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• To get onto land, plants evolved way to keep from drying out, to stand upright.
• Transport nutrients and water both over long distance and short distances.
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• At cellular level - plasma membrane allows for transport into cell (transport proteins).
• Some transport proteins act as selective channels - determine what can go into/out of cell.
• Plant cell - proton pumps function in pumping H+ ions out of cell.
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• Proton pump can aid in cotransport - H+ is pumped out of cell aiding in pumping in/out (against concentration gradient) of another substance (glucose)
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• Plants rely on osmosis to survive.• Direction of water movement
depends on solute concentration and physical pressure. (water potential)
• Water moves from high water potential to low water potential.
• Water potential measured in MPa - abbreviated psi.
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• Applying pressure to water can reverse movement of water.
• Using syringe (negative pressure) can force water to move upwards.
• Combined effects of pressure and solute concentrations on water potential are incorporated into equation: psi = psiP (pressure potential + psis (solute potential)
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• Flaccid cell, psip = 0.• Placed in solution with lower psi,
water will leave cell.• Cell will plasmolyze, shrinking
and pulling away from wall.• As cell swells, it will push against
wall, producing turgor pressure.
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• Placed in pure water - cell will have lower water potential due to solutes and water will enter cell.
• Walled cell with greater solute concentration than its surroundings will be turgid or firm. QuickTime™ and a
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• Aquaporins are specific transport proteins - aid in passive movement of water only.
• Cell wall gives plants shape, but not passing of materials.
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• Membrane that bounds vacuole (tonoplast) regulates molecular traffic between cytosol and contents of vacuole (cell sap)
• Plasmodesmata (connections between cells) connect symplast (cytoplasm stream)
• Cell walls of adjacent plant cells - apoplast. QuickTime™ and a
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• Because of distance water and nutrients need to travel between roots and leaves, simple diffusion not efficient enough.
• Water and solutes move through xylem vessels and sieve tubes by bulk flow, movement of fluid driven by pressure.
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• Tension allows for transport of materials.
• Transpiration forces water to move up plant in stream (negative pressure) - allows materials to move in bulk.
• Larger diameter of stem, faster material can move.
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Absorption of water by roots
• Water, mineral salts from soil enter plant through epidermis of roots, cross root cortex, pass into stele, then flow up xylem vessels to shoot system.
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Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 36.7
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• Much of absorption of water and minerals occurs near root tips - epidermis is permeable to water and where root hairs are located.
• Root hairs allow for maximum uptake.
• Most plants form partnerships with symbiotic fungi for absorbing water and minerals from soil.
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• Water, minerals in root cortex cannot be transported to rest of plant until they enter xylem of stele.
• Endodermis, innermost layers of root cortex, surrounds stele, is last checkpoint for absorption into vascular tissue.
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Transport of Xylem• Xylem sap flows into veins of leaf
providing them with water.• Plants lose water through
transpiration; water replaced through water transport.
• Xylem sap rises against gravity through pumping system.
• Accumulation of minerals in stele lowers water potential; generates positive pressure (root pressure) forces fluid up xylem.
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• Root pressure causes guttation - exudation of water droplets (seen in morning on tips of grass blades)
• Roots accumulate water during night, transpiration is low, water enters leaf at faster rate.
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• Xylem sap pulled through plant creating stream of water - cannot be broken.
• Cavitation (formation of water vapor pockets in xylem vessel) breaks chain.
• Occurs when xylem sap freezes in water.
• Cannot be fixed in trees, but stream can form around it.
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Control of transpiration• Guard cells control amount of
water lost during day (through stomata).
• Transpiration also cools plant down.
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• When transpiration exceeds delivery of water by xylem, (soil begins to dry out) leaves begin to wilt as cells lose turgor pressure.
• Guard cells control diameter of stoma by changing shape, widening or narrowing gap between 2 cells. QuickTime™ and a
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• Potassium helps in regulation of guard cells.
• Stomata open during day, closed at night to minimize water loss when too dark for photosynthesis.
• Regulated in 3 ways.• 1st - blue-red wavelengths signal
plant to start photosynthesizing.
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• 2nd - depletion of CO2.• 3rd - internal clock in plant cues
plant to start photosynthesizing - started at dawn.
• Opening and closing cycle of stomata is an example of circadian rhythm, cycles that have intervals of approximately 24 hours. QuickTime™ and a
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• Plants adapted to arid climates (xerophytes) - leaf modifications that reduce rate of transpiration.
• Some -smaller, thicker leaves.• Some - shed leaves during
extremely dry months. • Some - stomata concentrated on
lower (shady) leaf surface.
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Phloem sap
• Phloem transports organic products of photosynthesis throughout plant via translocation.
• Phloem sap - aqueous solution - sugar (mostly disaccharide sucrose) most abundant solute.
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• Xylem - unidirectional movement; phloem movement - variable.
• Sieve tubes carry food from sugar source to sugar sink.
• Sugar source - plant organ (especially mature leaves) where sugar is being produced by either photosynthesis or the breakdown of starch. QuickTime™ and a
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