Transcript Water Potential

Transport in Plants II
Water Balance of Plants
My empty water dish mocks me.
- Bob the Dog
Tutoring, 206
• Samantha D’Andrea,
• Mondays, 6 pm, AW 205,
• Will meet MLK Day.
Rubus spectabilis
Salmonberry
Rhizomes
Clones
Leaves Alternet
Salmonberry bird…
• Local (NW) flora,
• Ethnobotany,
• Fun!
Buy locally, or at Amazon ($16.47).
Transport
…molecular and ionic movement from
one location to another,
–
–
–
–
–
H2O,
Sugars and other organics,
Ions,
Gases,
Proteins, RNA, Hormones, etc.
+ Everything
Proteins/RNA/Hormones etc.
Today
• Water,
• Water Potential,
– free energy of water,
• Water Relations in Plant Cells,
• The uptake of water by plant roots.
Water
• Dipole,
• Hydrogen bonding,
– Adhesion,
– Cohesion,
– high Specific Heat,
– high Latent Heat of
Vaporization,
– etc.
Water and Plants
…to begin with.
•
Up to 80 - 95% FW of Plant Tissues,
•
Needed for the proper conformation of
all macromolecules,
•
Constitutes the environment for nearly
all biochemical reactions in the cell,
•
Reagent in many reactions, (e.g.
hydrolysis),
•
Necessary for tissue mechanics,
•
Evaporative cooling,
•
Bulk flow…
What Drives the Movement of Water?
Gravity,
Pressure,
Osmosis,
(entropy-driven).
Combinations?
Water Potential (Y)
Free Energy Status of Water in Plants
• The water potential of a sample is the sum of three major
component potentials: gravitational, osmotic, and pressure.
– Gravitational potential (YG) depends on the position of the water in a
gravitational field,
• negligible at the level of the cell. Is significant in taller plants and trees.
– Osmotic potential (YS) depends on the concentration of dissolved
substance in the water.
– Pressure potential (YP) depends on the hydrostatic pressure on the water.
Y =
Yg +
Y =
YS
+ YP
YS + YP
Y = YS + Y p
Y (Units)
• Y is the free energy of a water sample per unit mass,
– J m-3,
…expressed as units of pressure,
• 1 megapascal (MPa) = 10 bars, ~ 10 atmospheres, 7500 mmHg.
• Standard (Y0) = pure water at ambient pressure = 0 MPa.
Y=
YS + Y p
Solute (or) Osmotic Potential
• Represents the effect of dissolved solutes on water
potential,
YS = -RT cs
R: gas constant (8.32 J mol-1 K-1)
T: absolute temperature (K)
cs: concentration (mol L-1)
YS
Solute (or) Osmotic Potential
…of solutions at 25oC,
– 0.1 mol L-1 glucose = -0.24 Mpa,
– 0.1 mol L-1 NaCl = -0.48 Mpa,
– 0.1 mol L-1 CaCl2 = -0.72 Mpa.
Why?
Entropy effect: the mixing of solutes and water increases the disorder of the
system, thus lowering the free energy.
Y= YS + Yp
Pressure Potential
• Hydrostatic pressure represents the physical pressure on a
solution, or by the solution,
– Positive pressure raises the pressure potential,
– Negative pressure (tension) reduces pressure potential,
• The positive hydrostatic pressure within plant cells is
referred to as Turgor Pressure.
Y = YS + Yp
examples
Pressure
Tension
Plant Cell Water Relations
Q: If a membrane was placed between
these solutions, which way would the
water move?
A: Water moves toward the
compartment with the lowest Y.
Practice
Turgid/Plasmolysis
Turgid: Firm. Walled cells become
turgid as a result of the entry of water.
Plasmolysis: Shrinking of a cell due
to water leaving the cell.
Turgid Cell
-DY
+DY
+ + DY
(Yinside - Youtside = DY )
Turgid
Plasmolyzed
Pressure Probes
Careful measurement of plant
cell membrane permeability
to water suggested that
transport across the
membrane was too rapid for
simple diffusion.
