More Related Content
What's hot
What's hot (20)
Similar to 2014 HAPS Osmosis Workshop
Similar to 2014 HAPS Osmosis Workshop (20)
Recently uploaded
Recently uploaded (20)
2014 HAPS Osmosis Workshop
- 1. Is Osmosis the Diffusion of Water?
- 2. 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.
- 3. If you have any questions or comments, please contact me (Phil Tate) at: ptate4@gmail.com 806-789-4486 4423 110th St. Unit 22 Lubbock, TX 79424
- 4. 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.
- 5. 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.
- 6. 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.
- 7. • A selective permeable membrane selects or regulates what passes through the membrane. • Plasma membranes are selectively permeable.
- 8. Some characteristics of selectively permeable plasma membranes: • Water passes through, but not all solutes. • Rate of transport is controlled. o Opening and closing channels o Increasing or decreasing transport proteins • Direction of transport can be determined by the orientation of transport proteins. • Active and secondary active transport moves substances.
- 9. Selectively permeable = semipermeable Semipermeable ≠ selectively permeable
- 10. Sliding wall Sliding wall Semipermeable membrane fixed in position Water Sugar Osmosis Demonstration
- 11. Water flows to the right and both walls move to the right Volume decreases Sugar solution Volume increases
- 12. Left compartment Right compartmentMembrane Sugar molecule Pore
- 13. Left compartment Right compartmentMembrane Water molecule
- 14. Pisto n 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.
- 15. What causes the water to move? • Diffusion: random movement of the molecules? • Pressure: organized movement of the molecules?
- 16. 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
- 17. 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.
- 18. 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.
- 19. Concentration gradient = (c2 – c1)/(d2 – d1) = Δc/Δd Pressure gradient = (p2 – p1)/(d2 – d1) = Δp/Δd c2 c1 d2d1
- 21. 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.
- 22. Pressure and concentration are related by the ideal gas law: PV = nRT where P = pressure V = volume n = amount of the gas (mol) R = universal gas constant T = temperature (K)
- 23. PV = nRT P = (n/V) RT P = cRT where c = concentration = n/V = amount/volume
- 24. 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.
- 25. The van’t Hoff equation states that osmotic pressure is related to the concentration of the impermeable solute: P = cRT (ideal gas) Π = icRT where Π = osmotic pressure i = van’t Hoff factor c = concentration of the solute R = universal gas constant T = temperature (K)
- 26. 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.
- 27. 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)
- 28. The value of i can be determined by measuring osmotic pressure: Π = icRT i = Π/cRT
- 29. The value of i can be determined from the freezing point depression of water: i = ΔTf /Kf c where i = van’t Hoff factor ΔTf = freezing point depression of water Kf = cryoscopic constant for water (1.853 K kg/mol) c = concentration of solute
- 30. The concentration of particles (ic) in a solution determines the solution’s colligative properties. • Osmotic pressure • Freezing point depression • Boiling point elevation • Vapor pressure
- 31. 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
- 32. 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
- 33. 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.
- 34. 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.
- 35. 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.
- 36. “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.
- 37. Homework assignment P = Permeating solute in test solution NP = Nonpermeating solute in test solution X = Impossible combination * = Solution containing an isosmotic concentration of NP to which some P is added Source: Doemling DP. Isotonic vs isomotic solutions. A clarification of terms. JAMA. 1968 Jan 15;203(3):232-3. PMID: 5694052. Hypotonic Isotonic Hypertonic Hyposmotic P & NP X X Isosmotic P NP X Hyperosmotic P NP & P* NP
- 38. 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.
- 39. 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.
- 40. The evidence against diffusion: • Tritiated water experiments • Movement against a water concentration gradient
- 41. 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.
- 42. Membrane ΔP = 0 Movement by diffusion TOH TOH TOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH TOH TOH TOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH HOH Membrane ΔP >0 Movement 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.
- 43. 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.
- 44. 55.1 55.2 55.3 55.4 55.5 55.6 55.7 0 0.2 0.4 0.6 Waterconcentration(mol/L) Solute concentration (molality) NaF Na2SO4 Water moves by osmosis against its water concentration gradient into a NaF solution. Therefore, movement can not be by osmosis.
- 45. 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.
