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OSMOSIS: THE EGG-CELL-ENT LAB EXPERIMENT
Things to Do to a Naked Egg
Principles Investigated
Living cells are enclosed by a semipermeable membrane that controls the flow of liquids and of dissolved solids and gases into and out of the cell. The membrane forms a selective barrier between the cell and its environment, allowing some substances to pass through the membrane while blocking the rest. Without this selectivity, substances critical to the life of the cell would diffuse out of the cell and toxic materials would enter the cell.
Osmosis is the movement of a liquid solvent through a membrane separating two solutions of different concentrations. The molecules move from the area of higher concentration to the area of lower concentration of the solvent. Dissolved solids (solutes) are kept from passing.
Standards – 10th Grade Biology
1a. Students know cells are enclosed within semipermeable membranes that regulate their interaction with their surrounding.
Connections-Geometry
9.0 & 11.0 While discussing the plasma membrane (1a), students could compute the volumes and surface area (geometry 9.0) of the eggs and analyze the effects of surface area to volume ratio on cell size (geometry 11.0).
Expected Learning Outcomes
After viewing a short video and conducting a series of lab experiments students should be able to:
Define osmosis and explain why it is biologically important
Explain the difference between isotonic, hypotonic and hypertonic conditions
Predict what will happen to a cell in isotonic, hypotonic and hypertonic conditions
Need To Know: Demo
Students are shown a series of "naked eggs." Some eggs are colored, others glow in the dark, they bounce. When the colored eggs are cut, some show a thick coat of color, while in others the coat color is thin.
What is going on?
Need To Know: Applications to Everyday Life
Students are told a story about the “Hold Your Wee for a Wii” contest held in 2007 by a California radio station (107.9). One of the contestants died of water intoxication.
What happened?
The Lesson
Background: What students need to understand
The Basics of Osmosis
The Experiment
Purpose: To investigate the process of osmosis using a living system.
Procedures:
Materials: Three raw eggs, 250cc white cider vinegar, 125cc distilled water, 125cc Karo high fructose corn syrup, three clear plastic cups, one 500mL graduated cylinder, one 12-inch ruler, a piece of string, one pair of disposable laboratory gloves, four paper towels, SaranTM wrap, one Sharpie, and a laboratory scale to weigh the eggs.
1. Use a pencil to label each egg as follows: Egg 1, Egg 2, and Egg 3.
2. Measure the mass of each egg (in grams) and record the data in the table provided.
3. Measure the circumference of each egg (in centimeters, widest point) and record the data in the table provided.
4. Place each egg in labeled plastic cups. Cover Egg 1 and Egg 2 with 125cc white vinegar. Do not pour any vinegar over Egg 3. Cover the cups with SaranTM wrap and store the cups in the lab refrigerator for 48 h. Notice the bubbles of carbon dioxide forming on the outside of the eggshells of Egg 1 and Egg 2.
5. After 48 h, carefully remove each egg from its cup and rinse with fresh water. Pat the eggs dry and measure their masses and circumferences as described in Steps 2 and 3.
6. DO NOT BOUNCE THE EGGS
7. Rinse the cups with tap water and place each egg back in its own rinsed cup. Cover Egg 1 with 125cc distilled water and Egg 2 with 125cc Karo corn syrup. Do not pour anything over Egg 3. Cover the cups with SaranTM wrap and store the cups in the lab refrigerator for another 48 h.
8. After 48 h, remove the three eggs from their cups, carefully rinse Egg 1 and Egg 2 with water, pat them dry, and measure their masses and circumferences.
9. DO NOT BOUNCE THE EGGS
10. Compare the mass and circumference of bloated Egg 1 (from the distilled water) with the original mass of the egg in its shell (Steps 2 and 3). Compare the mass and circumference of shriveled Egg 2 (from the corn syrup) with the original mass of the egg in its shell.
Discuss with students which way water moved – from inside the egg to the outside or from outside the egg to the inside – and why this occurred.
11. YOU MAY NOW BOUNCE THE EGGS
12. Make sure to have a few back-up eggs in vinegar just in case one of the “naked” eggs gets punctured.
Results
Hypotonic, Control and Hypertonic Eggs
Student prior knowledge
Students have learned that cells are the basic unit of life. They know the names and functions of major plant and animal cell organelles.
Students know that all cells are bound by a semipermeable membrane and that this membrane protects the contents of the cell from the exterior environment. Students know that animal and plant cells get their nutrients, oxygen and water from outside the cell and that cells must transport waste from inside the cell to the outside.
Students know that diffusion is the net movement of molecules down a concentration gradient and that this process allows small molecules like oxygen and carbon dioxide to cross the plasma membrane. Students also know that polar molecules, such as sugars and proteins, cannot cross the plasma membrane freely.
Explanation
Vocabulary:
Dissolving the shell: Eggshells are made almost entirely of calcium carbonate (CaCO3) crystals. Vinegar is dilute acetic acid (CH3COOH). When the eggs are soaked in vinegar, the acid in the vinegar dissolves the egg shell. The equation for this reaction is
CaCO3(s) + 2CH3COOH(aq) ---> Ca++(aq) + 2CH3COO-(aq) + H2CO3(aq)
The bubbles that form on the eggshell when exposed vinegar are carbon dioxide (CO2) bubbles that occur when the carbonic acid (H2CO3) molecules break up:
H2CO3 ---> H2O + CO2
Osmosis: As the eggshells dissolves and the inner membrane is exposed osmosis begins to occur. Water diffuses through the egg membrane down a concentration gradient. The concentration of water molecules outside the egg sitting in the vinegar is greater than the concentration of water inside the egg. Thus, water diffuses through the membrane into the egg and the egg begins to bloat.
