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Steroids—these are completely different from triglycerides and phospholipids. They involve four carbon rings that are linked together. Figure 4 shows what cholesterol, a common steroid lipid, looks like:
The hormones estrogen and testosterone are also steroids. What these have in common is that they are made with the same four carbon rings but have different side chains. Cholesterol, in particular, is part of the cell membrane of many animal cells.
PROTEINS Proteins are chains of amino acids, which involves a wide variety of different “nitrogenous” molecules, which contain nitrogen. There are peptides, which are short chains of amino acids, also referred to as oligopeptide chains. There are also polypeptides, which are also just called “proteins”, having very long chains. There are 22 amino acids on earth with only 20 actually encoded for by the genetic code. The remaining two, selenocysteine and pyrrolysine are incorporated into proteins by synthetic mechanisms. Some are considered essential to humans because they must be taken in through food and are not encoded by the human genetic code. There are
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