THE EGG-CITING EGG-SPERIMENT!
Knight Foundation Summer Institute
Arthurea Smith, Strawberry Mansion Middle School
Liane D'Alessandro, Haverford College
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):
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 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:
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.
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 16.5 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.
for each group of students:
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.
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:
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."
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).
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).