Transcript Understanding Our Environment

Water In Plants
Chapter 9
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Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Diffusion
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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 is balanced by the
resistance to expansion of the cell wall.
- Turgor Pressure
 Pressure Potential
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Osmosis
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Water Potential of a plant is essentially its
osmotic potential and pressure potential
combined.
Water flows from the xylem to the leaves,
evaporates within the leaf air spaces, and
transpires through the stomata into the
atmosphere.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Osmosis
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Molecular Movement
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Plasmolysis
 Loss of water through osmosis is
accompanied by shrinkage of protoplasm
away from the cell wall.
Imbibition
 Colloidal material and large molecules
usually develop electrical charges when
they are wet, and thus attract water
molecules.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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.
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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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Required for Reproduction or Display
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission
Required for Reproduction or Display
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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, and each other, create a certain
amount of tension.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Mineral Requirements for Growth
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Essential Elements
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Macronutrients and Micronutrients
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Macronutrients are used by plants in greater
amounts.
 Nitrogen, potassium, calcium, phosphorus,
magnesium, and sulfur.
Micronutrients are needed by the plants in
very small amounts.
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
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
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission Required for Reproduction or Display
Stern - Introductory Plant Biology: 9th Ed. - All Rights Reserved - McGraw Hill Companies