Transcript
Is Osmosis the Diffusion of Water?
This slide show was used at the annual Human Anatomy & Physiology (HAPS) conference in Jacksonville, Florida on May 27, 2014.
You are welcome and encouraged to use the information and images in this slide show in your classes for educational purposes.
Additional explanations and references are in the notes.
If you have any questions or comments, please contact me (Phil Tate) at:ptate4@gmail.com806-789-4486
4423 110th St. Unit 22Lubbock, TX 79424
Teach the tip, but know the iceberg.
I have removed the image of the iceberg from this slide show, which I am making available to others. Using the image once for educational purposes is within copyright rules.For source of the image and an explanation of how it was created, see the notes.
From a sample of eight A&P textbooks:• Osmosis (Gr., pushing) is the (passive)
movement (net movement, diffusion, net diffusion, net flow) of water across a selectively permeable membrane.
• In the definition of osmosis, or elsewhere, these texts state that the movement of water occurs by diffusion.
The best term to describe the membrane is semipermeable, not selectively permeable.• A semipermeable membrane allows water to
pass through the membrane, but blocks, or partially blocks, the passage of at least one solute.
• Examples of semipermeable membranes are plasma membranes, cell junctions, basement membranes, and artificial, nonliving membranes.
• A selective permeable membrane selects or regulates what passes through the membrane.
• Plasma membranes are selectively permeable.
Some characteristics of selectively permeable plasma membranes:• Water passes through, but not all solutes.• Rate of transport is controlled.
o Opening and closing channelso Increasing or decreasing transport proteins
• Direction of transport can be determined by the orientation of transport proteins.
• Active and secondary active transport moves substances.
Selectively permeable = semipermeableSemipermeable ≠ selectively permeable
Sliding wall Sliding wallSemipermeable membrane fixed in position
Water Sugar
Osmosis Demonstration
Water flows to the right and both walls move to the right
Volume decreases
Sugar solution
Volume increases
Left compartment Right compartmentMembrane
Sugar moleculePore
Left compartment Right compartmentMembrane
Water molecule
Piston
The piston produces a pressure that prevents water and wall movement.
Water Sugar solution
The osmotic pressure of the sugar solution is equal to the piston pressure that prevents the movement of water into the sugar solution.
What causes the water to move?• Diffusion: random movement of the molecules?• Pressure: organized movement of the molecules?
Helium diffuses throughout the inside of the ball. This is disorganized random motion.
On the average, the helium moves toward the ground. This is organized motion caused by a force.
Inject helium
More formally:• A force is a push or pull that causes, or could
cause, an object to change speed, direction, or shape.
• Pressure is the force per unit area on a surface.
The movement of molecules is often described in terms of gradients.• A concentration gradient is the difference in
concentration between two points, c1 and c2, divided by the distance between them.
• A pressure gradient is the difference in pressure between two points, p1 and p2, divided by the distance between them.
Concentration gradient = (c2 – c1)/(d2 – d1) = Δc/Δd
Pressure gradient = (p2 – p1)/(d2 – d1) = Δp/Δd
c2
c1 d2d1
Increaseconcentration difference
Decreasedistance
c2
c1 d2d1
c2
c1d1 d2
c2
c1 d1 d2
For movement of water across a semipermeable membrane, the thickness of the membrane does not change. • Concentration gradients change because of
change in concentration.• Pressure gradients change because of change in
pressure.
Pressure and concentration are related by the ideal gas law:
PV = nRTwhere
P = pressureV = volumen = amount of the gas (mol)R = universal gas constant T = temperature (K)
PV = nRT
P = (n/V) RT
P = cRTwhere
c = concentration = n/V = amount/volume
Properties of an ideal gas:• Molecules have the same mass, but no
significant volume.• Molecules move randomly within a container.• Collisions between molecules and the container
wall are elastic, meaning there is no loss of energy during collisions.
• The only forces molecules exert upon each other occurs during collisions.
The van’t Hoff equation states that osmotic pressure is related to the concentration of the impermeable solute:
P = cRT (ideal gas)Π = icRT
whereΠ = osmotic pressurei = van’t Hoff factorc = concentration of the soluteR = universal gas constant T = temperature (K)
Note the introduction of the van’t Hoff factor.• For molecules, such as sugar, the expected i = 1.• For an ionic compounds, such as NaCl, the
expected i = 2.• This was one of the key pieces of evidence that
ionic compounds dissociate.