- 46. Sliding wall Sliding wall Semipermeable membrane fixed in position Water Sugar Osmosis Demonstration
- 47. P1 P2 P3 P4 Left compartment Right compartmentMembrane P1 = P2 = P3 = P4 = Atmospheric pressure Pore
- 48. P1 P2 P3 P4 Left compartment Right compartmentMembrane P1 = P2 = P3 = P4 = 1 AtmospherePressure(atmospheric) 1.0
- 49. Left compartment Right compartmentMembrane Sugar molecule Water molecule Pore
- 50. Left compartment Right compartmentMembrane Low pressure zone
- 51. P1 P2 P4 Left compartment Right compartmentMembrane P3 P2 > P3 Water moves through the pore Osmotic pressure Low pressure zone Pressure(atmospheric) 1.0
- 52. Water flows to the right and both walls move to the right Volume decreases Sugar solution Volume increases
- 53. Pisto n 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.
- 54. P1 P2 Left compartment Right compartmentMembrane P3 Low pressure zone Osmotic pressure P4 P2 = P3 Water movement stops Pressure(atmospheric) 1.0
- 55. 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 water Solution
- 56. P4 Water compartment Solution compartmentMembrane P3 P1 = P2 = P3 < atm Water does not move through the pore Osmotic pressure Low pressure zone Pressure(atmospheric) 1.0 P1 P2 P4
- 57. Sugar added to water diffuses to produce a sugar solution. There is no pressure change as predicted by the van’t Hoff equation.
- 58. 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.
- 59. It is much more complicated! • I have described a simple, physics explanation. • Many other explanations have been proposed.
- 60. 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.
- 61. 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)
- 62. 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
- 63. To get a copy of this PowerPoint • Email me at ptate4@gmail.com • Subject: Osmosis
Editor's Notes
- The iceberg philosophy of teaching: teach the tip, but know the iceberg. “The Essence of Imagination” by Ralph A. Clevenger (1999). Marketed by Successories (http://www.successories.com). See http://www.snopes.com/photos/natural/iceberg.asp for an explanation of how this image was created.
- Marieb, E., and Hoehn, K. (2013) Human Anatomy & Physiology, 9th ed. Pearson, New York, NY, 1107 pp. Martini, F., Nath, J., and Bartholomew, E. (2012) Anatomy & Physiology, 9th ed. Pearson, New York, NY, 1114 p. McKinly, M., O’Loughlin, V., and Bidle, T. (2013). Anatomy & Physiology: An Integrative Approach, 1st ed. New York, NY, 1169 pp. Saladin, K. (2012) Anatomy & Physiology, 12th ed. McGraw-Hill, New York, NY, 1136 pp. Tate, P. (2012) Principles of Anatomy & Physiology, 2nd ed. McGraw-Hill, New York, NY, 845 pp. Thibodeau, G. and Patton, K. (2007) Anatomy & Physiology, 7th ed. Mosby, St. Louis, MO, 1228 pp. Tortora, G. and Derrickson, B. (2006) Principles of Anatomy and Physiology, 11th ed. John Wiley & Sons, Hoboken, NJ, 1146 pp. VanPutte, C., Regan, J., and Russo, A. (2011) Anatomy & Physiology, 9th ed. McGraw-Hill, New York, NY, 1110 pp.
- Selectively permeable membranes are semipermeable membranes because water, but not all solutes pass through. Semipermeable membranes can not alter the rate of transport. The rate of transport is determined by differences in pressure and solute concentrations. For semipermeable membranes, the transport is the same in either direction. Semipermeable membranes can not move substances against their concentration gradients by expending energy.
- Physicists, chemists, the primary literature, and industry use the term semipermeable.
- A cylinder with two freely sliding walls. The ends of the cylinder are open to the atmosphere. A fixed semipermeable membrane (brown) separates the cylinder into two compartments, each containing pure water (blue). A sugar cube (green) is added to the right compartment at the beginning of the demonstration.
- The sugar cube dissolves and sugar molecules diffuse through the water, forming a sugar solution. Eventually, osmosis begins, and there is movement of water through the membrane from the left to the right compartment, decreasing the volume of the left compartment and increasing the volume of the right compartment. The two walls freely slide as the compartment volumes change and both walls move to the right because water is practically incompressible and unstretchable
- Sugar molecules do not pass through membrane pores.
- Water molecules can pass through membrane pores.
- Water movement into the right compartment can be prevented by positioning a piston against the right wall and applying exactly enough pressure to prevent the walls from moving. The pressure that stops the movement of water is, by definition, the osmotic pressure of the solution.
