PROTOPLASM 29 



but are first digested to the constituent amino acids to enter the cell. 

 Subsequently each cell combines the amino acids into the proteins which 

 are characteristic of that cell. Thus, a man eats beef proteins in a steak, 

 but breaks them down to amino acids in the process of digestion, then 

 rebuilds them as human proteins. 



Proteins and amino acids may serve as energy sources in addition to 

 their structural and enzymatic roles. Most animals eat more proteins than 

 are needed for the maintenance of protoplasm. The extra amino acids 

 undergo the process of deamination in which the amino group is re- 

 moved, then the remaining carbon skeleton enters the same metabolic 

 paths as glucose and fatty acids and eventually is converted to carbon 

 dioxide and water by the Krebs tricarboxylic acid cycle (p. 72) and 

 associated paths. The amino group is excreted as ammonia, urea, uric 

 acid or some other nitrogenous compound, depending on the kind of 

 animal. In prolonged fasting, after the supply of carbohydrates and fats 

 has been exhausted, the proteins of protoplasm itself are used as a source 

 of energy. Animal cells can synthesize some, but not all, of the different 

 kinds of amino acids; different species differ in their synthetic abilities. 

 Man, for example, is apparently unable to synthesize eight of these; they 

 must either be supplied in the food eaten or perhaps synthesized by the 

 bacteria present in the intestine. Plant cells apparently can synthesize 

 all of the amino acids. The ones which an animal cannot synthesize, but 

 must obtain in its diet, are called essential amino acids. It must be kept 

 in mind that these are no more essential tor protein synthesis than any 

 other amino acid, but are simply essential constituents of the diet, with- 

 out which the animal fails to grow and eventually dies. 



Nucleic Acids. The biological importance of the fifth major 

 group of organic compounds found in protoplasm, the nucleic acids, has 

 been fully appreciated only in recent years. These complex molecules, 

 as large as or larger than most proteins, were first discovered in 1870, 

 when Miescher isolated them fiom the nuclei of pus cells. Nucleic acid 

 molecules contain carbon, hydrogen, oxygen, nitrogen and phosphorus; 

 they gained their name from the fact that they are acidic and were first 

 identified in nuclei. They contain nitrogenous organic bases (purines 

 and pyrimidines), five-carbon sugars (ribose or desoxyribose) and phos- 

 phoric acid. For a long time it was thought that there were but two kinds 

 of nucleic acid-one containing the sugar ribose and called ribose nu- 

 cleic acid or RXA and found in cytoplasm, and one containing de- 

 soxyribose and called desoxyribonucleic acid or DNA and located in the 

 cell nucleus. Since 1948 experiments have made it clear that there are 

 many different kinds of RNA and of DNA. RNA and DNA are now 

 used as generic terms for a large class of substances which differ in their 

 details of structure and specificity. 



It is now clear that DNA is responsible for a large part, perhaps all, 

 of the specificity and chemical properties of the genes, the units of 

 heredity located in the nucleus. Ribonucleic acid plays an important 

 role in the svnthesis of protein and perhaps of other large molecules as 

 well. The building blocks of nucleic acids are nucleotides, just as amino 

 acids are the units of protein molecules. A nucleotide contains one 



