Some Final Comments on Metabolism

From lanosterol, the pathway for steroid biosynthesis continues on to yield cholesterol. Cholesterol then becomes a branch point, serving as the common precursor from which all other steroids are derived. 

Compare the structures of lanosterol and cholesterol, and catalog the changes needed for the transformation. 

In the last several chapters, we ve had a brief introduction to metabolism. There are, however, many more pathways we ve not mentioned and thousands of biological molecules whose biosynthetic schemes have been 

If you were to look at the steps of vitamin B12 biosynthesis, you would see exactly the same kinds of reactions we ve been seeing throughout the text— nucleophilic substitutions, eliminations, aldol reactions, nucleophilic acyl substitutions, and so forth. There are, of course, some complexities, but the fundamental mechanisms of organic chemistry remain the same, whether in the laboratory with smaller molecules or in organisms with larger molecnles. 

Phospholipids are important constituents of cell membranes and are of two kinds. Glycerophospholipids such as phosphatidylcholine and phospha-tidylethanolamine are closely related to fats in that they have a glycerol backbone esterified to two fatty acids (one saturated and one unsaturated) and to one phosphate ester. Sphingomyelins have the amino alcohol sphingosine for their backbone. 

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