Download water potential - Industrial ISD

Objective 7
TSWBAT compare
hypoosmotic, hyperosmotic,
and isoosmotic solutions and
predict the path of movement
of water and solutes in given
examples.
Osmosis is the passive transport of water
• Differences in the relative concentration of dissolved
materials in two solutions can lead to the movement
of ions from one to the other.
• The solution with the higher concentration of solutes is
hypertonic.
• The solution with the lower concentration of solutes is
hypotonic.
• These are comparative terms.
• Tap water is hypertonic compared to distilled water but
hypotonic when compared to sea water.
• Solutions with equal solute concentrations are isotonic.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Imagine that two sugar solutions differing in
concentration are separated by a membrane that
will allow water through, but not sugar.
• The hypertonic solution has a lower water
concentration than the hypotonic solution.
• More of the water molecules in the hypertonic solution
are bound up in hydration shells around the sugar
molecules, leaving fewer unbound water molecules.
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• Unbound water molecules will move from the
hypotonic solution where they are abundant to the
hypertonic solution where they are rarer.
• This diffusion of water across a selectively
permeable membrane is a special case of passive
transport called osmosis.
• Osmosis continues
until the solutions
are isotonic.
Fig. 8.11
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• The direction of osmosis is determined only by a
difference in total solute concentration.
• The kinds of solutes in the solutions do not matter.
• This makes sense because the total solute concentration
is an indicator of the abundance of bound water
molecules (and therefore of free water molecules).
• When two solutions are isotonic, water molecules
move at equal rates from one to the other, with no
net osmosis.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Objective 8
TSWBAT relate osmotic
potential to solute
concentration and water
potential.
Differences in water potential drive water
transport in plant cells
• The survival of plant cells depends on their ability to
balance water uptake and loss.
• The net uptake or loss of water by a cell occurs by
osmosis, the passive transport of water across a
membrane.
• In the case of a plant cell, the direction of water
movement depends on solute concentration and physical
pressure, together called water potential, abbreviated by
the Greek letter “psi.”
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• Water will move across a membrane from the
solution with the higher water potential to the
solution with the lower water potential.
• For example, if a plant cell is immersed in a solution
with a higher water potential than the cell, osmotic
uptake of water will cause the cell to swell.
• By moving, water can perform work.
• Therefore the potential in water potential refers to the
potential energy that can be released to do work when
water moves from a region with higher psi to lower psi.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Plant biologists measure psi in units called
megapascals (abbreviated MPa), where one MPa
is equal to about 10 atmospheres of pressure.
• An atmosphere is the pressure exerted at sea level by an
imaginary column of air - about 1 kg of pressure per
square centimeter.
• A car tire is usually inflated to a pressure of about 0.2
MPa and water pressure in home plumbing is about
0.25 MPa.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• For purposes of comparison, the water potential of
pure water in an container open to the atmosphere
is zero.
• The addition of solutes lowers the water potential
because the water molecules that form shells around the
solute have less freedom to move than they do in pure
water.
• Any solution at atmospheric pressure has a negative
water potential.
• For instance, a 0.1-molar (M) solution of any solute
has a water potential of -0.23 MPa.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• If a 0.1 M solution is separated from pure water by
a selectively permeable membrane, water will
move by osmosis into the solution.
• Water will move from the region of higher psi (0 MPa)
to the region of lower psi (-0.23 MPa).
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In contrast to the inverse relationship of psi to
solute concentration, water potential is directly
proportional to pressure.
• Physical pressure - pressing the plunger of a syringe
filled with water, for example - causes water to escape
via any available exit.
• If a solution is separated from pure water by a
selectively permeable membrane, external pressure on
the solution can counter its tendency to take up water
due to the presence of solutes or even force water from
the solution to the compartment with pure water.
• It is also possible to create negative pressure, or tension
as when you pull up on the plunger of a syringe.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The combined affects of pressure and solute
concentrations on water potential are incorporated
into the following equation:
psi = psiP + psis
• Where psiP is the pressure potential and psis is the solute
potential (or osmotic potential).
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• If a 0.1 M solution (psi = -0.23 MPa) is separated
from pure water (psi = 0 MPa) by a selectively
permeable membrane, then water will move from
the pure water to the solution.
• Application of physical pressure can balance or even
reverse the water potential.
• A negative potential can decrease water potential.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 36.3
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• Water potential impacts the uptake and loss of
water in plant cells.
• In a flaccid cell, psiP = 0 and the cell is not firm.
• If this cell is placed in a solution with a higher solute
concentration (and therefore a lower psi), water will
leave the cell by osmosis.
• Eventually, the cell will
plasmolyze, shrinking
and pulling away from
its wall.
Fig. 36.4a
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• If a flaccid cell is placed pure water (psi = 0), the
cell will have lower water potential due to the
presence of solutes than that in the surrounding
solution and water will enter the cell by osmosis.
• As the cell begins to swell, it will push against the wall,
producing a turgor pressure.
• The partially elastic wall
will push back until this
pressure is great enough
to offset the tendency
for water to enter the
cell because of solutes.
Fig. 36.4b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• When psip and psis are equal in magnitude (but
opposite in sign), psi = 0, and the cell reaches a
dynamic equilibrium with the environment, with
no further net movement of water in or out.
• A walled cell with a greater solute concentration
than its surroundings will be turgid or firm.
• Healthy plants are turgid
most of the time as
turgor contributes to
support in nonwoody
parts of the plant.
Fig. 36.5
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Objective 9
TSWBAT describe the
effects of water gain or
loss in animal and plant
cells.
Cell survival depends on balancing water
uptake and loss
• An animal cell immersed in an isotonic environment
experiences no net movement of water across its
plasma membrane.
• Water flows across the membrane, but at the same rate in
both directions.
• The volume of the cell is stable.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The same cell is a hypertonic environment will
loose water, shrivel, and probably die.
• A cell in a hypotonic solution will gain water,
swell, and burst.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• For a cell living in an isotonic environment (for
example, many marine invertebrates) osmosis is
not a problem.
• Similarly, the cells of most land animals are bathed in
an extracellular fluid that is isotonic to the cells.
• Organisms without rigid walls have osmotic
problems in either a hypertonic or hypotonic
environment and must have adaptations for
osmoregulation to maintain their internal
environment.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• For example, Paramecium, a protist, is hypertonic
when compared to the pond water in which it lives.
• In spite of a cell membrane that is less permeable to
water than other cells, water still continually enters the
Paramecium cell.
• To solve this problem,
Paramecium have a
specialized organelle,
the contractile vacuole,
that functions as a bilge
pump to force water out
of the cell.
Fig. 8.13
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The cells of plants, prokaryotes, fungi, and some
protists have walls that contribute to the cell’s
water balance.
• An animal cell in a hypotonic solution will swell
until the elastic wall opposes further uptake.
• At this point
the cell is
turgid, a
healthy
state for
most plant
cells.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Turgid cells contribute to the mechanical support
of the plant.
• If a cell and its surroundings are isotonic, there is
no movement of water into the cell and the cell is
flaccid and the plant may wilt.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In a hypertonic solution, a cell wall has no
advantages.
• As the plant cell looses water, its volume shrinks.
• Eventually, the plasma membrane pulls away from
the wall.
• This
plasmolysis
is usually
lethal.
Fig. 8.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings