The Chemical and Physical Structure of Protoplasm - 89 



CH 



OH 



N'V\ 



NH, 



<? 



adenine 



H 2 N X ^N^ X N' 

 H 



guanine 



CH 



N 



HO /C ^N x 



CH 



II 

 CH 



cytosine 



Three organic bases, represented in both DNA and RNA. 

 Note how structure of the purines (adenine and guanine) 

 differs from that of the pyrtmidines (e.g. cytosine). 



OH 



I 



N^C- Ch3 



I II 



HO N^ 



thymine 

 (in DNA only) 



OH 

 I 

 <^C \ 

 N CH 



^ 



.CH 



N' 

 H 



uracil 

 (in RNA only) 



Two pyrimidine bases 



Fig. 4-19. Organic bases (purines and pyrimidines) are important constituents of the various nucleotides 

 (Fig. 4-18). The nucleotides, in turn, provide building units that go into the macromolecular structure (Fig. 4-20) 

 of the nucleic acids (DNA and RNA). 



termine the structure and activity of all pro- 

 tein components (including enzymes) in the 

 cell. 



Other Protoplasmic Components. In addition 

 to the proteins, carbohydrates, lipids, and 

 other previously mentioned substances, pro- 

 toplasm always contains smaller amounts of 

 other, usually simpler, organic compounds. 

 This miscellaneous group of compounds in- 

 cludes: (1) various organic wastes, such as 

 urea (p. 342); (2) metabolic energy transmit- 

 ters, such as ATP (Fig. 4-21); (3) hormones, 

 such as adrenalin (p. 406); and (4) vitamins, 

 such as thiamine (p. 346). Many of these 

 miscellaneous substances are of great physio- 

 logical significance, as will be explained in 

 later chapters. 



Summary of Chemical Composition. All 

 living cells are composed of water, inorganic 



salts, proteins (including DNA and RNA 

 nucleoproteins), carbohydrates, and lipids, 

 together with smaller amounts of other sub- 

 stances. The average amounts of these com- 

 pounds are shown in Table 4-7, although cer- 

 tain cells may vary considerably from the 

 average. 



Table 4-7— Average Composition of 

 Protoplasm 



Percent 

 Kinds of Substances by Weight 



Water 80 



Inorganic salts 1 



Proteins 15 



Lipids 3 



Carbohydrates plus other sub- 

 stances 1 



100 