- Basketball analogy: Suppose you are holding a basketball. The air inside the basketball is composed of gas molecules moving randomly. If you inject helium into the basketball, the concentration of helium is higher at the injection site than elsewhere. The helium will diffuse throughout the inside of the basketball until it is uniformly distributed. Think of this as disorganized, random motion. If you drop the basketball, the random motion of the gas molecules does not stop. However, on average, all of the helium molecules are moving in the same direction toward the ground because of the force of gravity. Think of this as organized motion (Nelson, 2008, p. 8). Nelson P. Biological Physics, updated 1st ed., 2008. W.H. Freeman and Co., New York. 630 pp. Source of image: http://www.stockfreeimages.com/10027869/Basketball-abstract.html
- Examples of force involving contact between objects are pushing open a door, muscles pulling on bones to move limbs, and a ball bouncing off a wall. Examples of force that do not involve contract between objects are electric forces, such as between protons and electrons, and the forces produced by gravity and magnets. Pressure is the force per unit area on a surface. For example, if you balance your textbook on your head, gravity pulls on the mass of the book to produce a force called weight. The pressure you feel is the weight of the book divided by the surface area of the scalp that is in contact with the book. The more books you balance on your head, the greater the pressure. In the same manner, hydrostatic pressure is the pressure produced by a column of water. The higher the column, the greater the weight, and the greater the hydrostatic pressure. Diving to the bottom of a swimming pool, the increased hydrostatic pressure can be felt on the eardrums. Atmospheric pressure is the pressure produced by a column of air at sea level. Atmospheric pressure on a mountain top is less than at sea level because the height of the column of air decreases with altitude.
- An easy way to visualize a gradient.
- Two ways to increase the concentration gradient: (1) Increase the concentration difference and (2) decrease the distance.
- The ideal gas law combines the results of other laws (Silverberg 2000, p. 187): Avogadro’s law: V ∝ n, where n is the amount in moles Boyles law: V ∝ 1/P Charles law: V ∝ T Silverberg MS. Chemistry: The Molecular Nature of Matter and Change, 2nd ed., 2000. McGraw-Hill, Dubuque, IA. pp. 1086.
- These conditions are approximated for gases with a dilute concentration.
- In 1887, van’t Hoff proposed that the osmotic pressure for an ideal solution could be calculated from the concentration of the impermeable solute. In 1901, he was awarded the first Nobel Prize in Chemistry for his work. The equation indicates that for a given temperature, say room temperature, that osmotic pressure is a function of concentration, that is, the number of ions or molecules in a liter of solution. van’t Hoff JH. The role of osmotic pressure in the analogy between solutions and gases. (1887) In: J Memb Sci 1995 100:39-44.
- When sugar dissociates each sugar molecules is a unit. When NaCl dissociates in water, each ion acts as a unit.
- An osmole (Osm) is 6.022 x 1023 (Avogadro’s number) solute particles. One mole of a substance is the weight in grams of the atoms in its chemical formula. The number of osmoles of a substance is equal to the number of moles multiplied by the expected i. The osmotic concentrations of body fluids are typically expressed as milliosmole (mOsm), where a mOsm is 1/1000 of an osmole.
- The ideal gas law is an example of a phenomenological equation. It is a description of what was observed when the variables of pressure, temperature, volume, and amount were experimentally manipulated. Although it was not based on underlying theoretical considerations, there is now a molecular interpretation. Within the limits of an ideal (dilute) gas (or solution), it is accurate.
- Start comparison at 0.3 mOsm, the approximate concentration of a typical cell. Hamer J, Wu YC. Osmotic Coefficient and Mean Activity Coefficients on Uni-univalent Electrolytes in Water at 25 oC. J Physical and Chemical Reference Data Reprints 1972;1(4):1047-1099. ( i = osmotic coefficient x expected i)
- Based on freezing point depression from Lide DR (ed). CRC Handbook of Chemistry and Physics, Internet Version 2005, <http://www.hbcpnetbase.com>, CRC Press, Boca Raton, FL, 2005.
