Plant Physiology

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Plant Physiology
Water and Plant Cells
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• Water and plant cells
• I. Background on water in plants
• II. The properties of water
• III. Understanding the direction of water
  movement Water potential

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• Water
• Plant cells are mostly water 80 - 95
• of the mass of growing cells,
• Wood (Sapwood 35-75)
• Seeds 5-15
• Living cells must maintain a positive
• water pressure, or turgor to grow
• and function properly.
• Cell walls
• build Internal Hydrostatic Pressure
• Turgor Pressure
• Cell enlargement
• Gas Exchange in Leaves
• Transport in Phloem
• Transport across
  membrane
• Rigidity and Mechanical
  
• stability to
  nonlignified plant tissues

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• Water
• Plants lose large quantities of water in
  transpiration, the evaporation from the interior
  of leaves through the stomata is referred to as
  Transpiration.
• CO2 H2O
• 100 water exchange in one hour
• Water loss through leaf surface
• Heat dissipation by leaves through transpiration
• Half of the heat by sunlight is dissipated by
  transpiration
• Latent Heat of Vaporization

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Plant - Water Relations
Corn yield as a function of water availability
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Plant - Water Relations
Productivity of various ecosystems as a
function of annual precipitation
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Water passes easily through biological membranes,
particularly through Aquaporins - low resistance
pores.
Formed by Aquaporins
Aquaporins are proteins embedded in cell
membrane that regulate flow of water. Prof
Peter Agre and discovery of Aquaporins?
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II. The properties of water
Polar molecule that forms hydrogen bonds. 1)
good solvent 2) cohesive properties - attraction
to like molecules 3) adhesive properties -
attraction to unlike molecules
The polarity of water molecules results in
hydrogen bonding
Covalent bond
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Properties of water, continued

• Cohesion is the attraction of like molecules (H2O
  here) that gives water its tensile strength.
• Adhesion is the attraction of unlike molecules.
  Water adheres to cell walls, soil particles,
  glass tubes, etc.
• Adhesion explains capillarity surface tension.
• Surface Tension is caused by cohesive forces
  within liquid molecules.
• All of above forces give rise to a phenomenon
  called Capillarity (the movement of water along a
  capillary tube)

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• III. What factors determine the direction of
  water movement (through the soil, between cells,
  from roots to leaves, from leaves into air)?
• Gravity
• 2. Pressure
• 3. Concentration

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100
90
water flows upward in trees. How does this work?
80
70
Height, meters
60
50
40
Gravity causes water to move downward unless it
is opposed by an equal and opposite force.
30
20
10
0
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Pressure Water moves from regions of higher to
lower pressure garden hose straw through xylem
of plants
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Water moves from higher to lower pressure
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Water pressures in plant cells can be positive
(turgor), or negative, (tension). Living cells
0 MPa to 3 MPa) Dead xylem cells 0 MPa,
to as low as -12 MPa.
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3) Concentration Water moves by diffusion from
regions of higher to lower water
concentration. Solutes added to pure water
dilute the water concentration.
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Osmosis is the diffusion of water across a
selectively permeable membrane from a region of
higher to lower water concentration.
How does reverse osmosis purify water?
A process by which a solvent passes through a
porous membrane in the direction opposite to that
for natural osmosis.
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The concept of water potential, Y, brings
together the influences of gravity, pressure, and
concentration (solutes) in describing the energy
state of water and the direction of water
movement.
The water potential equation YW YS YP
Yg YW total water potential YS solute
potential YP pressure potential Yg
gravitational potential
All units will be pressure, pascals, Pa. MPa is
megapascal, 106 Pa
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Change in water status causes physiological
changes
Measure of the rate of passage of CO2 entering
or water vapors exiting through stomata
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Further readings

• Water and Plant Cells, Chapter 3, Plant
  Physiology by Taiz and Zeiger