Instructor Information
JCE Classroom Activity: #55
Diffusion of Water through a Differentially Permeable Membrane
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Maria Guadalupe Bertoluzzo,* Fabio E. Quattrin, Stella Maris Bertoluzzo, Ruben Rigatuso Facultad de Ciencias Bioquímicas y Farmacéuticas. Unr. Suipacha 531-(2000) Rosario, Argentina *
[email protected] photo by authors
Background To understand a biological system, it is necessary to know the fundamentals of the transport process through biological membranes. One transport mechanism is diffusion. This process is powered by random motion and progresses as dissolved species move from regions Copper(II) hexacyanoof high to low concentrations, leading to a situation where the species concentration is con- ferrate(II) membrane stant in time and space. A membrane is permeable to a substance when the substance can cross it by diffusion. If the membrane only allows certain substances to pass through, it is a differentially permeable membrane. If only the solvent and not the solute can cross the membrane, it is a semipermeable membrane. In the process of osmosis, solvent flows from regions of low to high solute concentrations through a semipermeable membrane.
Integrating the Activity into Your Curriculum This Activity allows students to watch and investigate osmosis. It reinforces the concept of transport in living cells. Instructors could contrast osmosis with diffusion, a principal method of movement of substances within biological cells. A demonstration model of osmosis (2) could be used to visually introduce the Activity. A demonstration similar to this Activity’s procedure has been published by Borgford and Summerlin (1).
perforated
About the Activity Copper(II) sulfate pentahydrate (CuSO4ⴢ5H2O) can be purchased from chemical supply companies and is available in hardware stores labeled for use as “root killer”. Potassium hexacyanoferrate(II) (K4Fe(CN)6) must be ordered from a chemical supply company. Copper(II) sulfate is toxic if swallowed. Potassium hexacyanoferrate(II) may be harmful if inhaled, and can cause irritation to the skin, eyes, and respiratory tract. Students should wear gloves and goggles when using these chemicals. Dispose of products according to local regulations. If your supply of potassium hexacyanoferrate(II) consists of very small pieces the size of grains of sand, you must make larger pieces prior to the Activity. For 1 or 2 larger pieces, dissolve 0.2–0.3 g of the solid (enough to cover the end of a spatula) in as little water as possible in a small container. Allow the water to evaporate overnight with the container tipped at an angle so the solution collects in one area. Scrape the resulting dried crystals off the bottom or side of the container. Students may need review to prepare the 5% (5 g solid/95 g water) and 20% (20 g solid/80 g water) solutions of copper(II) sulfate. In testing, 100 g of 5% yielded ~98 mL solution; 100 g of 20% yielded ~89 mL. Students can observe membrane growth in the 5% copper(II) sulfate solution without magnification. No appreciable membrane growth is seen in the 20% solution, allowing students to compare and contrast. Observations of onion cells under a microscope are an optional extension. Images of onion cells are in this issue of JCE Online.W
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
When a solution of potassium hexacyanoferrate(II), K4Fe(CN)6, comes in contact with a solution of copper(II) sulfate, CuSO4, a colloidal precipitate, copper(II) hexacyanoferrate(II), Cu 2Fe(CN) 6, is formed according to the reaction 2Cu 2⫹(aq) ⫹ [Fe(CN) 6] 4⫺(aq) → Cu2Fe(CN)6(s) (1). In this Activity, students investigate the process of osmosis through the differentially permeable membrane formed by this precipitate.
Answers to Questions 1. K4Fe(CN)6 begins to dissolve and react with the CuSO4 in solution. 2. You can see a reddish solid product form. 3. There is a flow of water to the inside of the membrane in the 5% solution, but not the 20% solution. The membrane in the 5% solution greatly increases in size, but not in the 20% solution. 4. Because of the flow of water through the membrane the volume inside increases, the fragile membrane breaks, and then a little solution escapes, contacts the CuSO4 solution, and more solid membrane forms. 5. In a 20% solution of CuSO4 a reddish membrane forms, but it grows very slowly.
References, Additional Related Activities, and Demonstrations 1. 2.
