Transport in Plants

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Transport in Plants

• Chapter 36

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Cellular Transport

• A. passive transport
• Driven by the principles of diffusion
• Much of the diffusion is facilitated
• Selective channels are usually gated and regulated

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Cellular Transport

• B. Active Transport
• Often uses proton pump
• Generates membrane potential
• may be used to drive active transport of other
  molecules (cotransport)

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Cellular Transport

• Water Transport
• By osmosis water moves from hypotonic to
  hypertonic solution in cells
• BUT in plant cell, cell wall provides a pressure
  component, therefore water potential must be used

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Water potential review

• What is ? of a container of distilled water that
  is open?
• If ? inside a cell is -0.7 MPa, and outside of
  the cell is -0.1 MPa, which direction will water
  move?
• If ?S-0.5MPa and ?P0.1MPa, will water move into
  or out of this cell if it is placed in an
  environment where ?-0.2 MPa?

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Water Potential

• Water moves from higher water potential (?) to
  lower water potential
• ?0 in pure water at atmospheric pressure
• ? and solute concentration have an inverse
  relationship
• ? and pressure have a direct relationship

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Plant Cells and Water Potential

• Plasmolysis protoplast pulls away from the cell
  wall due to loss of water
• The pressure exerted on the cell wall is 0
• Turgor pressure Pressure exerted on cell wall
  by protoplast due to influx of water
• The pressure exerted on the cell wall is positive

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Aquaporins

• 1990s Peter Agre (Johns Hopkins) discovers
  water pores in membranes that facilitate
  diffusion of water
• 2003 Nobel Prize in Chemistry along with Roderick
  MacKinnon
• Water is small and can diffuse through bilayer to
  some extent despite polarity
• Aquaporins speed up this diffusion
• May be gated to regulate water diffusion

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3 Cellular Compartments

• Apoplast continuum of cell walls and
  extracellular spaces
• Symplast continuum of cytosol linked by
  plasmodesmata
• Vacuole bound by membrane called tonoplast

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Lateral Transport

• Short distance horizontal movement
• Mechanisms
• Transmembrane (through membranes from one cell to
  the next) slow
• Symplastic (through one membrane into cell, then
  through plasmodesmata)
• Apoplastic (through cell walls)

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Absorption by Roots

• Root hairs increase surface area
• Mycorrhizae (fungal partners) enhance absorption
• Endodermis functions as selective region
• Symplastic continues through plasmodesmata
• Apoplastic must cross plasma membrane into
  cytosol due to Casparian strip
• Casparian strip is waxy and hydrophobic, traps
  water and minerals in the vascular tissue

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Bulk Flow

• Movement of a fluid driven by pressure
• In both xylem and phloem

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Long Distance Transport in the Xylem

• Root Pressure Pushing
• Transpiration Pulling

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Root Pressure

• Used by some plants, at night
• At night, plants still expend energy pumping
  minerals into xylem
• Accumulation of nutrients decreases ? , causing
  water to flow in by osmosis
• This upward push is root pressure
• Guttation exudation of water droplets seen in
  the morning

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Transpiration

• Major mechanism of movement
• Water is adhesive and cohesive
• As one water droplet moves, the next also moves
  (water in continuous column in xylem)
• As water evaporates out of the stomata, water
  below moves upward

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Guard Cells

• Guard cells regulate water loss.
• What conditions will promote the closing of guard
  cells?
• Buckle outward when turgid
• Regulate opening and closing by managing K

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Stomata
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Guard Cells

• Pump H out, K enters due to electric charge,
  water follows due to osmosis
• Close when K leaves (water follows)
• Aquaporins may vary permeability to regulate

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• Some leaf molds, which are fungi that parasitize
  plants, secrete a chemical that causes guard
  cells to accumulate potassium ions. How does this
  adaptation enable the leaf mold to infect the
  plant?

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Guard Cell Cues

• Light stimulates a blue light receptor in guard
  cells, activates H pumps
• CO2 depletion when Calvin cycle starts
• Can trick guard cells by placing in environment
  without CO2
• Internal Clock
• Open and close even in 24 hour dark
• Circadian rhythm

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What adaptations are seen for arid climates?

• thick leaves (low SA/vol)
• Thick cuticle
• Stomata on lower leaf surface
• Stomata in pores to shield wind
• Shed leaves in hot dry season
• Cacti have no leaves (adapt to spines)
• CAM photosynthesis

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Phloem Transport

• Translocation
• Sugar source to sugar sink
• Sugar loaded into sieve tube members by active
  transport
• Water follows by osmosis (increases pressure)
• At sink, sugar leaves sieve tube members (by
  diffusion or active transport)
• Water follows (decreases pressure)
• Water is recycled by xylem

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Oleander stomata in cypts
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Old Man cactus