CHEMICAL COMPONENTS OF CELLS: 

 MACROMOLECULES OF YEAST AND 

 THEIR SUBUNITS (I)* 



(Readings: Weisz, pp. 17-38 and 149-156; Villee, pp. 26-31. F. H. C. Crick, 

 "TTie Structure of the Hereditary Material," Sci. Am. 191, No. 4, Oct. 1954, 

 Reprint No. 5. P. Doty, "Proteins," Sci. Am. 197, No. 3, Sept. 1957, Reprint 

 No. 7.) 



A cell lives by virtue of its composition and 

 organization. Both are unique: the composition 

 in large part because of the universal presence 

 of certain classes of very large molecules, so- 

 called macromolecules, the largest and most 

 complex in all chemistry; and these are respon- 

 sible also for many of the most distinctive fea- 

 tures of cellular organization and behavior. 



Our approach to the macromolecules is enor- 

 mously simplified by three circumstances : (a) All 

 of them fall into three great classes — poly- 

 saccharides, nucleic acids, and proteins — com- 

 mon to all cells, and sharing common properties 

 within each class, (b) Each type of macromole- 

 cule is composed of a limited number of repeat- 

 ing subunits, bound together to form long 

 chains. The subunits of the polysaccharides 

 are sugars; those of nucleic acids, nucleotides; 

 and those of proteins, amino acids. Rather 

 than dealing with the individual atoms of which 



*An alternative or supplementary pair of exer- 

 cises on the biochemistry of milk will be found in 

 Appendix C. 



these molecules are composed, which may run 

 into many hundred thousands, we deal with the 

 much smaller numbers of subunits. (c) In all 

 types of macromolecule, the subunits are bound 

 to one another through the same device, the 

 elimination of a molecule of water between 

 each pair. Conversely every macromolecule 

 may be broken down into its subunits by the 

 reverse process, the insertion of a molecule of 

 water between each pair. The latter process is 

 called hydrolysis. Digestion is a series of such 

 hydrolyses, catalyzed by enzymes in the diges- 

 tive system, which cleave all the macromolecules 

 of the food into their constituent subunits : 



Sugars 



Nucleo- 

 tides 



Synthesis 

 -(n-l) H2O 



+(n-l)H20 

 Hydrolysis, 

 Digestion 



:< 



Polysaccharides 

 (glycogen, starch) 



Nucleic acids 



Proteins 



Amino 

 acids 



In this and the next laboratory session we 

 will separate the major types of macromolecules 



12 



