CELL METABOLISM 



73 



oxide is so familiar that it is perhaps only natural to infer that the 

 oxygen atoms in the carbon dioxide (CO2) are the same atoms that en- 

 tered the body as gaseous oxygen. This is not true, however, as an ex- 

 amination of Figure 4.2 will make clear. The oxygen atoms that enter 

 the body as oxygen unite with hydrogen to form molecules of water, 

 and leave the body as water. The oxygen atoms that leave the body in 

 carbon dioxide entered the body, by and large, in some substrate mole- 



Glycogen or Starch 



Glucose 



Amino acids 



Lactic acid± 



V" 



2O 



energy 1 



■H, 



.H 



Electron 



transmitting 



enzymes 



(cytochromes) 



-► Glucose phosphate 

 Fructose phosphate 

 Fructose diphosphate 

 2 Triose phosphates 



L 



► energy ^'^P —: 



— - ^m\\^^ 



Phosphoglycenc ac.d ^^^^„,,,^^ 



energy %^P # 

 "Pyruvic acid 



I 



)gl: 



1 



jvii 



1 



■^ CO2 



Acetyl coenzyme A- 



(2carbons)V 



4 



Oxalacetic acid"^ 

 (4 carbons) 



u 



Fatty 

 acids 



KREBS 



CITRIC ACID 



CYCLE 



Citric acid 

 (6 carbons) 



t^t 



CO: 



-T" 



-^ 



NET: CgHiaOe +6 Og— ► 6 COg + 6 HgO + energy 



Figure 4.2. A diagram of some of the steps in the glycolytic cycle (glucose to pyruvic 

 acid), the citric acid cycle and the cytochrome system. The symbol ~ P refers to energy- 

 rich phosphate bonds such as those in adenosine triphosphate (ATP) which can yield 

 their energy to drive cellular mechanisms. From this some appreciation can be gained 

 of the tremendous o\ersimplification involved in writing the over-all formula for the 

 oxidation of glucose given below. 