Osmotic concentration• A particle is defined as an atom, ion, or
molecule.• Osmotic concentration is expressed as osmoles,
where an osmole is Avogadro’s number of particles (6.022 x 1023).
• ic is the number of osmoles in a solution.o 1 mole of sugar = 1 osmole (1 x 1)o 1 mole of NaCl = 2 osmole (2 x 1)
The value of i can be determined by measuring osmotic pressure:
Π = icRT
i = Π/cRT
The value of i can be determined from the freezing point depression of water:
i = ΔTf /Kf c where
i = van’t Hoff factorΔTf = freezing point depression of waterKf = cryoscopic constant for water
(1.853 K kg/mol)c = concentration of solute
The concentration of particles (ic) in a solution determines the solution’s colligative properties.• Osmotic pressure• Freezing point depression• Boiling point elevation• Vapor pressure
Concentration i for NaCl i for KCl i for HCl
0.001 1.98 1.98 1.98
0.01 1.93 1.93 1.94
0.1 1.87 1.85 1.89
0.3 1.84 1.81 1.91
1.0 1.87 1.80 2.07
2.0 1.96 1.82 2.37
3.0 2.09 1.87 2.69
4.0 2.23 1.93 3.03
Effect of Different Electrolytes and Concentration (molality) on i
As concentration decreases, i approaches 2.
For a given concentration, i is different for different electrolytes.
As concentration increases, i becomes larger
Effect of Sucrose Concentration (molality) on i
Concentration i
0.09 1.02
0.122 1.02
0.289 1.03
0.476 1.05
1.026 1.12
1.948 1.23
Concentration vs. kind of particles• For an ideal gas or solution, the
concentration, not the kind, of particles determines osmotic pressure because the measured i approaches the expected i.
• For a real gas or solution, the concentration and the kind of particles determines the osmotic pressure.
Explanation for different i values:• The assumptions of the ideal gas law are violated.
o Increased concentration increases the part of the total volume occupied by particles.
o Particles interact with each other.• i values can be smaller or larger than expected.
o Oppositely charged ions tend to group together and the group becomes one particle.
o Polar molecules cause water to split into H+ and OH-.o Different part of large molecules may act as separate
particles.
ic using the measured i is the “effective” osmotic concentration of the particles in osmoles.• For solutions of physiological interest, the van’t
Hoff equation using the measured i works.• In practice, the osmolality of a fluid is measured.
For example, the osmolality of fluids in the kidneys.
“Osmotic” versus “tonic” terms.• Hypo-, hyper-, and isosmotic terms define the
osmotic concentration of solutions, assuming all the solutes are nonpermeable.
• Hypo-, hyper-, and isotonic terms define changes in cell volume.
• The terms are not equivalent if one or more of the solutes are permeable.
Homework assignment
P = Permeating solute in test solutionNP = Nonpermeating solute in test solutionX = Impossible combination* = Solution containing an isosmotic concentration of NPto which some P is added
Source: Doemling DP. Isotonic vs isomotic solutions. A clarificationof terms. JAMA. 1968 Jan 15;203(3):232-3. PMID: 5694052.
Hypotonic Isotonic HypertonicHyposmotic P & NP X XIsosmotic P NP XHyperosmotic P NP & P* NP
Comparing diffusion and pressure:• Diffusion is the net movement of a substance
from a region of higher concentration to an adjacent region of lower concentration of that substance.
• Diffusion results from the random movement (disorganized motion) of the particles, which is a function of their thermal energy or temperature.
• During osmosis, water moves by diffusion down its concentration gradient.
Pressure• Pressure is the force per unit area on a surface. • In osmosis, the surface area is the surface area
of all the pores in the membrane.• During osmosis, water moves down its pressure
gradient. • Osmosis is the bulk flow (organized motion) of
water due to pressure.
The evidence against diffusion:• Tritiated water experiments• Movement against a water concentration
gradient
Tritiated water experiments• Tritium (TOH) is regular water (HOH) in which a
hydrogen is replaced with tritium.• Tritium is a hydrogen isotope with two neutrons.• Tritium is radioactive and can be traced.
