BIO2202 Plant Physiology 1 Water Relations Lecture 2

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BIO2202 Plant Physiology 1 Water Relations -
Lecture 2

• Water Movement Osmosis

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Diffusion

• The process by which molecules intermingle as a
  result of their random thermal agitation
• Results in progressive movement of substances
  (e.g. gases, solutes, liquids) from regions of
  high concentration to regions of low concentration

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Ficks Law

• Js - Ds.?Cs
• ?x
• Js flux density i.e. rate of movement
• across a fixed unit of area
• Ds diffusion coefficient
• ?Cs difference in s between two
• ?x points x distance apart

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Diffusion rates depend on?

• Temperature
• Size of molecule
• Density of the medium
• Concentration gradient

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Comparative Rates of Diffusion
t C½ (distance)2 x K x D K1, D 10-9 m2.s-1
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Bulk (Mass) Flow

• Bulk movement of solutions in response to a
  pressure or energy gradient. E.g. water in a hose
  or drinking straw, river flow, rain.
• Rate of flow is very sensitive to radius

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Poiseuilles Equation

• Volume of flow rate ?r4 . ??P
• 8? ?x
• r radius
• ? viscosity

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What is a semipermeable membrane?

• A barrier impermeable to large solute molecules
  but permeable to small solute and solvent
  molecules. Selects on basis of size.
• E.g. dialysis tubing

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Differentially (or Selectively) Permeable
Membranes

• Characteristic of biological membranes
• Water passes readily, solutes slowly or not at
  all
• Depends on size, chemical charge properties
• Depends on interaction with specific membrane
  components e.g. channels, carriers.

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Osmosis

• Net movement of water across a membrane from a
  region of higher potential (e.g. more dilute
  solution) to a region of lower potential (e.g.
  more concentrated solution)

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Aquaporins

• Specialised membrane proteins which facilitate
  water movement across the membrane
• Form passive pores
• Efficiency of water movement can be altered by
  changing the phosphorylation state of aquaporins

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Taiz Zeiger, p. 68
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Water Potential

• A measure of the relative energy available for
  reaction or movement
• movement is always from a region of higher
  potential to a region of lower potential
• potential of pure water is defined as zero at
  ambient temp. pressure
• water potential (?W) is measured in megapascals
  (MPa).
• 1 MPa 10 bars

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