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Osmosis project of chemistry Document Transcript

  • 1. Osmosis ProjectWhat is Osmosis?Osmosis is basically the movement of water molecules from a dilute system solution to aconcentrated solution, through a partially permeable membrane. Water molecules are able topass through the cell membrane because they diffuse whereas sugar molecules are larger andcannot diffuse as easily therefore not being able to pass through. Cell membranes are like viskingtubes because they will let some substances through but not others. They are partially permeablemembranes.Osmosis is a very important process which enables plants to support themselves by absorbingwater and minerals through a partially permeable membrane. Plants are often surrounded by afilm of water and a solution. Cell membranes often separate the two and Osmosis will occur. Hotwater diffuses and enters at a faster rate because there is more energy whereas cold waterenters at a slower rate because there is less energy.What do we have to do?To carry out the Osmosis project we have to measure the amount of water and solution thatenters carrot tissue through the partially permeable membrane. We will change the strengths ofthe solution and then weigh the carrots to see”OsmosisOsmosis is thediffusion of a solventthrough asemipermeable membrane from a region oflow soluteconcentration to a region of high solute concentration. The semipermeablemembrane is permeable to the solvent, but not to the solute, resulting in achemicalpotential difference across the membrane which drives the diffusion. That is, the solvent flowsfrom the side of the membrane where the solution is weakest to the side where it is strongest,until the solution on both sides of the membrane is the same strength (that is, untilthechemical potential is equal on both sides).Osmosis is an important topic in biology because it provides the primary means bywhich water is transported into and out of cells.Contents [hide]1 Explanation2 Example of osmosis3 Chemical potential4 Osmotic pressure5 Reverse osmosis6 See alsoExplanation
  • 2. Solutes, such as proteins or simple ions, dissolve in a solvent such as water. This raises theconcentration of the solute in these areas. The solvent then diffuses to these areas of highersolute concentration to equalize the concentration of the solute throughout the solution.Example of osmosisA practical example of this osmosis in cells can be seen in red blood cells. These contain a highconcentration of solutes including salts and protein. When the cells are placed in solution,water rushes in to the area of high solute concentration, bursting the cell.Many plant cells do not burst in the same experiment. This is because the osmotic entry ofwater is opposed and eventually equalled by the pressure exerted by the cell wall, creatinga steady state. In fact, osmotic pressure is the main cause of support in plant leaves.When a plant cell is placed in a solution higher in solutes than inside the cell osmosis out ofthe cell occurs. The water in the cell moves to an area higher in solute concentration, and thecell shrinks and so becomes flaccid. This means the cell has become plasmolysed - the cellmembrane has completely left the cell wall due to lack of water pressure on it.In unusual environments, osmosis can be very harmful to organisms. For example, freshwaterand saltwater aquarium fish placed in water with an different salt level (than they are adaptedto) will die quickly, and in the case of saltwater fish rather dramatically. In addition, the use oftable salt to kill leeches and slugs depends on osmosis.Osmotic pressureAs mentioned before, osmosis can be opposed by increasing the pressure in the region of highsolute concentration with respect to that in the low solute concentration region. The force perunit area required to prevent the passage of water through a semi-permeable membrane andinto a solution of greater concentration is equivalent to the osmotic pressure of the solution,or turgor. Osmotic pressure is a colligative property, meaning that the property depends onthe concentration of the solute but not on its identity.Increasing the pressure increases the chemical potential of the system in proportion to themolar volume (δμ = δPV). Therefore, osmosis stops, when the increase in potential due topressure equals the potential decrease from Equation 1, i.e.:Where δP is the osmotic pressure and V is the molar volume of the solvent.For the case of very low solute concentrations, -ln(1-x2) ≈ x2 and Equation 2 can berearranged into the following expression for osmotic pressure:
  • 3. Demonstrating with eggsAn egg contains a semipermeable membrane underneath the shell, which can be used todemonstrate osmosis. The eggshell is mostly made ofcalcium carbonate which will dissolve inacid. Vinegar or hydrochloric acidare suitable. Stronger acids will dissolve the shell faster, butare more corrosive. Vinegar takes several days.Method• Place three eggs in a beaker, cover them with acid and weigh them down so they dontfloat above the surface.• Allow them to remain in the acid until the shells completely dissolve.• Then place each of the eggs in a different liquid:o Place one in watero Place one in an isotonic sucrose solution, [about 0.3M]o Place one in a 1M sucrose solution• Leave overnight.