How Does a Chick Breathe Inside Its Shell?

Abstract

Objective

Determine if the pores in a chicken egg shell allow water to enter by weighing eggs before and after hard boiling.

Credits

Haleh Khoshnevisan, MedImmune

Sandra Slutz, PhD, Science Buddies

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Introduction

Every animal requires oxygen to live. When animals, including humans, breathe in, oxygen enters the lungs, where it is shuttled into the blood stream and distributed to all the different parts of the body. The oxygen is used in an internal chemical reaction called metabolism to provide the animal with energy. The process of metabolism also produces a waste gas called carbon dioxide. In order to get rid of this waste gas, the blood stream carries the carbon dioxide back to the lungs where it is collected and finally breathed out.

Animals that grow inside their mothers, like humans, get their oxygen from their mothers. The blood stream of the baby animal and the mother are connected through an umbilical cord, which allows the baby to collect oxygen that his or her mother breathes in and use the mother's lungs to get rid of the carbon dioxide. But how do animals that grow in a shell and do not have umbilical cords, like chickens, take in oxygen and get rid of carbon dioxide?

Bird and reptile eggs have a hard shell. Directly under the shell are two membranes. When the eggs are laid by the mother, they are warmer than the air, and as they cool, the material inside the egg shrinks a little bit. This shrinking pulls the two membranes apart, leaving behind an air cell, also called an air sack, that is filled with oxygen. As the animal develops, it needs the oxygen replenished so it can continue to grow, and it needs the carbon dioxide it's making to be able to escape from the air cell. So, how does this happen? Well, if you examine a chicken egg carefully with a magnifying glass, you'll see that there are tiny little holes, called pores, in the shell. A chicken egg shell has more than 7,000 pores! Do you think that the pores could be a way for carbon dioxide to escape and fresh air to get in? In this science project you'll determine if substances can move in and out of an egg through these pores. To do this you'll weigh several raw chicken eggs. Then you'll cook them in water and weigh them again. If the pores really do allow materials to cross back and forth between the inside of the egg and the outside environment, then the air inside the egg should be replaced by water, and water is heavier than air. Ready to find out the answer? Let's get cooking!

Figure 1. This diagram shows different parts of the egg. Notice the air cell, which forms between the inner and outer membranes.

Terms and Concepts

• Oxygen
• Metabolism
• Carbon dioxide
• Umbilical cord
• Shell
• Air cell, also called an air sack
• Pore

Questions

• Why do animals need oxygen?
• Why do animals need to get rid of the carbon dioxide that their bodies make?
• What are the different parts of a chicken egg?

Bibliography

These websites have more information about breathing, oxygen, carbon dioxide, and metabolism.

• The Franklin Institute Science Museum. (2008). Respiratory System: Oxygen Delivery System. Retrieved May 22, 2008 from http://www.fi.edu/learn/heart/systems/respiration.html
• Inselman, L. (2006, August). Your Lungs & Respiratory System. Retrieved May 22, 2008 from http://www.kidshealth.org/kid/htbw/lungs.html
• Ophardt, C. (2003). Carbon Cycle. Retrieved May 22, 2008 from http://www.elmhurst.edu/~chm/vchembook/306carbon.html

• North Carolina Egg Association. (n.d.) Parts of an Egg. Retrieved May 22, 2008 from http://www.ncegg.org/EggInformation/TeachYourself/PartsofanEgg/tabid/134/Default.aspx
• 4-H Virtual Farm. (n.d.) The Parts of the Egg. Retrieved May 22, 2008 from http://www.ext.vt.edu/resources/4h/virtualfarm/poultry/poultry_eggparts.html

• National Center for Education Statistics. (n.d.). Create a Graph. Retrieved May 22, 2008 from http://nces.ed.gov/nceskids/CreateAGraph/default.aspx

Materials and Equipment

• Wax crayon
• Scale; you need one that will be able to distinguish changes as small as 0.1 grams (g). Your school might have a triple-beam balance that will measure such a small change; alternatively, some electronic kitchen scales have this kind of accuracy.
• Eggs (5)
• Large pot with lid
• Water
• Stove
• Timer
• Tongs or large spoon
• Strainer
• Dish towel or paper towel
• Lab notebook
• Graph paper

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Experimental Procedure

1. With the wax crayon, gently number the raw eggs 1 through 5 so that you can keep track of which egg is which throughout the experiment.
2. Using the scale, weigh each egg, one at a time. Record your data in a data table, like the one below, in your lab notebook.
   1. Make sure the scale reads "0" before you put each egg on the scale. If you don't do this each time, your measurements might not be accurate.
3. Cook your eggs until they are "hard boiled." There are many ways to cook a hard boiled egg. If you have a method you like, use that with an adult's help. Otherwise, have an adult help you try this one:
   1. Place the eggs in a large pot.
   2. Fill the pot with enough water to cover all of the eggs.
   3. Using the stove, bring the water and eggs to a boil.
   4. Put the lid on the pot and allow it to boil hard for 1 minute.
   5. Turn off the stove. Leave the water and eggs in the covered pot for 25 minutes.
   6. After 25 minutes, the eggs will be cooked through (hard-boiled).
   7. Using a large spoon or tongs, carefully take each egg out of the pot, one at a time, and place it in a strainer. Then you can rinse them with cold water so that they are cool enough to handle. If you use tongs, don't squeeze too hard—you don't want to crack the shells or you can't count those eggs in your experiment!
   8. Whichever method you use to cook your eggs, watch them for a few minutes as they begin to cook. For example, in the method described above you'd watch the eggs as the water and eggs heat up to a boil. Do you see anything happening? If so, record your observations in your lab notebook.
4. Gently dry off the hard-boiled eggs using a dish towel or a paper towel.
5. Re-weigh each egg. Don't forget to set your scale to "0" after you weigh each egg. Record the data in your data table.
   1. Note: if any of the eggs cracked during the cooking process, they cannot be used. Don't use either the raw or the boiled egg data in any of your data analysis.
6. Did the weight of the egg increase, decrease, or not change at all? If there was a change, calculate the difference by subtracting the raw weight from the hard-boiled weight.

   	Raw Weight (g)	Hard-Boiled Weight (g)	Was there a change in weight? (increase/decrease/no change)	Change in Weight (g)	Observations During Heating
   Egg #1
   Egg #2
   Egg #3
   Egg #4
   Egg #5

7. If you observed changes in the weight of the eggs after cooking, make a bar graph showing your data. Your graph should have a bar for each egg, and the bar should show the change in weight between the hard-boiled egg and the raw egg.
   1. You can make the graph by hand or use a website like Create a Graph to make the graph on the computer and print it.
8. Look at your graph. Do you see a consistent trend? Does hard-boiling the egg cause either an increase or a decrease in the weight of the egg? If so, can you explain why? How does what you observed while the egg was cooking help explain your data? Based on your data, do you think it is likely that oxygen and carbon dioxide can travel through the egg shell?

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Variations

• Do fresh eggs and aged eggs behave similarly? Buy a dozen eggs whose expiration date is at least two weeks away. Try this experiment with half of the eggs right away. Let the other six eggs age in the refrigerator for two weeks. Repeat the experiment with the aged eggs. How does the data compare between the fresh and the aged eggs?
• Do other bird eggs give you similar results? Try it and find out! You can often find duck and quail eggs at Asian grocery stores.
• For more interesting egg-based science fair projects, try Egg Substitutes.

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