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WATER AND SUGAR TRANSPORT
Chapter 32
In a hot summer day, an oak tree may lose up to 55 gallons of water in a single day.
Water loss due to transpiration is inevitable for plants. The stomata must be opened in order to
obtain carbon dioxide and release the oxygen produced during photosynthesis.
WATER POTENTIAL AND CEL-TO-CELL MOVEMENT
Osmosis is passing of solvent (e.g. water) molecules through a semipermeable membrane.
Water moves from the region of high water concentration to region of low water concentration.
Note that high concentration of water means low concentration of solutes.
The difference in solute concentration of different sides of a membrane is called the solute
potential or osmotic potential.
Water inside the cell pushes against the cell membrane, which in turns pushes against the cell
wall creating what is call, the turgor pressure.
The stiff wall produces a counteracting equal force against the turgor pressure called the wall
pressure.
Cells that are firm with these two forces acting against each other are said to be turgid.
The sum of these forces is called pressure potential.
Water potential
Water potential is the tendency of water to move from one location to another.
Water potential is a measure of the solute potential and pressure potential of the cell.
Water potential is expressed by the Greek letter Ψ (sigh).
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Ψ = Ψ s + Ψp
Water potential is a form of potential energy.
Water potential is a measure of the cell's ability to absorb water by osmosis.
It also measures the water's tendency to evaporate from the cell.
Water potential of pure water is 0 by convention, because it has no solutes.
When there is a solute, the water potential is expressed with a negative sign because the water
potential is less than in pure water. The solutes make the water in the cell less likely to move
out.
The pressure potential (Ψp ) pushes on the water inside the cell and makes it more likely to
move out of the cell. The pressure potential carries a positive sign.
The energy of pure water is 0 megapascals ( 1 MPa = 10 atmospheres or 14.5 pounds/inch2).
When solutes dissolve in water, the free energy of water decreases and the water potential
becomes a negative number.
Water moves from the region of high water potential (very negative) to the region of lower water
potential (less negative).
When a cell loses water, the cell membrane separates from the cell wall and the cell becomes
flaccid. This state is called plasmolysis.
The effects of water movement.
Water potential can also be measured for tissues, organs and systems.
Water moves from tissues with high water potential to surrounding tissues with lower water
potential.
The air around the plant and the soil have water potential.
Water in the soil contains solutes and it can be under pressure.
The water potential of air, soil and plant tissues change constantly under the influence of wind,
sun, rain, etc.
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The water potential in the soil is usually high relative to plant tissues.
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The water potential of the air is usually low relative to plant tissues.
There is a series of water potential differences between the soil, plant tissues and air called the
water potential gradient.
TRANSPIRATION AND WATER MOVEMENT FROM ROOTS TO LEAVES.
The loss of water from leaves, stems and other aerial parts of the plant is called transpiration.
Cohesion-Tension Theory
Also known as the Transpiration-Cohesion Theory.
Water molecules are polar and attract each other. They form hydrogen bonds with adjacent
molecules.
Water molecules are also attracted by molecules other than water, e.g. molecules on the wall of
the tracheids and vessel elements.
This attraction by hydrophilic walls is called adhesion.
Mesophyll cells are surrounded by a film of water. This film of water lines the air spaces in the
mesophyll: the water-air interface.
At the water-air interface a meniscus is formed because the outer layer of water molecules are
pulled in only one direction while those below are equally pulled from several directions. The
meniscus is formed that way.
When water leaves the meniscus surface, the meniscus becomes more concave and its surface
tension increases.
Water is being pulled on by adhesive and cohesive forces creating a negative pressure. This
negative pressure pulls water molecules from places where the pressure is greater: the
cytoplasm of the mesophyll cells and xylem cells.
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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.
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.
According to the Cohesion-Tension Theory, plants do not spend energy in transpiration.
Limiting water loss.
How do plants cope with soils that have very low water potential?
1. Morphological adaptations that limit water loss.
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Thick cuticle
Sunken stomata
2. Physiological adaptations that allow plants to function when water content is low.
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CAM and C4 photosynthesis
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Changes in solute potentials in roots
Increasing the concentration of solutes causes the solute potential to drop below the soil solute
potential causing water to move from higher to lower concentration (of water) following the water
gradient created by the solutes.
The anatomy of phloem tissue.
The phloem has two important cells, the sieve-tube elements and the companion cells.
Characteristics of these cells:
Sieve tube members
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Living cells at maturity.
Lack nucleus and other organelles at maturity.
Elongated cells, cylindrical, joined end to end.
Secondary cell wall present.
End walls are sieve plates with holes.
Cytoplasm extends from one cell to the next through the holes of the sieve plate.
Conduct the products of photosynthesis.
The most important photosynthate transported in the sieve-tube elements is sucrose, a
disaccharide.
Companion cells
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Living cells at maturity.
Associated to a sieve tube members by means of plasmodesmata.
Assists in moving sugars in and out of sieve tube members.
The nucleus is thought to direct the activity of both cells.
The Pressure-Flow Hypothesis.
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.
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.
Energy has to be spent in order to bring sugar into the sieve-tube elements against the
concentration gradient.
2. It requires ATP and a membrane transport system.
3. As a result water moves into the sieve tubes by osmosis increasing the hydrostatic pressure
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. Water is collected by the xylem and recycled.
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.
Cotransport.
In cotransport, and ATP-powered system transports ions or molecules and indirectly powers the
movement of other solutes by maintaining a concentration gradient.
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ATP is used to create a gradient of ions or molecules.
The protein H+-ATPase hydrolyzes ATP into ADP and P, and uses the energy to
pump H+ across the membrane to the outside of the cell.
This establishes a difference in H+ and charge across the membrane. This difference
in charge is called an electrochemical gradient.
The tendency of the protons is to move back into the cell.
They do so through the opening provided by another protein called the
cotransporter.
When these ions move back to the lower concentration area in the cytoplasm, they
carry with it the molecules of sucrose against the solute concentration gradient.
ATP energy is used indirectly.
Phloem loading of sucrose
Experiments have shown that H+-ATPase, proton pumps, are located on the membrane of the
companion cells.
1. Sucrose is transported from the source cell into the companion cells using the
cotransport system (e.g. H+-ATPase, etc).
2. Once inside the companion cell, the sucrose moves into the sieve-tube elements
through plasmodesmata, direct cytoplasmic connections.
Phloem unloading of sucrose
The mechanisms for unloading sucrose varies according to the different types of sinks (e.g. root
cells, leave cells, etc.) in the plant.
In the leaves of some plants like sugar beets, sucrose flows along the concentration gradient
created between the high concentration in the sieve-tube elements through the companion cell
into the leave cell where sugar is rapidly used in cellular respiration and other cell activities
(passive transport).
In the roots of the sugar beet, sucrose moves passively from the sieve-tube elements to the
companion cells then into the root cell.
The root cell has an organelle that stores large amounts of sucrose. A proton pump located in
the membrane surrounding the organelle hydrolyzes ATP and brings sucrose into the organelle
by means of cotransport (active transport).