Figure 10.—Absorption spectra of soil extract (NaOH). band of light and pass it through a substance and find that 100 percent of the light has passed through, we say that the ultraviolet was not absorbed. This means that the substance does not have vibrations of the same frequency as the ultraviolet. Now, if we pass a band of ultraviolet of another frequency and we find that only 10 percent of the light passes through the substance, then we know that the light was absorbed and that the substance had a vibration of the same frequency as that particular band of ultraviolet.
Proteins are large, complex molecules consisting of amino acids linked together in long chains that are folded into characteristic shapes. The amino acids are held together by peptide bonds, and their combination joined in such a fashion is called a peptide. A combination of two amino acid molecules is a dipeptide; three are a tripeptide; many are a polypeptide. A protein is a large polypeptide, or a combination of several polypeptides. When a protein is hydrolyzed, it is split into peptides, and these are then split into amino acids. (This is what happens in the stomach and intestine when proteins are digested.)
