Have you ever thought of a chicken egg as one big cell? Of course it is made up of many, many actual cells. But you can use it as a model to explore how different fluids get transferred from across cell membranes—and to other cells. This process is happening right now on a much smaller scale in your own body! But to see it with the naked eye we can try this macro-activity!
Every cell of our bodies is enclosed in a plasma membrane, a complex boundary made of lots of different layers. This separates the cell contents from the surrounding world outside the cell, which is often composed of fluid. These membranes play a large role in maintaining a proper balance of contents inside the cell (homeostasis) by controlling what passes through them. One of the most important characteristics of plasma membranes is their selective permeability, which means that they allow some substances to pass freely yet restrict others. See how an egg can do this on a scale you can observe!
- One egg
- A few cups or jars—large enough to hold an egg and enough liquid to submerge it
- White vinegar—enough to submerge an egg
- Large serving spoon
- Water—enough to submerge an egg
- Corn syrup—enough to submerge an egg
- Place the egg in a cup or jar and then pour in enough white vinegar to completely cover it. Observe the egg. Do you see tiny bubbles forming on the shell?
- Cover the egg and leave it for 24 hours.
- The next morning ladle the egg out of the vinegar with the big spoon. Is the shell different? How so?
- Dump the old vinegar and cover your now-squashy egg with fresh vinegar.
- Let the egg sit another 24 hours.
- Remove your egg and observe it again. Has it changed again?
- On the third morning spoon out the egg and rinse it in the sink under the tap. Be careful: because the eggshell has been dissolving, the egg membrane may be the only thing holding it together. The membrane is not as durable as the shell. How does the egg’s exterior look now?
- Put the naked egg in a cup of water and let it rest for eight hours. Does the de-shelled egg look the same? How has it changed?
- Carefully remove the egg and put it in a cup of corn syrup for another eight hours. Has the appearance of the egg changed again? How?
Observations and results
When you first place the egg in vinegar, carbon dioxide is produced as the acidic vinegar reacts with the calcium of the shell. As the egg sits in the vinegar, the shell is slowly dissolving away, producing a "de-shelled" egg. The eggshell should be totally gone by the third day, with a blob of egg white and yolk remaining inside a thin membrane.
The de-shelled eggs are good models of human cells. After the eggshell is gone, a white membrane remains. (It's actually two membranes, but they're held tightly together.) This membrane, like those of human cells, is selectively permeable, and the entire egg can be a model for a single cell.
Substances that can pass easily through the egg’s membrane will follow the principle of diffusion: They will move from the side of the membrane where they are at higher concentration to the side where they are at lower concentration. This exchange will continue until the concentrations on both sides of the membrane are equal.
Water can pass freely through a cell's membrane. (The movement of water across a membrane is a special version of diffusion called osmosis.) When an egg is soaked in a solution in which the concentration of water is higher than that inside the egg, water moves from the outside solution, across the membrane and into the egg. As a result, the egg plumps up and grows larger. This is what happened when the egg was in water.
When an egg is soaked in a solution where the concentration of water is lower than that inside the egg, however, water moves from the egg, across the membrane, and into the outside solution. As a result, the egg loses mass and may end up looking like a deflated balloon. This is what happened when the egg was in corn syrup.
The plasma membranes of our cells behave much like those of the eggs. Water, oxygen and nutrients must pass through the plasma membrane into our cells and waste must exit. Oxygen diffuses into red blood cells in our lungs and they transport it to our tissues, where the gas diffuses into other cells. Water in the stomach and intestines moves via osmosis into the bloodstream.
More to explore
The Science of Eggs, from Exploratorium
Take an Egg for a Spin, from Exploratorium
Fast Pickling, from Oakland Discovery Center
This activity brought to you in partnership with Exploratorium