This membrane, like those in human cells, is selectively permeable, allowing certain substances to pass through while blocking others. Substances that can pass easily through the membrane of the egg will follow the principles of diffusion. They will move through the membrane from the side where they are at a higher concentration to the side where they are at a lower concentration click to enlarge the diagram below.
This movement will continue until the concentration on both sides is the same. While random molecular motion will cause individual molecules to continue moving back and forth across the membrane, the overall concentration on each side will remain in equilibrium, with equal concentrations on both sides. Movement of a solvent such as water across a semipermeable membrane from a less concentrated solution to a more concentrated one is called osmosis.
When an egg is soaked in a solution that has a higher solute concentration the relative amount of dissolved stuff than the solute concentration inside the egg, water moves out of the egg and into the solution see diagram below. As a result, the egg loses mass and ends up looking deflated. An egg naturally has a lot of stuff inside, so the outside solution has to be very concentrated for this to happen.
By contrast, when an egg is treated with distilled water, or a dilute salt solution, the solute concentration is higher inside the egg than out, so the water moves into the egg, increasing its mass.
It may be easier to think about osmosis in terms of water concentration rather than solute concentration. If the solute concentration is high, then the water concentration will be low by comparison. This should cause water to move from the egg into the solution, and the egg should lose mass.
In addition, the egg may appear white and rubbery. Alcohol that diffuses into the egg can denature the proteins, unraveling their three-dimensional structure and causing them to coagulate or join together.
Egg proteins turn from translucent to white when they are denatured. In cooking, temperature is used to denature these proteins, but you may have noticed that alcohol has also "cooked" the egg and caused it to look hard-boiled.
The plasma membranes of your cells behave much like those of the egg. All of the trillions of cells in your body are like busy seaports with materials coming in and going out. Water, oxygen, and nutrients must pass through the plasma membrane into your cells, and wastes must leave. When the concentration of oxygen is higher in your lungs than it is in your blood, for example, the oxygen diffuses into red blood cells through capillary walls.
Your flowing blood then transports that oxygen to your tissues. From there, the oxygen diffuses into other cells to be used in cellular respiration.
Through a similar process, water in the stomach moves into the bloodstream and is then carried to the cells, where it supports a variety of essential bodily functions. Predict what would happen if you placed the shrunken eggs in plain water overnight. Do the experiment and explain your results.
In this activity, not only can you measure how much material moved into or out of a treated egg, but you can also chemically determine whether molecules moved across the membrane. When using this activity with large groups of students or multiple classes, have each group apply only one treatment, and then analyze the data collected from all groups. Having each small group design an experiment with one egg will allow you to do the activity with fewer eggs per class, and collecting several sets of data will enable students to identify any outliers.
This Snack is an excellent activity for introducing diffusion, osmosis, and the semipermeability of membranes and allows learners to engage in the NGSS Science and Engineering Practices. The process is really very simple. Carefully place the egg in a cup and fill the cup with vinegar so that the egg is completely covered. Just get enough vinegar in the cup to mostly cover the egg.
Now the hard part — you will have to wait as the acetic acid in the vinegar begins to react with the calcium in the egg shell. In just a short while, you should see some bubbles appearing on the outside of the egg. These are bubbles of carbon dioxide gas from the reaction. It can take hours before a good portion of the shell is removed. A good sign of progress is a white frothy scummy layer on the top of the surface of the vinegar.
After a day of soaking you can carefully remove the egg from the vinegar. I would suggest pouring the liquid into another cup and catching the egg in your hand. At this point you may be able to literally rub the shell off the egg with your fingers. It will rub off as a white powdery substance. Depending on your particular egg, you may already have a naked egg. However, I would suggest you fill a cup with fresh vinegar and soak the egg for at least one more day.
After two days of soaking you should have a pretty cool Naked Egg. Notice that the egg is a bit bigger than when you first started. This is because some of the vinegar and some of the water in the vinegar has moved through the membranes to the inside of the egg. The membranes are semi-permeable and allow water to move through them. This is called osmosis. Naked eggs are cool, but experimenting with them is even cooler.
You have probably already noticed that your naked egg is kind of rubbery. How far above the table can you drop your egg and have it survive by bouncing? I would suggest you start at one inch, then try two inches, and so on. Keep in mind that eventually this is going to get messy when it the membrane breaks. You might want to do this experiment outside.
If you want to see your egg get really big, simply put it in a cup filled with water. This process of water moving through a membrane is called osmosis. Osmosis equalizes — or makes the concentration of water on both sides of the egg membrane the same. This means the egg will swell as the water moves inside and get larger.
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