GENERAL ZOOLOGY 



upon the absence or presence of oxygen; these are the anaerobic (without 

 oxygen) phase and the aerobic (with oxygen) phase. 



Considering first the anaerobic phase, most of the reactions involve addi- 

 tion, subtraction, or rearrangement of phosphate groups in the molecule 

 (Fig. 2.9). The first step in carbohydrate metabolism is the addition of a 

 phosphate group to glucose. This reaction is called a phosphorylation and is 

 catalyzed by a specific enzyme. In the course of the reaction, ATP donates 

 its terminal phosphate group to the glucose, so that the resulting hexosephosphate 

 is a high-energy compound. Similarly, another phosphate group from another 

 ATP molecule is added to form a hexose diphosphate. This six-carbon com- 

 pound is then split, under the influence of another specific enzyme, to yield 

 two molecules of a three-carbon compound designated as a triose. Each of 

 these contains one high-energy phosphate bond and next obtains a low-energy 

 phosphate group from the inorganic store. Under the influence of a difl'erent 

 enzyme, two hydrogen atoms are removed from the triose diphosphate. This 

 reaction is known as a dehydrogenation and occurs only when coenzyme I, 

 or DPN (p. 33), is present to accept, or combine with, the released hydro- 

 gen atoms. As a result of changes occurring in the molecule during the 

 dehydrogenation, it now contains two high-energy phosphate bonds. In a 

 series of reactions the three-carbon compound is converted to pyruvic acid 

 which is the typical end product of anaerobic carbohydrate metabolism. 

 However, some cells are capable of modifying pyruvic acid under anaerobic 

 conditions. For example, muscle cells form lactic acid from it, and yeast 

 cells convert it to alcohol. 



During the conversion of each molecule of glucose to two molecules of 

 pyruvic acid, some free energv is harnessed by means of coupled reactions 

 (p. 28). Thus, four molecules of ATP are formed during the series of steps 

 which make use of energy from two molecules of ATP to run the reactions; 

 the cell has made a net gain of two ATP molecules. If glycogen, stored in 

 the cell, is the starting material, only one molecule of ATP is required to 

 drive the anaerobic phase .so that the net energy gain is represented by three 

 molecules of ATP. 



During the aerobic phase of carbohydrate metabolism, pyruvic acid is 

 broken down into carbon dioxide and water, and the bulk of the energy 

 originally stored in the hexose molecule is released in small packets. The 

 first step involves removal of one molecule of carbon dioxide from the pyruvic 

 acid to form acetic acid, a two-carbon compound; this enzymatic reaction 

 requires the coenzyme cocarboxylase (p. 32). Next acetic acid enters into 

 combination with coenzyme A (p. 33), in the presence of ATP, to form 

 acetyl coenzyme A. Only then does the carbohydrate food material enter the 

 series of reactions known as the tricarboxylic acid (TCA) cycle which is the 

 pathway for oxidation of not only carbohydrates but lipids and proteins, as 

 well. 



The reactions of the TCA cycle involve formation of citric acid, a six- 

 carbon acid, from acetyl coenzyme A and a four-carbon acid, oxaloacetic 



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