- For an ideal gas or solution, i = expected
- Increased concentration increases particle volume – Some definitions of ideal gas assume that particles have “point” mass. But particles do occupy space. As concentration increases, there are more particles occupying space and proportionately more of the solution volume is due to the particles. As particle density increases, the likelihood of interaction increases. Particle interaction is due to attractions between oppositely charged ions and intermolecular forces, such as hydrogen bonds. Cannon J, Kim D, Maruyama S, Shiomi J. Influence of ion size and charge on osmosis. J Phys Chem B. 2012 Apr 12;116(14):4206-11. doi: 10.1021/jp2113363. Epub 2012 Mar 23. PMID: 22397596. Zhao K, Wu H. Size effects of pore density and solute size on water osmosis through nanoporous membrane. J Phys Chem B. 2012 Nov 15;116(45):13459-66. doi: 10.1021/jp3076595. Epub 2012 Nov 6. PMID: 23116121.
- The calculated number of osmoles uses the expected i. The effective number of osmoles uses the measured i. The term “effective” is usually not stated. The “effective” concentration is equivalent to the concentration of an ideal gas or solution. See the discussion of osmotic coefficients in Koeppen BM, Stanton BA. Berne & Levy Physiology, 6th ed., 2009. Mosby, St. Louis. 848 pp. The osmotic coefficient (φ) is 100 times the measured i divided by the expected i. The osmotic coefficient is the deviation from expected. The effective osmotic concentration is then φic, where i is the expected i. I decided to not use osmotic coefficient because it introduces another term and because the van’t Hoff factor by itself is what is presented in typical chemistry texts. At higher concentrations, the van’t Hoff equation is not a good predictor of osmotic pressure (see Grattoni A, Merlo M, Ferrari M. Osmotic pressure beyond concentration restrictions. J Phys Chem B. 2007 Oct 11;111(40):11770-5. Epub 2007 Sep 19. PMID: 17880133).
- The “tonic” terms make sense when infusing solutions into patients. Sometimes the goal is not to change cell volume and an isotonic solution is used. Sometimes the goal is to change cell volume. For example, a hypertonic solution is used to to reduce cerebral edema. When describing osmosis in the kidneys, water is moving, but cells are not swelling or shrinking. The “osmotic” terms make more sense in this situation.
- Work through the table to verify that the “tonic” and “osmotic” terms are not always equivalent.
- The diffusion hypothesis.
- The pressure hypothesis.
- Even through an artificial lipid membrane, osmosis is greater than diffusion. Jansen M, Blume A. A comparative study of diffusive and osmotic water permeation across bilayers composed of phospholipids with different head groups and fatty acyl chains. Biophys J. 1995 Mar;68(3):997-1008. PMID: 7756562. Kramer EM, Myers DR. Five popular misconceptions about osmosis. Amer Assoc Phys Teachers. 2012 August;80(8):694-699. http://dx.doi.org/10.1119/1.4722325. Kramer EM, Myers DR. Osmosis is not driven by water dilution. Trends Plant Sci. 2013 Apr;18(4):195-7. doi: 10.1016/j.tplants.2012.12.001. Epub 2013 Jan 5. PMID: 23298880. Mauro A. Nature of solvent transfer in osmosis. Science. 1957 Aug 9;126(3267):252-3. PMID: 13454805. Mauro A. Some properties of ionic and nonionic semipermeable membranes. Circulation 1960 May;21(5):845-854. Doi: 10.1161/01.CIR.21.5.845. Rich GT, Sha’afi RI, Barton TC, Solomon AK. Permeability studies on red cell membranes of dog, cat, and beef. J Gen Physiol. 1967 Nov;50(10):2391-405. PMID: 6063688
- See figure 5A, p. 291 of Hammel HT, Schlegel WM. Osmosis and solute-solvent drag: fluid transport and fluid exchange in animals and plants. Cell Biochem Biophys. 2005;42(3):277-345. PMID: 15976460.
- This argument has not been refuted, as far as I know. This argument against diffusion seems to have a loop hole. Bogner P, Miseta A, Berente A, Schwarcz A, Kotek G, Repa I. Osmotic and diffusive properties of intracellular water in camel erythrocytes: effect of hemoglobin crowdedness. Cell Biol Int. 2005 Sep;29(9):731-6. PMID: 15951204. Cameron IL, Fullerton GD. Lack of appreciation of the role of osmotically unresponsive water in cell volume regulation. Cell Biol Int. 2014 May;38(5):610-4. doi: 10.1002/cbin.10238. Epub 2014 Jan 30. PMID: 24375657. Fullerton GD, Kanal KM, Cameron IL. On the osmotically unresponsive water compartment in cells. Cell Biol Int. 2006 Jan;30(1):74-7. Epub 2005 Dec 15. PMID: 16360324. Fullerton GD, Kanal KM, Cameron IL. Osmotically unresponsive water fraction on proteins: non-ideal osmotic pressure of bovine serum albumin as function of pH and salt concentration. Cell Biol Int. 2006 Jan;30(1):86-92. Epub 2005 Dec 22. PMID: 16376113. See figure 1.10, p. 119 for a model using osmotically inactive water. Fullerton GD, Cameron IL. Water compartments in cells. Methods Enzymol. 2007;428:1-28. Review. PMID: 17875409. Reid C, Rand RP. Fits to osmotic pressure data. Biophys J. 1997 Sep;73(3):1692-4. PMID: 19431907.
