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Water Potential Notes
Osmotic Potential:
- measure of tendency for H2O to cross a selectively permeable membrane from
low concentration to high concentration of solute
proportional to concentration of solute à [high
concentration = high osmotic pressure]
H2O Potential (Y):
High volume (concentration) H2O in one location represents great free
energy.
(Free energy = energy available to do work)
H2O moves from high free energy to low free energy
*compared to pure water at 1 atm. in steady temperature
Y water (pure) = 0
Y water (cell) < 0 (negative value -D
G)
Y = Y p + Y p
¯ Y p => ¯ y [
more neg. due to solute concentration]
Y p => y [
less neg. due to solute concentration]
y p
= Y ,
energy content of H2O, work ability
Y water (pure) = 0 [Add solute Y ¯ less than 0]
Solute in H2O lowers y , therefore, H2O
moves from low solute ( H2O) to high solute ( ¯ H2O)
Pressure applied increases y ,
y p à y
- therefore, if y p
applied ( atm. Pressure, H2O
moving into plant cell, H2O entering creates hydrostatic pressure pushing on
cell wall)
- the y increase ( y inside cell due to y pp cause intake of water to decrease and reach osmotic
potential) => turgid in plant cells (may lyse in animal cells)
- if negative pressure y p < 0 then partial vacuum (tension)
created. Uncommon in plants except in 2° xylem (dead cells)
Pure H2O
High Y y p or Y p (high pressure. Low solute concentration) hypotonic
D G .1
.2
.3
.4
.5 *isotonic y p1 = y
p2
.6
.7
.8
.9 [¯ Y p or ¯
Y p]
1.0 M solution hypertonic
(low pressure, high solute concentration)
¯ D G
low Y
If plant in solvent w/ ¯ Y => lose H2O, plasmolysis
If plant in solvent w/ Y => gain H2O, turgor
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