15 



X-ray Analyses of Proteins 

 and Nucleic Acids 



I. Protoplasm 



Living matter is often referred to as protoplasm. This is a loose definition; 

 it would be more exact to speak of the living cell and its contents, but 

 the word protoplasm is shorter and easier. Protoplasm contains many 

 types of structures and a wide variety of types of molecules. In this 

 chapter, two types or families of molecules, both found in protoplasm, 

 are characterized in terms of their physical shape and spatial arrange- 

 ment. One of these families, the proteins, occurs abundantly in all living 

 matter and forms an important part of all animal diets. The other, 

 the nucleic acids, is found only in relatively small quantities but is very 

 important for all life. Both families of compounds are high polymers, 

 that is, molecules composed of many small units called monomers. Pro- 

 teins and nucleic acids are large molecules with molecular weights as 

 high as 10 7 . These two families of molecules differ chemically in that 

 they are polymerized from different types of monomers. 



Some idea of the protoplasmic roles of proteins and nucleic acids may 

 be obtained by considering a typical cell. A composite cell is shown in 



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