• The next day remove the eggs and compare their size.You will find that the egg in water will have swollen considerably. If you carefully pierce themembrane with a needle a jet of water will shoot into the air. The egg in an isotonic solutionwill be approximately the same size, while the egg in 1M solution will have shrunk.Demonstrating osmosis with Potato slicesThere are a number of variations on this demonstration. Potato slices can be used, as also canraw potato chips (English) or French fries (American). Consistancy of sample can be ensuredby using a commercial potato chipper, or by using a cork borer of selected diameter. Othervegetables or fruit may, of course, be used. The changing dependent variable may be weightor length.Materials• Potato• Knife, cork borer, commercial chipper• Cutting board• Dilutions of sugar (sucrose), suggested range 0 - 0.6M• Suitable containers (beakers or test tubes or potato cylinders) to match selecteddilution range• Weighing balance / millimetre ruler• Paper towel• Marking pen/crayon• ForcepsNote: Ensure that the cork borer or chipper will cut chips or cylinders which fit into astandard test tube
  • 4. Method• Cut suitably sized slices or cylinders of potato. As far as is practical, all pieces shouldbe the same length, width, and thickness, the actual size depending on the chosencontainer (beaker or test tube).• Mark each container, pour in the appropriate dilution of sucrose.• Pat each piece of potato dry, and a) weigh it or b) measure its longest length. Notethe measurement.• When the piece has been measured, immerse it in one of your solutions.• Leave the potato in the sugar solutions for at least half an hour.• Use the forceps take out each chip in turn, carefully blot it dry without squeezing, andremeasure the piece.• Plot a graph of either absolute change or percentage change in weight/length vmolarity of the sugar dilutionTurgor(Redirected fromOsmotic pressure)Turgor (also called turgor pressure orosmotic pressure) is the pressure that can build in aspace that is enclosed by amembrane that ispermeable to a solvent such as water but not tosolutes.A biological cell, for example a plant cell, contains ions, sugars, amino acids, and othersubstances. In a hypotonic environment, water flows across the plasma membrane into thecell (since the concentration of water is lower inside the cell than outside), causing it toexpand. The cell wall of a plant cell restricts the expansion, causing the cell to press againstthe wall. The resulting pressure is called turgor.The osmotic pressure π of a dilute solution can be calculated using the formulawhereM is the molarityR is the molar gas constantT is absolute temperature (i.e. measured in kelvin).Examples of osmotic pressure• Hypertonic is a solution with higher solute concentration (higher osmotic pressure)than another thus water wants to move in.• Hypotonic is a solution with lower solute concentration (lower osmotic pressure) thananother thus water wants to move out of it.• Isotonic is solution with the same solute concentration (same osmotic pressure) asanother; no net movement of water.
  • 5. Calculating Osmotic PressureOsmotic PressureWe need to know the molar concentration of dissolved species in orderto calculate the osmotic pressure of an aqueous solution. We calculatethe osmotic pressure, (pi), using the following equation:Where:M is the molar concentration of dissolved species (units of mol/L).R is the ideal gas constant (0.08206 L atm mol-1K-1, or other valuesdepending on the pressure units).T is the temperature on the Kelvin scale.π = MRTiπ =.01082 atmR=0.082058 L-atm/mol-KT=25+273= 298i = 1 if the protein does not break up in solution and remains in 1 piece?substituting in th eosmotic pressure equation we getMolality = .01082 / ( .082058 X 298)= 0.000442476 moles/ Kgour solution is 4.3g / .29 l or 4.3 X 1 / .29 = 14.83 g / literif we assume density = 1 we also have 14.83 g of protein / kg of solutioncomparing the descriptions of 1 kg of the solution we have .0004425 moles = 14.83 g
  • 6. 1 mole weighs 14.83 / .0004425 = 33514 g/ mole !!Snapshots 1–3: determining molecular weights for three different solutesI. Osmotic Pressure via an Internal MeasurementMechanismNormallly when doing dialysis one fills the tube as full as possible so as to have thegreatest surface are for osmosis. The process will thus occur as rapidly as possible.Afterall, osmosis is not an overly fast process in the first place!When the contents are hypertonic, the tube becomes pressurized. In order to measurethat osmotic pressure, or turgor pressure, one is wont to think about affixing a manometeror other pressure gauge to the system. But how can one do that and yet maintain of tightseal that can withstand the expected pressures? Thus we move from science totechnology!However, if one were to fill the tube only one third full with a large balloon of trapped airabove it, a barometer of sorts is created because that air is compressible (see left figureto the right). The higher the osmolarity, the more water will try to flow into the tube, theliquid volume expands and that compresses the trapped air. All one needs to do is notethe amount of that compression, which is quite easy since the tube is a cylinder and rulermeasurements are not difficult to make.So much for the theory of this technology. Let us now turn to making it happen - the tricksof the trade, as it were.1. Make up a hypertonic solution. Table sugar is inexpensive and can form 40%solutions. However, because it is a small molecule, it will slowly drop inconcentration as osmosis continues. It would be better to use something likePEG (polyethylene glycol; aka "carbowax") having such a high molecular that itcannot escape the tubing.