- Reexamine the osmosis demonstration with a pressure explanation. A cylinder with two freely sliding walls. The ends of the cylinder are open to the atmosphere. A fixed semipermeable membrane (brown) separates the cylinder into two compartments, each containing pure water (blue). A sugar cube (green) is added to the right compartment at the beginning of the demonstration.
- As the demonstration begins, P1 = pressure of pure water away from the membrane. P2 = pressure of pure water at membrane pore on the left. P3 = pressure of solution at membrane pore on the right. P4 = pressure of solution away from the membrane. All pressures are equal to atmospheric pressure because both ends of the cylinder are open to the atmosphere.
- Graph of pressures.
- At first, the sugar cube dissolves and sugar molecules diffuse through the water, forming a sugar solution. But there is no movement of water until the solute molecules reach the membrane. When sugar molecules hit membrane pores, they rebound back into the solution where they primarily hit water molecules, which are much more numerous than the sugar molecules. Thus, the fixed-in-position membrane produces a force that pushes sugar molecules and then water molecules away from membrane pores. It seems paradoxical, but pores that only allow the passage of water repel water in the presence of a nonpermeable solute.
- As some of the water molecules are pushed away from pore openings, the force produced by water molecules hitting pore openings decreases. Since pressure is force per unit area, the pressure at pore openings decreases, producing low-pressure zones at the openings.
- This decrease in pressure (P4 – P3) is osmotic pressure (π). Water moves through pores into the right compartment because the pressure on the pure water side of pores is greater than the pressure in the low-pressure zones (P2 > P3). Note that P2 does not decrease because the right compartment contains only water and no solutes.
- The sugar cube dissolves and sugar molecules diffuse through the water, forming a sugar solution. Eventually, osmosis begins, and there is movement of water through the membrane from the left to the right compartment, decreasing the volume of the left compartment and increasing the volume of the right compartment. The two walls freely slide as the compartment volumes change and both walls move to the right because water is practically incompressible and unstretchable
- Water movement into the right compartment can be prevented by positioning a piston against the right wall and applying exactly enough pressure to prevent the walls from moving. The pressure that stops the movement of water is, by definition, the osmotic pressure of the solution.
- The increased pressure produced by the piston increases pressure in the right compartment (P4) and in the low-pressure zones (P3). When the pressure in the low-pressure zones equals the pressure on the pure water side of the pores in the left compartment (P2 = P3), water movement through the pores stops.
- Is there any evidence that osmotic pressure causes a decrease in pressure at the membrane? The Pfeffer-type osmometer, U-tube, and thistle tube oppose osmotic water flow by increasing pressure. In the Hepp-type osmometer, the volume of the water chamber does not change and water does not move out of the chamber because water is basically unstretchable. For a similar effect, recall holding the end of a fluid-filled straw. Solute reflected from the membrane produces a force and a decrease in pressure that would move water across the membrane. Assuming that this decrease in pressure is transmitted through the pore to the pure water chamber, a decrease in pressure below atmospheric pressure can be measured in the chamber. Mauro A. Osmotic flow in a rigid porous membrane. Science. 1965 Aug 20;149(3686):867-9. PMID: 14332848.
- Assuming pressure is transmitted through pores, this shows than osmotic pressure can be a negative pressure (i.e., below atmospheric pressure).
