Fluid transport in plants

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Fluid transport in plants
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Water potential

• Water potential free energy of water
• Pure water 0 megapascals (MPa)
• 9.87 atm.
• Water with dissolved solids has negative free
  energy
• Water moves from a region of higher water
  potential to that of a lower

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Osmosis

• Diffusion of water through a membrane
• Cell membranes are semipermeable

solute
membrane
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Osmosis

• Increased pressure would cause the cell membrane
  to expand
• What would happen of the cell were constricted by
  a rigid wall?

solute
membrane
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Osmosis

• Outward pressure would eventually be matched by
  pressure from the wall
• Water would stop flowing in, or flow to an area
  of less concentration/pressure

solute
membrane
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Direction of phloem transport

• Sucrose is the sugar moved in the phloem
• Sucrose may move in two directions, but always
  source to sink

Moving from the leaf toward the root and the
growing tip
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Mass flow in phloem
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Leaf x-section

• Sugars move from cell to cell by diffusion
  through plasmo- desmata and active transport

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Xylem/phloem transport rates
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Properties of water

• Adhesion - the attraction of water molecules to
  another substance
• Cohesion - the mutual attraction of water
  molecules to each other
• Tension - (negative pressure) - created by
  cohesion of water molecules
• Capillarity - rise of water in a column due to
  adhesion to surface of walls and cohesion of
  water molecules

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Capillary rise

• Because of the attraction of water to the tube
  walls (adhesion) to other water molecules
  (cohesion), the column moves up the tube
• What limits the diameter tube for which this will
  work?

small diameter tube
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Capillary xylem elements

• Xylem vessels and trachids are small capillary
  tubes
• Diameters range from about 50 µm to 400 µm
• Water in these tubes will rise by capillary
  action for 0.6 meters to 0.08 meters

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Cohesion-tension theory

• Water will only rise 10.4 meters in a tube with a
  vacuum in a closed top due to air pressure
• If there is a driving (pulling) force, water can
  be taken up to great heights in a tube
• Evaporation is the force that pulls water up
  the xylem column
• like squeezing the sponge

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Transpiration pull
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Breakage of the column

• If the tension becomes too great O2, CO2, and N
  come out of solution and form bubbles in the
  xylem elements
• Thus, the water column will break apart
• This is called cavitation and the bubble is an
  embolism
• Since the cohesive force is broken at this point,
  transpiration pull has no effect on the lower
  column

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Other possibilities

• Compensating Pressure Theory
• Martin J. Canny

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Dendrograph readings

• A very sensitive instrument can record rhythmic
  changes in stem diameter over a 24 hour period
• Once thought that the stem was pumping water
  upward
• Accounted for by changes in tension of the water
  column
• Smallest diameter in the day time

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Purpose of transpiration

• Mineral transport
• Cells supplied with adequate water
• Evaporative cooling
• This is probably the most important

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