  • 7. 2. How to entrap so much air in the tubing. After pouring in the desired amount ofhypertonic solution, blow a stream of air down into the tube, and then close thevery mouth of the tube. By spinning the tube around by the bottom end, a twist inthe tubing works its way down such that the trapped air fills out the "balloon." Donot twist so much as to start compressing the air. All you want is to have the airtake the wrinkles out of the tube. Now tie your knot and work that knot down tobottom of the twist, and pull the knot tight.3. The tube at this time might still be a bit limp. Using a few turns of a string aroundand around the tube well below the meniscus, tighten the string so the tube isjust barely rigid, and tie the string.4. It helps to keep the whole dialysis "sausage" submerged so that the upper partdoes not dry out and become brittle. There are two ways to keep the "sausage"totally submerged:a. tie two heavy lead fishing "sinkers" to each end of the sausageb. take a long container (perhaps a flower vase), fill it with water bysubmerging it horizontally in a bucket and then inserting the sausage intoit. Now lift the bottom of the "vase" and stand it up in the bucket. You willsee the trapped sausage bumping its "head" upon the bottom of thecompletely filled vase.Your system is ready to use, after you make two measurements:a. In millimeters, measure and record the distance between the top and bottom knots in thetubing. This measurement will later be used to adjust for the elasticity of the tubing as pressureincreases and the tubing stretches.b. In millimeters, measure and record the distance between the top knot and the meniscus.This measurement corresponds to the starting volume of the trapped air at one atmosphere ofpressure.Using this simple apparatus, there are now two approaches to determining osmoticpressure.i. One is simply to immerse the tube in water and follow the compression of the trapped air(above figure). The problem with this is that the concentration of the liquid inside the tubing iscontinuously being diluted with water as the run proceeds. Only when the compression ceasescan one then determine to concentration of solute that brought about that amount ofcompression. Often the solutes have no simple method for quantitation.ii. Another approach (right) is to compress the trapped air a known amount by adding morebelts around the bottom portion of the tubing (below the meniscus). When this pressurizedsystem is immersed in water, and the meniscus neither rises nor falls, then the osmotic pressureis known WITHOUT changing the concentration. But this will require a number of trials, as onemight try a series of set-ups each at a different pre-pressurization. Then one looks for the tubewith "no change."Calculation:Pressure = (To/Tx) x (Bo/Bx) x 14.2 psiAtm pressure x 14.2 = psi (pounds per square inch)
  • 8. Isotonic solutions are ones that contain a solvent, such as water,and a solute, such as table salt. The ions of salt have an ionic bond, sodium plus chloride.A saline solution is therefore considered isotonic.Normal saline solution (0.9% NaCl) is considered isotonic with blood (although itactually has a slightly higher degree of osmolality). Ringers lactate is also consideredisotonic.5% Dextrose solution is also considered hypotonic compared with blood, becausealthough it is isotonic while infusing, the dextrose is metabolized and free water is left,which is hypotonic.In the general sense, two solutions are isotonic when they contain the same amounts ofsolutes, or dissolved substances, and therefore have the same osmotic pressure. Ascommonly used in the medical field, though, isotonic solutions are solutions which have thesameconcentration of solute as the cells in the human body. A cell placed inan isotonic solution will neither gain nor lose water.When two aqueous solutions of different concentrations or tonicities are separated by asemi-permeable membrane such as a cell wall, water will migrate from the lessconcentrated, or hypotonic, side to the more concentrated, or hypertonic, side in an attemptto bring both sides into equilibrium. This process is known as osmosis. The greater thedifference in the two solutions concentrations, the higher the osmotic pressure will be, andthe quicker the osmotic transfer will be. It is the nature of osmosis that the identity of thesolute doesnt matter. Thus, salts, sugars, and other soluble compounds are all effective atregulating osmotic pressure. All may be used to prepare isotonicsolutions.Tonicity is of critical importance in biology and medicine because of its effect on living cells.Cells will only grow in isotonic solutions, and any drug administered intravenously must beadjusted to be effectivelyisotonic with human blood. A 0.9% solution of sodium chloride isconsidered isotonic with blood, although in fact its osmotic pressure is actually slightlyhigher.Hypertonic soln.A solution on which the concentration of solutes is greater than that of the cell that residedon the solutionIn healthcare you will often hear the words iso- and hypertonic solutions. A third option the"hypotonic" solution is also a possbility.As many people will know, the human blood contains both sodium, potassium, chloride salts.In healthcare a solution will be hypertonic when the amount of salts in it exceeds that ofhuman blood. It will be isotonic (iso meaning "same") when the amount of salts arecomparable to that of blood. Finally the solution will be considered hypotonic when theamount of salts present in the solution is less than that of blood.Hypertonic: More concentrated