Membrane
ΔP = 0Movement by diffusion
TOH
TOH
TOH
HOH
HOHHOH
HOH HOH
HOH
HOH
HOHHOH
HOHHOH
HOH
HOH
TOH
TOH
TOH
HOH
HOHHOH
HOH
HOH
HOH
HOH
HOH
HOH
HOHHOH
HOH
HOH
Membrane
ΔP >0Movement by osmosis
Movement of TOH by osmosis across cell membranes is two to six times greater than by diffusion.
In one artificial membrane, the rate was 730 times greater.
Movement against a concentration gradient• There are 55.5 moles of water in 1 L of pure
water.• When a solute is added to pure water, the mole
fraction (proportion) of water usually decreases.• Some solutes so strongly attract water that the
amount of water in 1 L increases.
0 0.1 0.2 0.3 0.4 0.5 0.654.854.9
5555.155.255.355.455.555.655.7
NaFNa2SO4
Solute concentration (molality)
Wat
er co
ncen
trati
on (m
ol/L
)
Water moves by osmosis against its water concentrationgradient into a NaF solution. Therefore, movement can not be by osmosis.
Wait a minute! That is not proof!• The water associated with the solute is
“osmotically unresponsive water.”• The actual concentration of the “available” water
in the solution is less than pure water, so diffusion could still occur with its concentration gradient.
Sliding wall Sliding wallSemipermeable membrane fixed in position
Water Sugar
Osmosis Demonstration
P1 P2 P3 P4
Left compartment Right compartmentMembrane
P1 = P2 = P3 = P4 = Atmospheric pressure
Pore
P1 P2 P3 P4
Left compartment Right compartmentMembrane
P1 = P2 = P3 = P4 = 1 AtmosphereP
ress
ure
(atm
osph
eric
)
1.0
Left compartment Right compartmentMembrane
Sugar molecule
Water molecule
Pore
Left compartment Right compartmentMembrane
Low pressure zone
P1 P2 P4
Left compartment Right compartmentMembrane
P3
P2 > P3 Water moves through the pore
Osmoticpressure
Low pressure zoneP
ress
ure
(atm
osph
eric
)
1.0
Water flows to the right and both walls move to the right
Volume decreases
Sugar solution
Volume increases
Piston
The piston produces a pressure that prevents water and wall movement.
Water Sugar solution
The osmotic pressure of the sugar solution is equal to the piston pressure that prevents the movement of water into the sugar solution.
P1 P2
Left compartment Right compartmentMembrane
P3
Low pressure zone
Osmoticpressure
P4
P2 = P3 Water movement stops
Pre
ssur
e (a
tmos
pher
ic)
1.0
Pfeffer-type osmometer
The pressure generated by the piston that prevents water movement is measured.
Hepp-type osmometer
Volume of water chamber can not change. Pressure across the membrane becomes negative (decreases below atmospheric pressure).
Pure waterSolution
P4
Water compartment Solution compartmentMembrane
P3
P1 = P2 = P3 < atmWater does not move through the pore
Osmoticpressure
Low pressure zoneP
ress
ure
(atm
osph
eric
)
1.0
P1 P2 P4
Sugar added to water diffuses to produce a sugar solution. There is no pressure change as predicted by the van’t Hoff equation.
Pressure changes only if a force acts.• The semipermeable membrane applies a force to
the solute particles.• Osmotic pressure is not generated until the
solute particles reach the membrane.• Random molecular motion (Brownian
movement) averages to zero.• The semipermeable membrane rectifies
Brownian movement, creating a net movement away from the membrane.
It is much more complicated!• I have described a simple, physics explanation.• Many other explanations have been proposed.
Take home message:• Semipermeable membrane is the best term.• The kind of particle affects osmotic pressure.• The van’t Hoff equation using measured values
of i works for physiological solutions.• “osmotic” and “tonic” terms are not equivalent.• Movement of water by osmosis is 2 – 6 times
greater than by diffusion.• Osmosis is the bulk flow of water due to a
pressure gradient.
Acknowledgements• Kramer EM, Myers DR (2012) Five popular
misconceptions about osmosis. • Hobbie RK, Roth BJ. (2007) Intermediate Physics
for Medicine and Biology• Nelson P. Biological Physics (2008)
Contact information:• Dr. Phil Tate: ptate4@gmail.com• Dr. Eric Kramer: ekramer@simons-rock.edu• Dr. Russel Hobbie: hobbie@umn.edu• Dr. Philip Nelson: nelson@physics.upenn.edu
To get a copy of this PowerPoint• Email me at ptate4@gmail.com• Subject: Osmosis
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