When the “naked” egg is placed in distilled water, an even greater difference in the concentration of water between the two systems is created. Thus, the egg left to soak in distilled water will get even bigger as water continues to diffuse through its membrane into the egg.
When the other “naked” egg is placed in corn syrup, the egg shrinks in size and begins to shrivel. The concentration of water molecules outside the egg sitting in corn syrup is now lower than the concentration of water inside the egg. Thus, water diffuses out of the egg through the egg’s membrane and into the syrup, down a concentration gradient , and the egg begins to shrivel.
Hypertonic vs. hypotonic: This experiment clearly illustrates the concepts of hypertonic and hypotonic solutions. The bloated egg sitting in distilled water shows that the solution inside the egg is hypertonic to the solution outside the egg. The shriveled egg sitting in corn syrup shows that the solution inside the egg is hypotonic to the solution outside the egg. (hyper = over; above; hypo = under; below; ton = tone; tension; something stretched)
Questions & Answers (for 10th graders)
1. Refer to the egg demonstrations in Need to Know: Demo. When the colored eggs were cut, some showed a thick coat of color, while in others the coat color was thin. What was going on?
The indicator dyes had different molecular weights. The yellow and red dyes had low molecular weights while the blue and green dyes had high molecular weights. Thus, the yellow and red dye traveled deeper into the egg than the blue and green dyes. Note that this is an example of dialysis, not osmosis, because it was the solutes and not the solvent that moved through the membrane into the egg.
Note that no water molecules moved into the hard-boiled egg. The cell membrane was not permeable to water and osmosis could not occur.
2. Refer to the “Hold Your Wee for a Wii” contest in Need To Know: Applications to Everyday Life. What happened?
Water intoxication — or hyponatremia — is known among athletes, usually extreme athletes, but it can happen to anyone who consumes too much water. When more water is consumed than the body can regulate, blood plasma rises and dilutes the sodium content in the serum. Sodium cannot cross freely across the membrane but water can, so water moves into the cells.
The contestant in the “Hold Your Wee for a Wii “ probable suffered from acute hyponatremia. In acute hyponatremia, sodium levels drop rapidly, resulting in potentially dangerous effects, such as rapid brain swelling, which can result in coma and death. The skull prevents the brain from expanding, the brain stem is compressed, respiratory arrest occurs, and the person dies.
3. Differentiate among isotonic, hypotonic, and hypertonic solutions in plant and animal cells.
Isotonic solution. Both animal and plant cells will be in dynamic equilibrium with the isotonic solution and there will be no net water gain.
Hypotonic solution. Both animal and plant cells will gain water by osmosis and swell. Animal cells that gain too much water may burst. Because plant cells have walls, they will gain in turgor pressure and will not burst.
Hypertonic solution. Both animal and plant cells will lose water.
Applications to Everyday Life
Medical Care
Blood. Red blood cells are stored in a slightly hypertonic plasma solution relative to the cells to prevent them from drawing in water and bursting. A similar solution is used to suspend the active ingredient of drugs injected intravenously into a patient.
Dialysis. Osmosis is also linked to dialysis. During dialysis an artificial kidney machine removes waste products from a patients' blood. Selectively permeable dialyzing membranes allow water, salts, and other waste dissolved in the blood, to pass through into a tank of distilled water. However, the red blood cells are too large to enter the dialyzing membrane so they return to the patient's body. Note that osmosis refers only to the transfer of solvents whereas dialysis deals with the transfer of solutes. The figure in the left shows human blood cells in various solutions and the figure on the right shows patients undergoing dialysis.
Preservation of Food
Osmosis is also used for the preservation of fruits and meats, although the process differs for the two.
Fruit are 75% water and need a combination of techniques to preserve them: they are first blanched to stop enzymatic action, then exposed to a hypertonic sugar solution for dehydration, then air dried to remove the excess water.
Meat is cured in a hypertonic brine solution that draws salt into it. The meat retains water in the cells but blood cells are drawn out. Both methods prevent the intrusion of bacteria.
Jamon Serrano
Salted Chinese duck eggs exemplify the effect of osmosis. In brining, the whites remain fluid while the yolks harden into a deep orange sphere while still raw.
Chinese salted duck eggs
Water Purification
Osmosis, or rather, reverse osmosis (RO) is one of the most economical and efficient method for purifying water.
Reverse osmosis removes impurities from water, including total dissolved solids (TDS), turbidity, asbestos, lead and other toxic heavy metals, radium, and many dissolved organics. RO also removes chlorinated pesticides and most heavier-weight VOCs. RO can even be used to filter radioactive plutonium or strontium from drinking water.
Portable Reverse Osmosis Water Filtration
References
1. Science Clarified. http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-2/Osmosis-Real-life-application.html#ixzz1KGnJHP41
2. Laschish, Uri. "Osmosis, Reverse Osmosis, and Osmotic Pressure: What They Are" (Web site). <http://members.tripod.com/~urila/>(February 20, 2001).
3. "Salt-Curing Meat in Brine." Walton Feed (Web site). <http://waltonfeed.com/old/brine.html> (February 20, 2001).
4. “Salt and the Ultraendurance Athlete.” (Web site) http://www.rice.edu/~jenky/sports/salt.html
5. Herr, N. & Cunningham, J. (1999). Hands-On Chemistry Activities with Real-life Applications: Easy- to-use Labs and Demonstrations for Grades 8-12. San Francisco: Jossey-Bass.
You Tube Videos on Osmosis
Details of the Egg Experiment
Hypotonic to Hypertonic and Vice-versa
Absorption of Dye from Water
Everything You Ever Need To Know About Diffusion & Osmosis
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