Download Understanding Our Environment

Water In Plants
Outline
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Molecular Movement
 Diffusion
 Osmosis
Water Movement
 Cohesion-Tension Theory
 Regulation of Transpiration
 Transport of Organic Solutes
 Pressure-Flow Hypothesis
Mineral Requirements for Growth
Molecular Movement
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Diffusion
 Movement of molecules from a region of
higher concentration to a region of lower
concentration.
- Move along a concentration gradient.
- Move until equilibrium reached.
Diffusion
Osmosis
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Osmosis is diffusion of water through a differentially
permeable membrane from a region where the
water is more concentrated to a region where it is
less concentrated.
 Water enters a cell by osmosis until the osmotic
potential (pressure required to prevent osmosis)
is balanced by the resistance to expansion of the
cell wall.
- Turgor Pressure
 Pressure Potential (is the turgor pressure
that develops against the walls as a result
of water entering the vacuole).
Osmosis
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Water Potential of a plant is essentially its osmotic
potential and pressure potential combined.
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If two adjacent plant cells have different water
potentials, water will move from from the cell having
the higher water potential to the cell having the
lower water potential.
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Water flows from the xylem to the leaves,
evaporates within the leaf air spaces, and
transpires through the stomata into the
atmosphere.
Osmosis
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Osmosis is the primary means by which
water enters plants from their environment.
Molecular Movement
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Plasmolysis
 Loss of water through osmosis is
accompanied by shrinkage of protoplasm
away from the cell wall.
Imbition
 Colloidal material (permanent suspension
of fine particles) and large molecules
usually develop electrical charges when
they are wet, and thus attract water
molecules which adhere to the internal
surfaces of the materials.
Fig. 9.5
Fig. 9.6
Molecular Movement
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Active Transport
 Plants absorb and retain solutes against a
diffusion, or electrical, gradient through the
expenditure of energy.
- Involves proton pump.
Water and Its Movement Through The Plant
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More than 90% of the water entering a plant
passes into leaf air spaces and then
evaporates through the stomata into the
atmosphere (Transpiration).
 Usually less than 5% of
water escapes through
the cuticle.
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Cohesion - Tension Theory
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When the negatively charged end of one
water molecule comes close to the positively
charged end of another water molecule,
weak hydrogen bonds hold the molecules
together.
 Water molecules adhering to capillary
walls of the xylem tracheids and vessels,
and cohering to each other, create a
certain amount of tension.
Cohesion - Tension Theory
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When water transpires, the cells involved
develop a lower water potential than the
adjacent cells.
 Creates tension on water columns,
drawing water from one molecule to
another, throughout the entire span of
xylem cells.
Regulation of Transpiration
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Changes in turgor pressure occur when
osmosis and active transport between the
guard cells and other epidermal cells cause
shifts in solute concentrations.
 When photosynthesis is not occurring in
the guard cells, potassium ions leave, and
the stomata close.
- An increase in potassium ions causes a
lowering of the water potential and
osmosis leading to turgid guard cells.
Fig. 9.13
Regulation of Transpiration
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Stomata of most plants are open during the
day and closed at night.
 Stomata of many desert plants open only
at night.
- Conserves water, but makes carbon
dioxide inaccessible during the day.
Humidity plays an inverse role in
transpiration rates.
 High humidity reduces transpiration, while
low humidity accelerates it.
Regulation of Transpiration
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If a cool night follows a warm, humid day,
water droplets may be produced through
hydathodes at the tips of veins of some
plants (Guttation).
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Transport of Organic Solutes in Solution
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One of most important functions of water in
the plant involves the translocation of food
substances in solution by the phloem.
 Most of our knowledge on this subject
came from studying aphids feeding on
phloem.
Fig. 9.16
Pressure-Flow Hypothesis
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Organic solutes flow from a source where
water enters by osmosis.
 Organic solutes are moved along
concentration gradients between sources
and sinks.
Fig. 9.17
Mineral Requirements for Growth
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Essential Elements
Macronutrients and Micronutrients
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Macronutrients are used by plants in greater
amounts and constitute 0.5% to 3.0% of the
plant’s dry weight.
 Nitrogen, potassium, calcium, phosphorus,
magnesium, and sulfur.
Micronutrients are needed by the plants in
very small amounts, often constituting only a
few parts per million of the dry weight.
Review
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Molecular Movement
 Diffusion
 Osmosis
Water Movement
 Cohesion-Tension Theory
 Regulation of Transpiration
 Transport of Organic Solutes
 Pressure-Flow Hypothesis
Mineral Requirements for Growth
Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display