CELL METABOLISM 71 



chrome enzymes. One of the enzymatic steps in the conversion of glucose 

 to lactic acid is inhibited by fluoride ion and another by iodoacetate. 

 These substances, and others, have been used as tools by biochemists to 

 investigate the properties and sequences of enzyme systems. 



Enzymes themselves may act as poisons if they get into the wrong 

 place. As little as 1 milligram of crystalline trypsin injected intra- 

 venously will kill a rat. Certain snake, bee and scorpion venoms contain 

 enzymes that destroy blood cells or other body tissues when injected into 

 the body of the prey. 



22. Respiration and Cellular Energy 



The term "respiration" originally meant simply inhaling and ex- 

 haling. It was thus a synonym of breathing and the term "artificial 

 respiration" reflects this usage. Later, respiration came to mean the 

 exchange of gases between the cell and its environment, the intake of 

 oxygen and the release of carbon dioxide. Most recently, as more of the 

 details of cellular metabolism have become known, it has come to mean 

 those enzymatic reactions in which oxygen is utilized by the cell, the 

 reactions by wliich substrates are oxidized and most of the energy is 

 made available to the cell. The term "fermentation" was originally 

 defined by Pasteur as "life without air" and is now used to refer to the 

 chemical reactions of substrate molecules which occur in the absence 

 of oxygen. 



The energy required by each cell in an animal or plant body must 

 be obtained by releasing the potential energy of a foodstuff molecule and 

 converting the energy into a form that is usable by the cell for its various 

 physiologic functions— contraction, conduction, secretion, or whatever. 

 The energy is released and converted into "energy-rich" phosphate com- 

 pounds, of which adenosine triphosphate, ATP, is of prime importance. 

 These energy-rich phosphate compounds do not, in general, pass from 

 one cell to another, but are formed and used within the same cell. Thus, 

 the energy for muscle contraction is not released from food molecules 

 in the stomach or liver and carried as "energy" to the muscle. Instead, 

 food molecules, such as glucose, are carried by the blood to all the cells 

 of the body. Then, within each cell, the glucose is metabolized, first to 

 pyruvic acid and then, if there is a supply of oxygen, to carbon dioxide 

 and water. If there is little or no oxygen, the pyruvic acid is converted 

 to lactic acid, alcohol, or some other substance. 



As the cellular metabolism of such diverse things as green plants, 

 rats, yeast, bacteria and sea urchins has been investigated, it has become 

 clear that the fundamental enzyme reactions in all cells are remarkably 

 similar. The steps by which glucose is converted to pyruvic acid, called 

 the glycolytic cycle, are the same not only in man and mouse, but in 

 moss and mold as well. This similarity of enzyme systems may simply 

 reflect the fact that all living things are related by evolutionary descent; 

 the system of glycolytic enzymes became established in the early forms 

 of life and has been transmitted to all the forms subsequently derived 

 from these. