Small rope-like structures called chalazae are part of the egg whites and they anchor the yolk to the inner shell membrane to keep the yolk centered. In the context of this article we are most interested in the albumin layer where we find the proteins that make up the foam needed for our angel food cake. 56 ()
More on amino acids Egg white is made up of albumin, a clear alkaline solution of about 10% proteins and 90% water mixed with traces of minerals (including sodium, potassium, chorine, sulfur, iron, phosphorus, calcium and magnesium), fats, vitamin and glucose. The albumin makes up about two-thirds of the weight of the egg’s contents, not including the shell. A little more than 50% of the egg’s protein is contained in the albumin. There is no cholesterol in the albumin. So, let’s consider the basic chemistry of amino acids. As your students will likely remember from biology class, proteins are made up of amino acids, so we need to understand a little about amino acid chemistry. All amino acids have the basic structure at left. They are a combination on an amine functional group (green in the diagram) and a carboxyl functional group (yellow) with a side chain (red) attached to the central carbon atom. The composition of the side chain determines the exact nature of the amino acid. For example, if the side chain is just one hydrogen atom then the amino acid is glycine. If the side chain is a methyl group (–CH 3 ) then the amino acid is alanine. We will have more to say about the side chains later. Below is a chart of the proteins and their structures. 57 ()
( (nutrient)#mediaviewer/File:Amino_acids.png ) 58
Amino acids combine to form proteins. Proteins are, then, polymers made up of protein units. The diagram at right shows the most common amino acid bonding mechanism. When two amino acid molecules approach each other and the carboxyl end of one collides with the amino end of the other the carboxyl unit loses a hydrogen atom and an oxygen atom, and the amino unit loses a hydrogen atom. The resulting molecule is called a dipeptide in which the two original amino acids are now linked by a covalent peptide bond (-CO-NH-) and a water molecule is also produced. The reaction is a condensation reaction. The reaction below is another way of looking at the reaction described above. The atoms shown in blue form the water molecule and the atoms shown in red are involved in the peptide bond. ( n.wikipedia.org/wiki/Peptide_bond ) So we can think about many amino acids linking together via peptide bonds to form longer chains. Chains with molecular masses of less than 10,000 Da (Daltons) are not considered proteins but are usually referred to as oligopeptide chains. Longer protein chains may have thousands of amino acids, as noted below. In addition, the resulting protein molecules can vary greatly in size and mass. For example, keratin, a smaller protein found in human hair and nails, has a molecular mass in the 50,000-70,000 Dalton range. On the other hand, titin, the
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