  • 9. Isotonic: Just as concentratedHypotonic: Less as concentratedThe stiffness of plantstems, roots, and leaves is due to the presence of water in their cells. Plantsexhibit turgor when they stand erect and return to their original position afterbeing bent This rigidity in plants is the result of the firmness of each water-filledcell.In this project, you will determine the changes in turgor pressure in plants as aresult of increases and decreases of water concentration in a plants cells.Factors affecting the absorption of water into cells, such as variations in celltypes, temperature, and permeability of the cell membrane, will be determined.You will also study the effect of turgor pressure on plant movement
  • 10. Getting StartedPurpose: To demonstrate the effects of turgor pressure on an animal cellmembrane.Materials• Baby food jar• White vinegar• Raw egg in shell• Refrigerator• 1-cup (250-ml) measuring cup• Distilled waterCAUTION: Always wash your hands after touching an uncooked egg. It maycontain harmful bacteria.Procedure1. Fill the jar with vinegar.2. Stand the egg in the jar of vinegar with the small end of the egg below the surface of thevinegar (see Figure 7.1).3. Put the jar and the egg in the refrigerator to prevent the egg from spoiling.4. After 24 hours, remove the egg and discard the vinegar.5. Carefully place the egg into the measuring cup without cracking the eggshell.6. Fill the cup with distilled water.
  • 11. 7. Put the cup in the refrigerator.8. Observe the egg for seven days.ResultsThe membrane exposed by the vinegar swells and finally ruptures. Cracks in theshell starting at the edge of the exposed membrane form and extend across theegg. (See Figure 7.2.)Why?The egg is a single cell surrounded by a cell membrane. This membrane—theshell membrane—surrounds and controls the passage of materials into and out ofthe egg.Membranes that are selective in what passes through them are calledsemipermeable membranes. Pores in the membranes are large enough to allowthe easy passage of water molecules, but they are too small to allow largermolecules such as fats and proteins to get through. The movement of waterthrough a cell membrane is called osmosis and occurs when there is a differencein the concentration of water on either side of the membrane.The swollen shell membrane ruptures when placed into a hypotonic solution (asolution with a higher water concentration than that of the area to which it iscompared). The water in the cup (100% water) is hypotonic to the fluid contentof the egg. As more water moves into the egg through the membrane, the cellbecomes crowded with excess molecules, which results in a buildup of pressure.This pressure caused by excess water is called turgor pressure. As the fluidcontent of the egg continues to increase, the pressure of the expanding shellmembrane breaks the hard eggshell. The thin, unprotective shell membranestretches under the pressure, creating a bulge that ultimately ruptures.
  • 12. Try New Approaches1. How do the results change when the egg is placed into a hypertonic solution (a solution with alower water concentration than that of the area to which it is compared)? Repeat the experimentreplacing the distilled water with a salt solution made with 1 cup (250 ml) of water and 1tablespoon (15 ml) of table salt (sodium chloride). Science Fair Hint: Use a data table torecord written descriptions and diagrams of observations made of eggs placed into hypertonicand hypotonic solutions.2. If more of the membrane is exposed, does the egg continue to swell and rupture when placedinto a hypotonic solution? Repeat the original experiment removing the entire shell from theegg by covering the egg with white vinegar for 24 hours. Measure the circumference of the eggbefore placing it into the vinegar (mixture of acetic acid and water) and before placing it intothe water. After placing it in the water, measure it daily for seven days or until the egg breaks(if it does). Use these measurements to determine the change in size of the cell due to osmosisand whether the water continues to enter the cell at an even rate each day.
  • 13. Each of these examples use cells, but the concept applies to other things (like water balloons,etc.):