156 - The Cell 



CARBOHYDRATES 

 (POLYSACCHARIDES) 



PROTEINS 



SIMPLE SUGARS 



X 



I 



I 

 I 



AMINO ACIDS 





"7 



HEXOSES; SOME 



AMINO ACIDS, e.g. 



ALANINE, SERINE, etc 



FATTY ACIDS; OTHER 



AMINO ACIDS, e.g. 



LEUCINE, TYROSINE, etc. 



CERTAIN AMINO 



ACID, e.g. PROLINE, 



ARGININE, etc. 



OTHER AMINO 



ACIDS, e.g. TYROSINE, 



ASPARTIC, etc. 



ACETYL - CoA 



KREBS 



(CITRIC ACID) 



CYCLE 



Fig. 8-13. Some major pathways of cellular metabolism. Schematic summary, after Krebs. 



reactions, in reverse, can lead to synthesis, 

 provided that a proper source of energy and 

 precursory substances are on hand. The pat- 

 terns of metabolism provide considerable 

 facility for the interconversion of substrates 

 — proteins into carbohydrates, carbohydrates 

 into fats, and so forth. This is possible be- 

 cause the pathways of protein, carbohydrate, 



and fat metabolism join each other at a com- 

 mon cross road, provided by acetyl coenzyme 

 A (Fig. 8-5). By reverse pathways material 

 derived from one source (for example, pro- 

 tein) may be diverted into another (for ex- 

 ample, carbohydrate), in such a manner that 

 the cell has great flexibility in its utilization 

 of different organic molecules. 



TEST QUESTIONS 



1. What important compounds are represented 

 by the following symbols: ATP. ADP, and 

 AMP? Specify the number (if any) of high- 

 energy phosphate bonds that are present in 

 each case. 



2. How do high-energy phosphate bonds differ 

 from ordinary phosphate linkages? What 

 symbols are used to distinguish between the 

 two types? 



3. What type of chemical reaction is represented 

 by the splitting of high-energy phosphate 

 bonds? Give one such reaction. 



4. Specify five different types of cellular ac- 

 tivity that derive energy from the hydrolysis 



of ATP. What other cellular processes may 

 be energized by ATP? 



5. To what extent is it true that many catabolic 

 reactions contribute energy toward main- 

 taining the ATP reserves of the cell? Ex- 

 plain. 



6. Differentiate between oxidation and reduc- 

 tion: 



a. in terms of die transfer of electrons 



b. in terms of hydrogen transfer 



7. Provide an example to illustrate the fact 

 that oxidations and reductions are insepara- 

 ble, mutually dependent processes. 



8. Under what circumstances will an oxidation- 



