90 - The Cell 



G C T G A 



Fig. 4-20. Molecular structure of a single strand of 

 nucleic acid (DNA). Black circles = sugar (deoxyri- 

 bose); white circles = phosphate; A, adenine; C, cyto- 

 sine; G, guanine; and T, thymine (Fig. 4-19). The chain, 

 when broken up, yields either nucleotides (phosphate- 

 sugar-base units) or nucleosides (slightly smaller sugar- 

 base units, devoid of phosphate). 



In the living cell the various compounds 

 may be united into larger complexes: pro- 

 teins with salts, carbohydrates, and lipids; 

 steroids with other lipids; carbohydrates 

 with inorganic salts, etc. Water associates 

 with virtually all compounds dissolved in it; 

 and in the case of such supermolecular aggre- 

 gates it is not possible to distinguish clearly 

 between chemical and physical combinations. 

 Such linkages tend to be disrupted by the 



very methods usually employed to extract 

 the protoplasmic substances from the cell, 

 and much still remains to be learned about 

 the chemistry of protoplasm. However, our 

 present knowledge makes it plain that the 

 complexity of the composition of protoplasm 

 is sufficiently great to account for the amaz- 

 ingly intricate behavior of living cells. 



PHYSICAL STRUCTURE OF PROTOPLASM 



Protoplasm is not just a mixture of chemi- 

 cal components; if such were the case, one 

 should be able to create a living cell by 

 merely adding the proper ingredients to a 

 test tube. This, however, would be like ex- 

 pecting an accurately timed watch to emerge 

 from a randomly piled assortment of cog- 

 wheels, screws, springs, etc. Obviously the 

 creation of any functioning mechanism de- 

 pends not only upon the materials of which 

 it is composed, but also upon how these ma- 

 terials are formed and fitted into a func- 

 tional relationship. 



In the last analysis, protoplasm, like other 

 matter, is particulate, that is, composed of 

 particles. In protoplasm, however, the par- 

 ticles display a tremendous variability of 

 size and shape: from single molecules and 

 ions, large and small, to various molecular 

 aggregates, visible and subvisible. The si/e 

 and shape of these particles greatly influence 

 their behavior; and consequently it is neces- 

 sary to study the dimensions very carefully. 



NH, 



I 

 HC % 



N' 



C 



II 



-N. 



V 



CH 



'N- 



Fig. 4-21. Adenosine triphosphate (ATP). This energy-rich compound is certainly the most important meta- 

 bolic energy transmitter in the cell. It can quickly transmit energy to a wide variety of cellular activities (as ex- 

 plained in Chapter 8). Note the special symbol (~). Th's denotes the two high-energy bonds, which transmit 

 all of the energy (p. 143). 



