|MadSci Network: Cell Biology|
The cell of a chicken egg is made up of about 94 percent calcium carbonate (Ca(CO3)2), 1 percent magnesium carbonate, 1 percent calcium phosphate, and about 4 percent organic matter, mainly protein. The egg shell has pores through which air, moisture, and even bacteria can pass. The shell represents about 11 percent of the weight of an egg. The shell is made up of three parts -- the mammillary or inner layer is adjacent to the shell membranes and has a "knob" like appearance. The middle layer, the bulk of the shell is made up of small calcite crystals, mostly randomly arranged with the pores running through it. The cuticle or top layer is a thin film of protein that covers the egg when it is layed, but which drys up and flakes off over time. The organization of the shell (i.e., the crystals of calcite) provides a rigid structure that protects against breakage in certain directions. On the other hand a small tap in some directions will crack the shell. The information about the shell was taken from the USDA's Egg Grading Manual.
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