1 of 5 11/1/2011 10:30 AM THE EGG-CITING EGG-SPERIMENT! Knight Foundation Summer Institute Arthurea Smith, Strawberry Mansion Middle School Liane D'Alessandro, Haverford College Introduction: Get ready to amaze your students and yourself with this experiment! Not only can you remove the shell of an egg without hard boiling it first but also you can make the egg shrink and swell!! Besides all the fun and excitement, though, this experiment demonstrates a very important process that occurs all the time in our bodies to keep us alive. Our bodies are made up of cells that are responsible for every job our body does. But for our cells to perform all these jobs properly, they need to bring in certain particles from outside the cell (just the way that an individual person usually needs help from other people to do a big project). One way for particles to enter the cell and help it do its job is to simply pass through the cell membrane. All living cells are surrounded by a membrane which separates the stuff inside the cell (water, nucleus, etc.) from the stuff outside the cell (water, other cells, nutrients, etc.) This membrane decides what can enter the cell and what should stay outside. One way that it prevents certain particles from entering is by having holes only large enough to allow small particles to enter. It is therefore called a semipermeable membrane. An analogy could be made to a basketball net. Only certain size balls will get through. That is, a tennis ball will go through the hoop, a basketball will go through the hoop, but a big beach ball will not. The same holds true with cell membranes. Many particles, such as sugars and salts that are dissolved in the water surrounding the cell try to enter the cell, but they are too big and therefore cannot pass through the membrane. To demonstrate this principle to the students, do the following activity as a warm-up (taken from The Human Body for Every Kid, p. 17): WARM-UP #1: 1/2 cup salt 1/2 cup dry beans jar with lid (quart or liter) colander large bowl Pour salt and dry beans into jar. Secure with lid and shake back and forth several times to thoroughly mix the contents. Hold colander over bowl as a helper student opens the jar and pours its contents into the colander. Gently shake the colander up and down several times over the bowl. Observe the contents of the bowl and colander. Clearly in this little demo, the colander acts as the cell membrane allowing only certain size particles to pass through to the bowl. Here, the salt is small enough to pass through, but the beans are not. In a cell, water, carbon dioxide, and oxygen are small enough to cross the membrane, but as mentioned above, larger particles cannot cross. Since the movement of water across a membrane is such an important process for cells to do
2 of 5 11/1/2011 10:30 AM their job properly, it has a special name - 0smosis. Another concept that must be understand before attempting to comprehend the main experiment is diffusion. Diffusion is the movement of particles from an area of high concentration to an area of low concentration until the concentration in both areas is uniform. A simple yet educational demonstration of this principle is described below: WARM-UP #2: ( 1 ) beaker or clear cup 1/2 cup of water 3-4 drops of food coloring paper and pencil to record observations Fill the beaker 1/2 way with the water. Add 3-4 drops of food coloring. Just let the jar sit. Do not swirl. Observe the beaker for a few minutes while recording what is happening to the color in the water. In this demonstration the food coloring diffuses throughout the water. When first dropped into the water, the part of the water that shows color has a high concentration of food coloring. Over time, the color begins to spread throughout the water to areas with no color (low concentration of food coloring). Eventually, the color is evenly spread throughout the entire beaker of water. The same theory explains how the smell of perfume diffuses throughout a room. EGG-SPERIMENT: At last to the main part of the experiment -- here an egg will be the model for a cell. First, the shell must be removed to expose the cell membrane. Vinegar takes care of this job very easily because it is an acid that reacts with the calcium in the shell to remove the shell. (Evidence of the reaction is provided by the bubbles that form when the egg is dropped in vinegar). Once the shell is removed, the concepts of diffusion and osmosis can be illustrated. The egg will be transferred from the beaker with vinegar to a beaker containing corn syrup or some other highly concentrated solution such as honey. In a few days, the egg will actually shrink because all the water inside the cell wants to move to an area of lower concentration (i.e. outside the cell). (When I did this experiment, the egg actually shrunk from 17.7 centimeters to 15 centimeters from Wednesday night to Saturday morning. But even more dramatic was how the egg felt before and after being placed in the corn syrup.) Once the egg shrinks, the egg can be put into a new beaker containing water. Here, the egg will swell up again because now there is a higher concentration of water outside the cell than inside. The new size is even bigger than the original size of the egg. Incidentally, the reason that only the water diffuses across the membrane while the sugar in the corn syrup does not is that the sugar particles are too big to cross the membrane. That is what the first warm-up experiment showed. Examples of the relevance of osmosis will be given in the "Extensions" section. Objectives: To understand one way that materials enter a cell To understand the function of the cell membrane To understand the principles of diffusion and osmosis
