Download Ch. 7 Osmoregulation in Cells

Osmoregulation in Cells
Ch. 7
AP Biology
Ms. Haut
Water Balance of Cells Without
Walls
• Behavior of cell in a solution depends on:
– Solute concentration
– Permeability of membrane
http://static.flickr.com/46/178037251_5dbba5a33c_o.jpg
Water Balance
• Cells without walls cannot tolerate excessive uptake or
loss of water
– Most terrestrial organisms are bathed in extracellular fluid that is
isotonic to the cells
– Organisms in hypertonic/hypotonic environments have
adaptations
Hypotonic solution
Isotonic solution
Hypertonic solution
Animal
cell
H2O
Lysed
H2O
H2O
Normal
H2O
Shriveled
Water Balance
• Paramecium—protist
that lives in pond
water
• Environment is
hypotonic to cell
• Contractile vacuole
pumps out excess
water to prevent lysis
http://www.daviddarling.info/images/contracti
le_vacuole.gif
Water Balance
http://cache.eb.com/eb/image?id=6541&rendTypeId=4
Water Balance of Cells With Walls
• Wall aids in maintaining water balance
• Wall expands just so much before it exerts a
back pressure that opposes further water uptake
Hypotonic solution
Isotonic solution
Hypertonic solution
Animal
cell
H2O
Lysed
Hypotonic
solution
Animal
cell
Plant
cell
H2O
H2O
H2O
Normal
Isotonic
solution
H2O
H2O
H2O
H2O
Shriveled
Hypertonic
solution
H2O
H2O
H2O
H2O
Turgid (normal)
Flaccid
Plasmolyzed
Lysed
Normal
Shriveled
Water Balance
http://www.uic.edu/classes/bios/bios100/mike/spring2003/plasmolysis.jpg
How Solutes and Pressure
Affect Water Potential
• Both pressure and solute concentration
affect water potential
• The solute potential of a solution is
proportional to the number of dissolved
molecules
• Pressure potential is the physical pressure
on a solution
Water Potential ()
• The free energy of water that is a consequence
of solute concentration and applied pressure
– Water will always move across the membrane
from the solution with the higher water
potential to the one with lower water potential.
Effect of solutes and pressure on
water potential
– Pure water has a water potential of zero, and
addition of solutes lowers water potential into
the negative range.
– Increased pressure raises the water potential
into the positive range.
– A negative pressure, may also move water
across a membrane; this bulk flow (movement
of water due to pressure differences) is
usually faster than movement caused by
different solute concentrations.
Water Potential ()
= P + S
P = Pressure potential
S = Solute potential or osmotic potential
Quantitative Analysis of Water
Potential
• The addition of
solutes reduces water
potential
Addition of
solutes
0.1 M
solution
Pure
water
H2O
 = 0 MPa
P = 0
S = –0.23
 = –0.23 MPa
Quantitative Analysis of Water
Potential
• Physical pressure increases water potential
Applying
physical
pressure
Applying
physical
pressure
H2O
H2O
 = 0 MPa
P = 0.23
S = –0.23
 = 0 MPa
 = 0 MPa
P = 0.30
S = –0.23
 = +0.07 MPa
Quantitative Analysis of Water
Potential
• Negative pressure decreases water potential
Negative
pressure
H2O
P = –0.30
S = 0
 = –0.30 MPa
P = 0
S = –0.23
 = –0.23 MPa
• Water potential affects uptake and loss of water
by plant cells
• If a flaccid cell is placed in an environment with
a higher solute concentration, the cell will lose
water and become plasmolyzed
Plasmolyzed cell
at osmotic
equilibrium
with its
surroundings
P = 0
S = –0.9
 = –0.9 MPa
0.4 M sucrose solution:
P = 0
S = –0.9
 = –0.9 MPa
Initial flaccid cell:
P = 0
 S = –0.7
 = –0.7 MPa
Distilled water:
P = 0
S = 0
 = 0 MPa
Turgid cell
at osmotic
equilibrium
With its
surroundings
P = 0.7
 S = –0.7
 = 0 MPa
Example
• 0.1M solution has a S of –0.23MPa; in the
absence of pressure, the water potential is –0.23
MPa.
 = P + S
 = 0 + (-0.23 MPa)
 = -0.23MPa
• So, if this solution is separated from pure water
( = 0 MPa) by a selectively permeable
membrane, water will diffuse into the solution
Water Balance
• Turgor loss in plants
causes wilting, which
can be reversed when
the plant is watered