Top Banner
Is Osmosis the Diffusion of Water?
63

2014 HAPS Osmosis Workshop

Apr 11, 2017

Download

Science

Philip Tate
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Page 1: 2014 HAPS Osmosis Workshop

Is Osmosis the Diffusion of Water?

Page 2: 2014 HAPS Osmosis Workshop

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.

Page 3: 2014 HAPS Osmosis Workshop

If you have any questions or comments, please contact me (Phil Tate) at:[email protected]

4423 110th St. Unit 22Lubbock, TX 79424

Page 4: 2014 HAPS Osmosis Workshop

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.

Page 5: 2014 HAPS Osmosis Workshop

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.

Page 6: 2014 HAPS Osmosis Workshop

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.

Page 7: 2014 HAPS Osmosis Workshop

• A selective permeable membrane selects or regulates what passes through the membrane.

• Plasma membranes are selectively permeable.

Page 8: 2014 HAPS Osmosis Workshop

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.

Page 9: 2014 HAPS Osmosis Workshop

Selectively permeable = semipermeableSemipermeable ≠ selectively permeable

Page 10: 2014 HAPS Osmosis Workshop

Sliding wall Sliding wallSemipermeable membrane fixed in position

Water Sugar

Osmosis Demonstration

Page 11: 2014 HAPS Osmosis Workshop

Water flows to the right and both walls move to the right

Volume decreases

Sugar solution

Volume increases

Page 12: 2014 HAPS Osmosis Workshop

Left compartment Right compartmentMembrane

Sugar moleculePore

Page 13: 2014 HAPS Osmosis Workshop

Left compartment Right compartmentMembrane

Water molecule

Page 14: 2014 HAPS Osmosis Workshop

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.

Page 15: 2014 HAPS Osmosis Workshop

What causes the water to move?• Diffusion: random movement of the molecules?• Pressure: organized movement of the molecules?

Page 16: 2014 HAPS Osmosis Workshop

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

Page 17: 2014 HAPS Osmosis Workshop

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.

Page 18: 2014 HAPS Osmosis Workshop

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.

Page 19: 2014 HAPS Osmosis Workshop

Concentration gradient = (c2 – c1)/(d2 – d1) = Δc/Δd

Pressure gradient = (p2 – p1)/(d2 – d1) = Δp/Δd

c2

c1 d2d1

Page 20: 2014 HAPS Osmosis Workshop

Increaseconcentration difference

Decreasedistance

c2

c1 d2d1

c2

c1d1 d2

c2

c1 d1 d2

Page 21: 2014 HAPS Osmosis Workshop

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.

Page 22: 2014 HAPS Osmosis Workshop

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)

Page 23: 2014 HAPS Osmosis Workshop

PV = nRT

P = (n/V) RT

P = cRTwhere

c = concentration = n/V = amount/volume

Page 24: 2014 HAPS Osmosis Workshop

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.

Page 25: 2014 HAPS Osmosis Workshop

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)

Page 26: 2014 HAPS Osmosis Workshop

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.

Page 27: 2014 HAPS Osmosis Workshop

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)

Page 28: 2014 HAPS Osmosis Workshop

The value of i can be determined by measuring osmotic pressure:

Π = icRT

i = Π/cRT

Page 29: 2014 HAPS Osmosis Workshop

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

Page 30: 2014 HAPS Osmosis Workshop

The concentration of particles (ic) in a solution determines the solution’s colligative properties.• Osmotic pressure• Freezing point depression• Boiling point elevation• Vapor pressure

Page 31: 2014 HAPS Osmosis Workshop

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

Page 32: 2014 HAPS Osmosis Workshop

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

Page 33: 2014 HAPS Osmosis Workshop

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.

Page 34: 2014 HAPS Osmosis Workshop

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.

Page 35: 2014 HAPS Osmosis Workshop

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.

Page 36: 2014 HAPS Osmosis Workshop

“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.

Page 37: 2014 HAPS Osmosis Workshop

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

Page 38: 2014 HAPS Osmosis Workshop

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.

Page 39: 2014 HAPS Osmosis Workshop

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.

Page 40: 2014 HAPS Osmosis Workshop

The evidence against diffusion:• Tritiated water experiments• Movement against a water concentration

gradient

Page 41: 2014 HAPS Osmosis Workshop

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.

Page 42: 2014 HAPS Osmosis Workshop

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.

Page 43: 2014 HAPS Osmosis Workshop

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.

Page 44: 2014 HAPS Osmosis Workshop

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.

Page 45: 2014 HAPS Osmosis Workshop

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.

Page 46: 2014 HAPS Osmosis Workshop

Sliding wall Sliding wallSemipermeable membrane fixed in position

Water Sugar

Osmosis Demonstration

Page 47: 2014 HAPS Osmosis Workshop

P1 P2 P3 P4

Left compartment Right compartmentMembrane

P1 = P2 = P3 = P4 = Atmospheric pressure

Pore

Page 48: 2014 HAPS Osmosis Workshop

P1 P2 P3 P4

Left compartment Right compartmentMembrane

P1 = P2 = P3 = P4 = 1 AtmosphereP

ress

ure

(atm

osph

eric

)

1.0

Page 49: 2014 HAPS Osmosis Workshop

Left compartment Right compartmentMembrane

Sugar molecule

Water molecule

Pore

Page 50: 2014 HAPS Osmosis Workshop

Left compartment Right compartmentMembrane

Low pressure zone

Page 51: 2014 HAPS Osmosis Workshop

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

Page 52: 2014 HAPS Osmosis Workshop

Water flows to the right and both walls move to the right

Volume decreases

Sugar solution

Volume increases

Page 53: 2014 HAPS Osmosis Workshop

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.

Page 54: 2014 HAPS Osmosis Workshop

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

Page 55: 2014 HAPS Osmosis Workshop

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

Page 56: 2014 HAPS Osmosis Workshop

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

Page 57: 2014 HAPS Osmosis Workshop

Sugar added to water diffuses to produce a sugar solution. There is no pressure change as predicted by the van’t Hoff equation.

Page 58: 2014 HAPS Osmosis Workshop

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.

Page 59: 2014 HAPS Osmosis Workshop

It is much more complicated!• I have described a simple, physics explanation.• Many other explanations have been proposed.

Page 60: 2014 HAPS Osmosis Workshop

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.

Page 61: 2014 HAPS Osmosis Workshop

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)

Page 62: 2014 HAPS Osmosis Workshop

Contact information:• Dr. Phil Tate: [email protected]• Dr. Eric Kramer: [email protected]• Dr. Russel Hobbie: [email protected]• Dr. Philip Nelson: [email protected]

Page 63: 2014 HAPS Osmosis Workshop

To get a copy of this PowerPoint• Email me at [email protected]• Subject: Osmosis