3 of 5 11/1/2011 10:30 AM To record careful and detailed observations of a scientific experiment Vocabulary: cell cell membrane semipermeable membrane concentration diffusion osmosis circumference for each group of students: (3) 250 ml beakers or big clear cups (1) egg (be careful handling the egg!) 250 ml vinegar 250 ml corn syrup or honey 250 ml water approximately 12" piece of string ruler paper and pencil to record observations 7. 8. 9. 10. Place egg into one of the beakers. (Be careful -- do not drop the egg in.) Fill beaker almost to the top with vinegar. Record observations immediately. What do you notice? Continue to record observations over the next three days. Be sure to note how the egg feels when you touch it each day. Don't squeeze too hard! It may seem disgusting to touch, but it actually feels quite amazing! Once the shell is removed (i.e. the egg is soft), gently rinse off the egg. Measure the circumference of the egg by wrapping the string gently around the middle of the egg. Hold your finger on the spot where the end of the string meets the rest of the string. Measure the distance between the end of the string and your finger with the ruler. Record measurement. Also record how the egg feels at this point. Transfer the egg to a new beaker. Fill this beaker almost to the top with the corn syrup or honey. Record the properties of the syrup (i.e. is it thick to touch? slow to pour? etc.). Each day for the next 3-4 days, record observations. First note if the syrup has changed any. Also. note how the egg feels. Compare these observations to those made in step 4 and step 6? Next, gently take the egg out of the beaker and rinse off. Remeasure the circumference as directed in Step Record measurement. Place the egg into a new beaker and fill this one almost to the top with water. In several hours (it's okay to wait until the next class period), come back and record observations again. How does the egg feel this time? How does it compare to the observation in step 7 and step 4? Remeasure the egg as directed above. Record measurement. How does this measurement compare to the measurements in step 4 and step 8?
4 of 5 11/1/2011 10:30 AM Assessments: Make sure the students can define the vocabulary given above. Also have them answer the following questions to turn in. They should try to work individually at first. Then they can discuss their observations and conclusions with their group or the whole class. Why was the shell removed first? How did the syrup change (see steps 6 and 7)? Explain why this change occurred? How did the egg change from the beginning of the experiment to the middle to the end. Explain why using the vocabulary above. What do you think would happen if the egg was placed in orange juice instead of corn syrup? Why? What do you think would happen if you repeated step 8 a couple of times except you used new corn syrup each time instead of the water? Why? What would happen to a plant if it was placed in a high concentration of salt water? Why? Extensions: Have the students design their own experiment by thinking of a different substance to put the egg in rather than the corn syrup or honey. They would have to develop their own hypothesis, record observations, compare the results of the new experiment with those of the original experiment, and draw conclusions. In Barbara Newman's and Stephanie Kramer's book Hands-On Life Science Activities for Middle Schools (p. 61), there is an interesting modification to this experiment using a potato instead of an egg and salt water instead of corn syrup. Another modification to the original experiment is to have the students weigh the egg during the observations in addition to measuring the circumference. Furthermore, in reference to question #6 of the "Assessments" section, the students could observe this situation by making use of the video microscope. Another interesting fact to share with the kids that relates to the concepts just learned is how a Paramecium lives. This organism lives in fresh water. Surely then, there is a higher concentration of water molecules outside the cell then inside the cell. Therefore, water diffuses across the membrane into the Paramecium. Ask the students what this means for the Paramecium. Well, clearly it means that the Paramecium would flood itself if it did not have some way to get rid of all the water rushing in. Luckily, it does have a mechanism to get rid of it. This excess water accumulates in a certain part of the Paramecium called the "vacuole" and ever so often it is expelled back into the environment. The idea of diffusion of materials across a membrane is essential to our respiratory system where carbon dioxide and oxygen diffuse across the membrane of a red blood cell and the membrane of air sacs in the lungs. Therefore, this experiment could lead to the study of the respiratory system. Philadelphia Science Content Standards: SCIENCE CONTENT STANDARD #1: NATURE OF SCIENCE This experiment satisfies Benchmark 1 for grades 5-8: "design, modify, and conduct an investigation through testing, revising, and occasionally discarding ideas. all of which lead to a better understanding of how things work." It also satisfies Benchmark 3 for grades 5-8: "collect and summarize data from an experiment and interpret the results in terms of the data."
5 of 5 11/1/2011 10:30 AM SCIENCE CONTENT STANDARD #3: LIVING ENVIRONMENT Benchmark 4 for grades 5-8 states that students should be able to "explain how cells carry on the many functions needed to sustain life". This experiment shows that one way cells are able to carry on the functions is through the movement of materials across a cell membrane (as in the respiratory system). Cross-references: This experiment is perfect for a unit on the cell -- cells of all organisms. It may also fit into a unit on water (how our bodies get water).