Download Chapter 03 Water and Plant Cells

Chapter 03
Water and Plant Cells
BIOL 5130/6130
Bob Locy
Auburn University
Chapter 03.01. Water in Plant Life - 01
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Water is essential to sustaining plant life.
ca 90% of the mass of a well watered plant is H20
ca 97% of the water taken up by the roots passes
through the plant and is transpired for evaporative
cooling. Only 2% is retained for growth, and 1% is
consumed in photosynthesis
Plant cells are pressure-regulated to sustain turgor
pressure, while animal cells are volume-regulated
Dilemma – Plants must open stomates and loose
water to acquire CO2
Gradient for water loss ca 500x gradient for CO2
acquisition
Water use effeciency ratio is moles H20 lost per CO2
gained, is ca 500 in most situations
Chapter 03.01. Water in Plant Life - 02
Agricultural and Ecological significance
Chapter 03.02. The Structure and Properties of Water - 03
02.01. The polarity of water molecules gives rise to
hydrogen bonds
02.02. The polarity of water makes it an excellent solvent
Chapter 03.02. The Structure and Properties of Water - 04
02.03. The thermal properties of water result from hydrogen
bonding
02.04. The cohesive and adhesive properties of water are
due to hydrogen bonding
Chapter 03.02. The Structure and Properties of Water - 05
02.05. Water has a high tensile strength
•  Demonstrates tensile strength
•  cavitation
Chapter 03.03. Water Transport Processes - 06
03.01. Diffusion is the movement of molecules by random
thermal agitation
Chapter 03.03. Water Transport Processes - 07
03.02. Diffusion is rapid over short distances but extremely
slow over long distances
Fick’s (First) Law:
Where :
Js = rate of transport (flux density)
Δcs
Ds = diffusion coeficient
Js = -Ds -----Δx
Δcs = concentration gradient
Δx = distance
Chapter 03.03. Water Transport Processes - 08
03.03a. Pressure-driven bulk flow drives long-distance
water transport
•  Bulk flow involves the mass movement of all
molecules not just solute molecules, like water
movement in a pipe.
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Poiseuille’s Equation describing bulk flow:
Volume flow rate =
( )( )
Where:
r = radius of tube
η = viscosity of fluid
ΔΨp = pressure gradient
Δx = distance
πr4
----8η
ΔΨp
-----Δx
Chapter 03.03. Water Transport Processes - 09
03.03b. Pressure-driven bulk flow drives long-distance
water transport - Continued
•  Units of Pressure
Chapter 03.03. Water Transport Processes - 10
03.04. Osmosis is driven by a water potential gradient
•  Osmosis involves diffusion across a semipermeable
membrane – allows transport of water and small
uncharged solutes, but usually not large or charged
molecules.
•  In osmosis, both the concentration gradient and the
pressure gradient drive flow.
03.05. The chemical potential of water represents the freeenergy status of water
•  Chemical potential is a measure of the pressure and
concentration effects.
•  Plant physiologists prefer to measure chemical
potential as water potential = chemical potential of
water / partial molar volume of water
Chapter 03.03. Water Transport Processes - 11
03.06. Three major factors contribute to cell water potential
•  Osmotic potential or solute potential, Ψs, or the solute
concentration effect.
•  For ideal (dilute solutions) can be estimated with
the van’t Hoff equation: Ψs = -RTcs, where R =
universal gas constant, T = temperature in degrees
Calvin, and cs is the solute concentration.
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Hydrostatic Pressure potential, Ψp, inside a plant cell
this is often referred to as the turgor pressure.
Gravity potential, Ψg, defined as Ψg =ρwgh where ρwg
= 0.01 MPa M-1.
Water Potential Equation: Ψw = Ψs + Ψp + Ψg
Chapter 03.03. Water Transport Processes - 12
03.06. Three major factors contribute to cell water potential Continued
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Sometimes in thin layers interacting with soil particles
or cell walls a “matrix” potential can be generated.
This is called Ψm, but it is difficult to estimate or
determine.
Water Potential Equation
Ψw = Ψs + Ψp + Ψg + Ψm
Most of the time Ψg & Ψm will be so small that they can
be ignored. In this case the water potential equation
reduces to:
Ψw = Ψs + Ψp
Chapter 03.03. Water Transport Processes - 13
03.07. Water enters the cell along a water potential
gradient
Chapter 03.03. Water Transport Processes - 14
03.08. Water can also leave the cell in response to a water
potential gradient
Chapter 03.03. Water Transport Processes - 15
03.09. Small changes in plant cell volume cause large
changes in turgor pressure
Chapter 03.03. Water Transport Processes - 16
03.10. Water transport rates depend on driving force and
hydraulic conductivity
Chapter 03.03. Water Transport Processes - 17
03.11. Aquaporins facilitate the movement of water across
cell membranes
Chapter 03.03. Water Transport Processes - 18
03.12. The water potential concept helps us evaluate the
water status of a plant
Chapter 03.03. Water Transport Processes - 18
03.13. The components of water potential vary with growth
conditions and location within the plant
•  At the cellular level, plants can control aspects of
cellular water relations. However, at the organismal
level plants live with the environmentally derived
conditions, and adjust very little to moderate water
relations as we will see in chapter 4.
•  As discussed above, plants have the ability to
control hydraulic conductivity of the plasma
membrane.
•  They can also control the solute potential of the
inside of the cell. This permits them to adjust the
driving force for water acquisition.
•  Plants have the capacity to regulate osmotic
potential across a vast range of circumstances and
osmotic potentials.
END
Chapter 03
Water and Plant Cells
Supplemental topics and study questions
Can be found at:
http://4e.plantphys.net/chapter.php?ch=3