I call students to the gathering area and review the different parts of the cell. We recall the baggie cell models we made during our last lesson, that are now displayed in our science center.
I tell students that today, we will look at some of the functions of a cell. We do not have a science lab and a microscope per student, but we can still see some of a cell’s functions through macroscopic modeling. We review the terms, microscopic and macroscopic before we move on to the activity.
I tell students that we will complete two tasks today. I will split the class in half and one half will follow instructions for the first demonstration, while I work on a group demonstration with the second half of the class. The self-directed demonstration needs to be set up and then observed for a period of time before results are obtained. I tell students that they will have a science puzzle to work on, to build vocabulary.
I divide students into groups of 4. One person is the materials person, one is the scribe, one is the observer, one is the group reader. I have the materials laid out on the science table. The reader reads the instructions to the group. The materials person gathers the required materials from the science table. Each group needs a small cup of iodine (pre-measure 1 tsp into Dixie cups), cornstarch (pre-measure 1 tsp into Dixie cups), ½ cup of water, 1 Ziplock sandwich bag, a stopwatch, and a glass beaker.
The reader, re-reads the instructions as group members follow them. Once the experiment has been set up, the observer starts a two-minute timer and begins to describe what he/she sees, while the scribe writes it on the recording sheet. Every two minutes, the observer describes what is occurring, while the scribe records it on the recording sheet. This process is followed for 15-20 minutes. This should be sufficient time to see the diffusion process of the iodine permeating the plastic membrane.
The explanation for this experiment is:
Diffusion is the movement of molecules from crowded areas to areas where there is more space. Osmosis happens when you have diffusion. Osmosis is the movement of a substance through a membrane that allows some materials through and not others. This process moves from an area of high concentration to an area of low concentration through the membrane. Therefore, osmosis is a type of diffusion.
This demonstration needs to be prepared about 2-4 days in advance. If it’s being doing as a teacher demonstration, as recommended in this lesson, you will need four to six uncooked, fresh eggs.
Before beginning the experiment, measure the diameter of the eggs and the weight of the eggs. Submerge the eggs in vinegar. This should remove the egg shells in about 24 hours. If the shells are not completely removed in 24 hours, replace the vinegar and soak for an additional 24 hours. Once the shells are completely removed, measure the circumference and weight of three of the eggs. Place one in colorless corn syrup, one in water, and leave the third in vinegar. Within a day, the egg in corn syrup will lose water to the syrup. It will dramatically shrink in weight and circumference. The second egg will gain water from the water bath. It will dramatically increase in size and weight. The third and fourth egg, in vinegar, will remain fairly similar in size and weight. During this time, you should place these eggs in sealed containers to control odor. I usually take pictures morning and night to show the changes in the eggs.
On the day of the demonstration, I tell students that I have previously removed the shells from two eggs and I would like them to observe the results. I show students the two eggs in vinegar. I place the de-shelled eggs in a Ziplock baggie and allow students to hold the eggs in their flattened palm to feel the osmotic pressure of the egg through the thin membrane. I use baggies to prevent the spread of Salmonella poisoning that is a risk with raw eggs. It is also a great way to prevent mess if an egg ruptures.
I turn down the lights at this point, and point a flashlight through the egg so students can see the yolk and the thin membrane holding the egg together.
I ask students:
At this point I show students the egg in corn syrup and the egg in water. I ask them what they think the main chemical component inside the membrane might be? I ask them how we might test to see if movement of water is occurring through the membrane? I hope that students will point out the “water” went into one egg and out of the other. Thus, the membrane is permeable to some materials, but not to others.
An optional demonstration of how thin that membrane is, is to blow an egg empty, as one would for decorating it, and then soaking it in vinegar. The only thing left behind is the membrane. I love to show students this as it is so thin. However, cells are so much smaller and thus their membranes are so much thinner, yet they remain intact so well despite the battering we give them during everyday living.
This look inside an egg give students a simple model of what a cell might look like if they were as big as an egg. We do not have all the cell structures, but is great for showing a general model and modeling the process of material, and thus information exchange that occurs inside an animal’s body. This serves as great background for our information exchange unit later in the year.
I switch the class groups and repeat the demonstration, while the other half works on the independent investigation.
NOTE: have another 2 eggs in vinegar, as back-up in case they rupture during the first demonstration.
After both groups have completed both investigations, I call students back to the gathering area. We talk about how the water and iodine permeated the baggie plastic and turned the cornstarch solution a brown or purple color. I tell students that iodine is used as an indicator of starch. I demonstrate this by dropping a couple of drops onto a cut, raw potato. The potato instantly turns purple/brown to indicate the presence of starch in the potato. We discuss that the iodine solution moved into the bag, the starch solution did not move out of the bag. The iodine molecule is much smaller than the starch molecule and thus only the smaller molecule can move through the “intermolecular” holes in the plastic.
In the egg demonstration, we saw a model of a cell in the form of a “naked” egg. Students were able to observe the permeability of the egg membrane as a demonstration of how materials pass into and out of a cell or the membranes surrounding cell structure membranes, within the cell.
I tell students that information, inside a body, is passed through chemical and electrical messages. These demonstrations show how chemical information is transferred between cells to ensure efficiency and survival.
NOTE: There is more than osmosis occurring in the “naked” egg demonstration. However, this information in not necessary to share with fourth graders. Further investigation of cell structures occurs in middle and high school. This could be an area of research for advanced students who need more advanced content.