...one way to measure water permeability.
Aquaporins
Volume flow rate =
viscosity (h)
pr4
DYp
8h
Dx
pressure gradient
distance
• 38 different genes code for 38
different aquaporin proteins
(octimers) in Arabidopsis,
…integral membrane proteins that
form a water pore across the
membrane.
• These genes are expressed in
different tissues, and expression is
partially under environmental
control,
• Co-ordinated control of
aquaporins regulate plant cell
permeability to water.
Class Quiz (+2 / -2)?
extra credit?
Two good
examples as to
when an
organism might
use this protein.
What type of transport?
What drives the transport?
Relatively fast or slow?
What type of transport?
What drives the transport?
Relatively fast or slow?
Transport in Plants III
Water Balance of Plants II
Plants suck.
- Anonymous 206 Student
To Do
Transport of Xylem Sap,
Transpiration and control,
Evolution of water transport
and adaptations.
Plant Water Relations
xylem
Process
Osmosis, etc.
Driving Force
DY
Diffusion
Dcwv
Bulk Flow
DY
p
Transport of Xylem Sap
DRIVING FORCES
•
Root pressure,
– sometime +DY from the soil/water matrix, but
usually zero or negative,
– active transport of ions into the root creates large
gradients, thus - DY.
•
Transpiration-Cohesion-Tension,
– water vapor diffuses from leaf-cell surfaces to
surrounding air,
– a water column extends from the root to this
interface, and is held together by cohesion,
– the tension that forms, “pulls” water through the
plant.
Root Pressure
Solute Accumulation in Xylem
Guttation
•
Absorption and active transport of
ions in the root create a -Dys and
thus a lower Y,
–
lower Y provides a driving force for
water uptake, and a thus +DYp,
•
Cut stems exude sap (as high as
0.05 - 0.5 MPa),
•
Guttation: specialized cells release
root pressure at vein endings in leaf
margins,
– hydathodes, specialized cells,
(including guard cells).
Transport of Xylem Sap
DRIVING FORCES
•
Root pressure,
– sometime +DY from the soil/water matrix, but
usually zero or negative,
– active transport of ions into the root creates large
gradients, thus - DY.
•
Transpiration-Cohesion-Tension,
– water vapor diffuses from leaf-cell surfaces to
surrounding air,
– a water column extends from the root to the leaf
interface, and is held together by cohesion,
– the tension that forms, “pulls” water through the
plant.
Big
Picture
Evolution of Vasculature
Poisuille’s Equation
Volume flow rate =
viscosity (h)
pr4
DYp
8h
Dx
pressure gradient
distance
1. Create a tube, make it bigger.
2. Lower the viscosity.
3. Create and maintain a pressure gradient.
Evolution of Vasculature
Fick’s Law and Organisms
50m
Euglena oxyuris
Bryophyte s
50m
Choleochaete orbicularis
Trees ?
Bryophytes
(0.5 mm - 50 cm)
Simple vasculature
~ 500 mya
Haldrom/Leptom
• Conducting tissues in bryophyte
stems with the following cell
types;
– Hydroids: elongated cells
lacking protoplasts at maturity,
lack lignification and secondary
cell walls,
– Leptoids: elongate cells with
reduced cytoplasm....
Seta w/ Capsules
Lignin
Secondary Walls and Lignin
•
Secondary Walls: provide rigid
support for conductive tissue,
– maintain higher -Yp,
•
Lignin: highly branched phenolic
polymer, may be associated with
cellulose and proteins. Deposited in
secondary walls, adds strength,
– maintain higher -Yp,
– limits “leaking”,
…add structural potential, facilitating
upward growth.
Castor Bean Stem
Xylem Cells
Bordered Pits
Pits: microscopic regions where the
secondary wall of a xylem cell is
absent, and the primary wall is thin
and porous.
Big
Picture
Wednesday
• Transport of Xylem Sap,
• Control of Transpirations,
• Evolution of water transport and adaptations,
• Phloem.
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