Plant Transport

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Plant Transport

• How does water get from the roots of a tree to
  its top?
• Plants lack the muscle tissue and circulatory
  system found in animals, but still have to pump
  fluid throughout the plants body

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Plant Transport

• Water first enters the roots and then moves to
  the xylem, the innermost vascular tissue
• Plants need water
• As a starting product for photosynthesis
• As a solvent to dissolve chemicals
• For support
• To pay for water lost by transpiration

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Plant Transport

• Heartwood is the xylem that has died much darker
• Sapwood is the younger, outermost wood that has
  not yet become heartwood conducts water from the
  roots to the leaves, and to store

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Plant Transport

• Water movement (transport) occurs at three
  levels
• Cellular
• Lateral transport (short-distance)
• Whole plant (long-distance)

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Plant Transport Cellular level

• Diffusion movement from an area of high
  concentration to an area of lower concentration
• Plays a major role in bulk water transport, but
  over short distances
• Although water diffuses through cell membranes,
  ions and organic compounds rely on membrane-bound
  (protein) transporters

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Plant Transport Cellular level

• Some protein transporters form channels that
  allow molecules to diffuse (passive transport)
• Others require energy to move minerals and other
  nutrients against a concentration gradient
  (active transport)

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Plant Transport Cellular level

• Proton pumps hydrolyze ATP and use the release
  energy to pump hydrogen ions (H) out of the
  cell makes a proton gradient that is higher in
  H outside of the cell (? membrane potential)
• Makes the inside of a cell more negative than the
  outside, driving the transfer of positive ions
  (e.g., K ions)

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Plant transport Cellular level

• Osmosis passive transport
• Transport of water (and its solutes) across a
  semi-permeable membrane

• Unlike animal cells, plants have cell walls and
  this affects osmosis
• Water potential, ? (Greek letter psi), is used to
  predict which way water will move
• Water will move from solution with higher ? to a
  solution with lower ?

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Water Potential, ?

• Water moves from higher ? to lower ?
• The addition of solutes lowers ?
• Increasing pressure raises ?
• In essence, ? measures the ability of soil water
  to move (into or out of the plant)
• Low osmotic concentration high ?
• High osmotic concentration low ?

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Water Potential, ?

• If a single plant cell is placed into water, then
  the concentration of solutes inside the cell is
  greater than that of the external solution, and
  water will move into the cell by the process of
  osmosis
• The cell expands and presses against the cell
  wall, a condition known as turgid (swollen), due
  to the cells increased turgor pressure

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Water Potential, ?

• ? is measured in units of pressure
• Pure water at standard temperature and pressure
  has a ? of zero
• The addition of solutes to water lowers its ?
  (makes it more negative), just as an increase in
  pressure makes it more positive
• Water will move from higher ? to lower ?

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Water Potential, ?

• Water will spontaneously flow from a high
  potential to a low potential, like a ball rolling
  down a hill
• ? are usually negative
• ? is measured as pressure potential, ?P and
  solute potential, ?S
• The total potential energy of water in the cell
  ?P ?S

http//www.steve.gb.com/science/water_potential.ht
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• Pressure potential, refers to the turgor
  pressure resulting from pressure against the cell
  wall

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• Water pressure also arises from an uneven
  distribution of a solute on either side of a
  membrane, which results in osmosis
• Solute potential, ?S describes the smallest
  amount of pressure needed to stop osmosis
• Water flows from a solution with the less
  negative ?S to the more negative ?S

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A watered plant regains its turgor
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Water Potential, ?

• Water potential of the soil is negative, but not
  as negative as the cell, due to the high content
  of solutes
• Water moves from high (less negative) to low
  (more negative) water potential

http//www.steve.gb.com/science/water_potential.ht
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Plant Transport Long distance

• Evaporation of water in a leaf creates a negative
  pressure (negative water potential) in the xylem,
  which literally pulls water up the stem from the
  roots

http//www.steve.gb.com/science/water_potential.ht
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Plant Transport Long distance

• Water moves from the soil into the roots only if
  the soils water potential is greater
• It then moves along gradients of successively
  more negative water potentials in the stems,
  leaves and air

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Water and Mineral Absorption

• Most of the water absorbed the plant comes in
  through root hairs extensions of root epidermal
  cells
• Root hairs are almost always turgid, because
  their water potential is greater than that of the
  surrounding soil
• Collectively, have enormous surface area
• And dont forget the mycorrhizae

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Root tips
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Water and Mineral Absorption

• Minerals are absorbed at the root hair
• Minerals may either follow the cell walls or
  spaces in between them, or go directly through
  the plasma membrane of the cells
• They will, however, eventually reach the
  endodermis, where their entry is blocked by
  casparian strips a waxy material that surrounds
  endodermal cells, before reaching the xylem

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Water and Mineral Absorption

• Apoplast route movement through cell walls and
  the spaces between cells
• Symplast route cytoplasm continuum between
  cells
• Transmembrane route membrane transport b/w
  cells and across membranes of vacuoles within the
  cells provides the greatest control over which
  substances enter and leave

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Casparian strip

• Transport into the endodermis is selective
• Passage through the cell walls blocked by
  casparian strips
• Substances must enter the cells of the endodermis
  in order to pass into xylem
• Allows selectivity

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Root hair
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Xylem

• Xylem sap brings minerals to leaves and water to
  replace what is lost by transpiration
• Moves at rates of 15 meters/hour travels
  vertically up distances of 100 meters in the
  tallest trees
• At night, when transpiration is low or absent,
  root pressure caused by the accumulation of ions
  in the roots, causes more water to enter the root
  hair cells by osmosis

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Xylem

• Under certain circumstances, root pressure is so
  strong that water will ooze out of a cut plant
  stem for hours or even days
• When root pressure is very high, it can force
  water up to the leaves, where it may be lost
  (guttation)
• Guttation produces what is more commonly called
  dew on leaves

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Phloem

• Carbohydrates manufactured in leaves and other
  green parts are distributed through the phloem to
  the rest of the plant
• Translocation
• Phloem sap consists primarily of sucrose (30),
  as well as hormones, amino acids, and minerals
• Phloem sap travels from sugar sources to sugar
  sinks (non-green parts, growing shoots and roots,
  and fruits)

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Maple syrup is sap!

• Sap in maple trees remains frozen during the
  winter
• Begins to flow again when weather warms, and is
  triggered by cold nights and warmer days
• A hole is tapped into the tree allowing sap to
  drain
• Sugar maples have the greatest amount of sugar in
  the sap produce 20 gallons of sap (2 quarts of
  syrup)

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Maple syrup is sap!

• The sap that produces maple syrup flows through
  the sapwood the living portion of the xylem

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Just in case youre not starving yet