate.

For C4 plants, the ratio is 300:1
Transpiration also brings mineral to all parts of the plant and helps in cooling the plant.
How stomata open and close
The stoma is the opening located between to dumbbell-shaped guard cells.
The guard cells are suspended over an air chamber by subsidiary epidermal cells.
Guard cells control the diameter of the stoma by changing shape.
When the guard cells become turgid, the stoma opens. When flaccid, the stoma closes.
Potassium ion mechanism.
The changes in turgor pressure in the guard cells are the result of the reversible uptake and loss
of K+.
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Light triggers an influx of K+ into the guard cells.
It occurs through active transport; ATP required. A proton pump is probably involved and
K+ move through channels driven by a membrane potential.
Osmotic pressure decreases and water moves into the guard cells.
The increase turgidity of the cells causes a change in shape and the stoma opens.
Most of the K+ are stored in the central vacuole. The tonoplast plays a role here.
Regulation of aquaporins may also be involved by varying the permeability of the
membranes to water.
Opening of the stomata is most pronounced in blue light, and to a lesser extent in red light.

Light  proton pump moves H+  K+ transported through  water diffuses into
out of the guard cell
specific K channels
the guard cells
 guard cells change shape and open the stoma.
The stoma may close by a reversal of the process when light decreases.
Loss of turgidity closes the stoma.
Stimuli to open and close the stomata:
1. Stomata are open or closed according to the physiological needs of the plant.
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Photosynthesis: depletion of CO2. A low concentration of CO2 in the leaf induces
stomata to open even in the dark.
Transpiration: loss of water causes loss of turgor throughout the plant.
The hormone abscisic acid is produced in response to water deficiency and causes the
guard cells to close.
Increase in temperature increases cellular respiration and CO2 production.
In mesophytes, the stomata are usually open during the day and closed at night.
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CAM plants do the reverse.
2. Light affects triggers the intake of K+ by the guard cells.
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There are blue-light receptors in the membrane of the guard cells that trigger ATPpowered proton pumps which in turn promotes the uptake of K+ ions.
3. An internal clock causes the stomata to open and close periodically.
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Cycle of 24 hours are called Circadian rhythms.
Xerophytes have leaf adaptations that reduce the rate of transpiration.
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Small, thick leaves or reduced to spines.
Thick cuticle
Stomata are concentrate on the lower, shady leaf surface, in pits: sunken stomata.
CAM pathway of photosynthesis. The stomata open during the night to incorporate CO2
into organic acids.
TRANSLOCATION OF PHLOEM SAP
Sucrose is the main sugar translocated in the phloem.
Sugars moves from the source where it is being produced, to the sink, where the sugars are
being utilized or stored.
Sucrose manufactured in mesophyll cells can travel via the symplast to sieve-tube members.
In some species, sucrose leaves the symplast and travels through the apoplast until is actively
incorporated into the sieve-tube members or by the companion cells that then pass the sucrose
to the sieve tubes through plasmodesmata.
Pressure flow theory.
This theory postulates that sugar moves in the phloem by means of a pressure gradient that
exists between the source, where sugar is loaded into the sieve tube members, and the sink,
where sugar is removed from the phloem.
1. Sucrose and other carbohydrates is actively loaded into the sieve tubes at the source by a
chemiosmotic mechanism.
2. It requires ATP.
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ATP supplies energy to pump protons out of the sieve tube members into the apoplast.
Creates proton gradient.
The gradient drives the uptake of sucrose into the symplast through channels by the
cotransport of protons back into the sieve tube members.
3. As a result water moves into the sieve tubes by osmosis increasing the hydrostatic pressure
in the sieve tubes that forces water to flow in the sieve tubes.
4. Sugar is actively or passively unloaded from the sieve tube into tissues at the sink.
5. As a result water leaves the sieve tubes at the sink decreasing the hydrostatic pressure
inside the sieve tubes.
6. A gradient is created between the source and sinks which drives the flow within the sieve
tubes.
Other substances transported in the phloem are hormones, ATP, amino acids, inorganic ions,
viruses and complex organic molecules like sugar-alcohol compounds.