Transcript Transport in Plants - Diablo Valley College
Transport in Plants
Chapter 36
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How does water get to the top of a tree?
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How does the leaf excess energy (sugars) get down to the roots?
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What things move –
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Into the plant ?
Out of the plant?
Inside the plant?
Overview
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Review Osmosis Movement across membranes Water Potential equation Water transport in Xylem
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Into the root
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Up the stem Out the leaf Stomata functioning Phloem transport
Membranes Movement
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Only water (somewhat) and gasses can move freely across membrane.
Charged and larger molecules need protein channels to pass membrane.
May move with it gradient passively May use ATP to actively pump molecules against their concentration /charge gradients across membranes.
Types of cellular transport across membranes
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Protons actively pumped out Cations (+) move across by facilitated diffusion due to membrane potential (charge gradient).
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Cotransport uses anion(-) gradient potential to move cations or neutral molecules across membrane.
Types of cellular transport across membranes
Fig. 8.12
Fig. 8.11
Water Potential
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Two components
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Concentration (solutes present) Pressure (resistance to cell wall, gravity, and physical pumping) Water Potential Ψ=Ψ (solute) + Ψ (pressure) Osmosis Reverse osmosis
Water Potential
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Ψ values measured in MPa Ψ(total)=Ψ (solute) + Ψ (pressure) Distilled water has a Ψ (solute)=0 Any concentration of a solute makes Ψ (solute)=negative The higher the concentration, the more negative Ψ (solute) Water moves to more negative Ψ (total) Ions/water can be stored in the tonoplast
Root pressure Leaf stem
Water Movement in xylem
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Xylem cells are dead and hollow.
Three stages:
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How water gets in to roots
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How water moves up the stem
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How water leaves the leaf Water loss controlled by stomata
Cell to cell water movement
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Symplast movement never leaves cytoplasm Apoplast moves from cytoplasm to cell wall Cellulose is hydrophilic
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Paper towels
Movement in root
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Cellulose wall or Lower Ψ (solute) of root hair cytoplasm draws water in from soil
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mycorrhizae Moves to endodermis
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Casparian strip apoplast route.
(waxy layer in cell wall) blocks
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Water must cross a membrane (solute) controlling Ψ Cells in Stele actively maintain low Ψ (solute) drawing water into xylem.
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Serpentine soils Rigid xylem walls build up pressure forcing water up – Root pressure
Fig. 36.7
exodermis root hair epidermis Newly forming Vascular cylinder cortex Casparian strip (gold) within all the abutting walls of cells of the endodermis
Guttation
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On dewy mornings root pressure may exceed evaporation forcing water up and out leaves.
Root pressure usually only loads xylem, not the major force in water movement up the stem
Movement up stem
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Water is pulled from above by tension
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negative Ψ (pressure) in the leaves Water molecules stick together cohesion Cell walls help support the weight of the water column by adhesion Due to hydrogen bonding
Surface tension- Transpirational Pull
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Each water molecule hydrogen bonds to four other molecules.
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Water resists stretching out Evaporation increase surface area of water in leaf- increasing its resistance Transpiration pulls water up from below creating tension , a negative Ψ (pressure)
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Extends down the roots to interface with soil Drier soils create even more tension Can break water column-
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Vapor plugs xylem vessel New xylem need cavitation Tracheids in conifers make them more drought tolerant
Fig. 36.10
Fig 36.11
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Water always moves to the lower Ψ (total)
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What has the lowest Ψ (total) ?
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Orientation of cellulose microfibrils allows guard cell to move apart when the uptake water turgor pressure.
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Fig 36.13a
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Guard cells actively accumulate K+ ions in tonoplast to open stomata.
Responds to tension in xylem; CO 2 ; Circadian rhythms; hormonal control Guard cells have chloroplasts
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Sugars loaded by cotransport in companion, or transfer (bundle sheath) cells.
Creates high concentration in sieve tube members in source.
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Sugars loaded at source by active transport.
Water moves in by osmosis, creates positive Ψ (pressure) Moves by bulk (pressure) flow Sugars unloaded at sink, less osmotic pull water moves out Fig. 36.17