Borgford, C. L.; Summerlin, L. R. Chemical Activities Teacher Edition; American Chemical Society: Washington, DC, 1988; pp 69–70. Morse, Joseph G. A Simple Demonstration Model of Osmosis. J. Chem. Educ. 1999, 76, 64–65. JCE Classroom Activities are edited by Nancy S. Gettys and Erica K. Jacobsen
JChemEd.chem.wisc.edu • Vol. 80 No. 9 September 2003 • Journal of Chemical Education
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JCE Classroom Activity: #55
Student Activity
Diffusion of Water through a Differentially Permeable Membrane Red blood cells placed in a concentrated salt solution shrink and shrivel (see photograph), but cells placed in distilled water swell and burst. The process involved is osmosis, in which a solvent moves through a semipermeable membrane to equalize the solution concentrations on either side of the membrane. This is a very important process in living systems. Solutions are isotonic when the concentrations on both sides are equal, hypertonic if the solution outside the membrane is more concentrated, and hypotonic if the solution inside the membrane is more concentrated. You can model the osmotic process with a nonliving system by creating a membrane from a colloidal solid, copper(II) hexacyanoferrate(II), Cu2Fe(CN)6. The membrane is formed by placing potassium hexacyanoferrate(II) crystals, K4Fe(CN)6, into an aqueous solution of copper(II) sulfate, CuSO4.
Try This
Be Safe! Copper(II) sulfate is toxic if swallowed. Potassium hexacyanoferrate(II) may cause irritation to skin, eyes, and respiratory tract. Use goggles and gloves. Avoid contact with eyes, skin, and clothing. Wash thoroughly after handling. Avoid breathing dust from these chemicals, keep the containers closed when not in use, and use with adequate ventilation.
photo by authors
You will need: copper(II) sulfate pentahydrate; two small, clear, colorless containers; balance; spatula; stirring rod; distilled water; potassium hexacyanoferrate(II); forceps; clock; and marker. Use distilled water for all solutions. __1. Label two clean, clear, colorless containers “5%” and “20%”. In the “5%” container, prepare 100 g of 5% by mass solution of copper(II) sulfate pentahydrate. In the “20%” container, prepare 100 g of a 20% by mass solution of copper(II) sulfate pentahydrate. __2. Place both containers on a solid, level surface against a dark background. __3. Using forceps, take a small piece (4–5 mm long) of potassium hexacyanoferrate(II), and drop it into the 5% solution. Drop a second piece (4–5 mm long) of potassium hexacyanoferrate(II) into the 20% solution. __4. Observe and record what happens in the two containers over a period of 15 minutes. Are your observations the same for both containers?
More Things To Try
Red blood cells in an isotonic solution. photo by authors
You will need: red onion; knife; microscope slide and cover slip; light microscope; distilled water; dropper; and a saturated solution of sodium chloride (NaCl, table salt). __1. Cut a 1 cm square from a red onion. Carefully peel off the thin purple film. __2. Spread the film on a microscope slide and add a cover slip. __3. With a dropper, add a drop of distilled water to the side of the cover slip. __4. View the slide under a light microscope. Describe and explain what occurs. __5. Repeat steps 1–4 using saturated NaCl solution instead of distilled water. How do these observations compare to your observations of potassium hexacyanoferrate(II) in copper(II) sulfate solutions?
Questions __1. What happens to the potassium hexacyanoferrate(II) solid when it is placed in the solution? __2. How can you tell that a new compound [copper(II) hexacyanoferrate(II)] forms when potassium hexacyanoferrate(II) and copper(II) sulfate react? __3. From your observations, can you say that there is a flow of water through the membrane in the 5% solution? The 20% solution? If so, in which direction? Explain. __4. Develop a hypothesis to explain how the solid membrane grows. __5. What differences could you see when you compared the 20% solution of CuSO4 with the 5% solution?
Red blood cells in a hypertonic solution of NaCl.
Information from the World Wide Web (accessed July 2003) 1. 2. 3. 4. 5.
Transport In and Out of Cells. http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html Osmosis. http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm Osmosis. http://www.purchon.com/biology/osmosis.htm Osmosis. http://physioweb.med.uvm.edu/bodyfluids/osmosis.htm Membrane Transport. http://physioweb.med.uvm.edu/bodyfluids/membrane.htm
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 80 No. 9 September 2003 • JChemEd.chem.wisc.edu