- Mauro A. Forum on osmosis. III. Comments on Hammel and Scholander's solvent tension theory and its application to the phenomenon of osmotic flow. Am J Physiol. 1979 Sep;237(3):R110-3. PMID: 474782. Nelson P. Biological Physics, updated 1st ed., 2008. W.H. Freeman and Co., New York. 630 pp. (see pp. 258-259)
- One can hardly go wrong with the force argument because it is basic physics. Water moves across a semipermeable membrane because a force is applied to the water. Pressure is force/unit area, so there is a pressure at pore openings. The tricky part is explaining what causes that force. What I have presented is a classic, physics Newtonian argument. It could be rephrased as momentum transfer (Ben-Sasson, et. al. 2003). If Δd (dx in calculus) is small enough, it has been proposed that diffusion could generate a pressure change (Benedek, et. al. 2000, pp. 212-228 and especially p. 222). This proposal is disputed (Kiil, 2003, p 110). It also has been suggested that if Δd is small enough, momentum transfer resembles diffusion (Zeuth, et. al. 2013, p. 5026). What goes on in the nano-world of cells and in aquaporins in which water molecules pass through single file is unknown because data collection at that scale has not been achieved. To paraphrase a well-know saying for Las Vegas, what happens in the pore stays in the pore (see Beckstein, 2008 for a more formal description). Alleva K, Chara O, Amodeo G. Aquaporins: another piece in the osmotic puzzle. FEBS Lett. 2012 Sep 21;586(19):2991-9. doi: 10.1016/j.febslet.2012.06.013. Epub 2012 Jun 20. PMID: 22728434. Beckstein O. Teaching old coefficients new tricks: new insight into the meaning of the osmotic and diffusive permeation coefficients. Biophys J. 2009 Feb;96(3):763-4. doi: 10.1016/j.bpj.2008.10.048. PMID: 19186119. Benedek GB, Villars FMH. Physics With Illustrative Examples From Medicine and Biology, 2nd ed., 2000. Springer-Verlag, New York. 640 pp. Ben-Sasson SA, Grovern NB. Osmosis: a macroscopic phenomenon, a microscopic view. Adv Physiol Educ. 2003 Mar;27(1):15-9. doi: 10.1152/advan.00015.2002. PMID: 12594069. Corrigendum. Adv Physiol Educ 2007 June;31(2):245. doi:10.1152/advan.50000.2007. Davis IS, Shachar-Hill B, Curry MR, Kim KS, Pedley TJ, Hill AE. Osmosis in semipermeable pores: an examination of the basic flow equations based on an experimental and molecular dynamics study. Proc R Soc A 2007 Mar;463:881-896. Doi:10.1098/rspa.2006.1803. Granik VT, Smith BR, Lee SC, Ferrari M. Osmotic pressures of binary solutions of non-electrolytes. Biomed Microdev 2002;4:309-321. Grattoni A, Merlo M, Ferrari M. Osmotic pressure beyond concentration restrictions. J Phys Chem B. 2007 Oct 11;111(40):11770-5. Epub 2007 Sep 19. PMID: 17880133. Guell DC, Brenner H. Physical mechanism of membrane osmotic phenomena. Ind Eng Chem Res. 1996;35:3004-3014. Kiil F. Kinetic model of osmosis through semipermeable and solute-permeable membranes. Acta Physiol Scand 2003;177:107-117. Kim KS, Davis IS, Macpherson PA, Pedley TJ, Hill AE. Osmosis in small pores: a molecular dynamics study of the mechanism of solvent transport. Proc Roy Soc 2005;461:273-296. Kramer EM, Myers DR. Five popular misconceptions about osmosis. Amer Assoc Phys Teachers. 2012 August;80(8):694-699. http://dx.doi.org/10.1119/1.4722325. Kramer EM, Myers DR. Osmosis is not driven by water dilution. Trends Plant Sci. 2013 Apr;18(4):195-7. doi: 10.1016/j.tplants.2012.12.001. Epub 2013 Jan 5. PMID: 23298880. Raghunathan AV, Aluru NR. Molecular understanding of osmosis in semipermeable membranes. Phys Rev Lett. 2006 Jul 14;97(2):024501. Epub 2006 Jul 10. PMID: 16907451. Yokozeki A. Osmotic pressures studied using a simple equation-of-state and its applications. Appl Energy 2006;83:15-41. doi:10.1016/j.apenergy.2004.10.015. Zeuthen T, Alsterfjord M, Beitz E, MacAulay N. Osmotic water transport in aquaporins: evidence for a stochastic mechanism. J Physiol. 2013 Oct 15;591(Pt 20):5017-29. doi: 10.1113/jphysiol.2013.261321. Epub 2013 Aug 19. PMID: 23959676.
- Thanks to Eric Kramer, David Myers, Russel Hobbie, and Philip Nelson.