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Water and Plant Cells
• Properties of Water
• Water Potential
• Components of Water
Potential
• Water relations
Water in plant life
• Corn yield as a function of water availability.
• Productivity of ecosystems as a function of
annual precipitation.
• Most water lost through transpiration.
Properties of Water
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Polar molecule
Thermal Properties
Specific Heat
Melting Point
Heat of fusion
Boiling point
Heat of vaporization
Solvent properties
• Dielectric constant
• Cohesion and
Adhesion
Water Transport Processes
• Diffusion
• Pressure-driven bulk
flown (Aquaporins)
• Osmosis
Water movement
• Fick’s law of diffusion
•
Js = - Ds (∆Cs / ∆x)
• Js is rate of transport (mol/m2s1)
• Ds = diffusion coefficient through a medium
• (∆Cs / ∆x) = concentration gradient of a substance
separated by distance x.
Water Potential
• Chemical activity of water
• Components of water potential
• Ψw = Ψs + Ψp + Ψg
Components of water potential:
Solutes (osmotic potential)
• Ψs = 0 for pure water
Or always negative
• Ψs = - R T Cs
• R = 8.32 J/mol K
• T = absolute
temperature
• Cs = solute
concentration (mol/L)
Pressure potential
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•
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Hydrostatic pressure (Ψp )
Positive (turgor pressure)
Negative (tension)
Ψp = 0 for water at ambient pressure (0 MPa)
Gravity
• Ψg = pwgh
• Pw = density of water
• g = acceleration due to gravity
• pwg = 0.01 MPa m-1
• At cell level we can ignore gravity or for
small plants.
Concepts of water potential
Plant cell volume
• Small changes in
volume cause large
changes in turgor
• 10-15% change in
volume, turgor goes to
zero
• Modulus of elasticity (ε)
or rigidity of cell wall.
• High ε wall is stiff, low ε
wall is soft
Physiological changes during
drought