Objective 7 TSWBAT compare hypoosmotic, hyperosmotic, and isoosmotic solutions and predict the path of movement of water and solutes in given examples

Objective 7

TSWBAT compare hypoosmotic, hyperosmotic, and isoosmotic solutions and predict the path of movement of water and solutes in given examples.

• 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.

Osmosis is the passive transport of water

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.


• 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

• 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.


Objective 8

TSWBAT relate osmotic potential to solute concentration and water potential.

• 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.”

Differences in water potential drive water transport in plant cells


• 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.


• 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.


• 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.


• 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).


• 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.


• 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).


• 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.



Fig. 36.3

• 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

• 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

• 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

Objective 9

TSWBAT describe the effects of water gain or loss in animal and plant cells.

• 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.

Cell survival depends on balancing water uptake and loss


• 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

• 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.


• 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

• 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

• 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

• 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

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Objective 7 TSWBAT compare hypoosmotic, hyperosmotic, and isoosmotic solutions and predict the path of movement of water and solutes in given examples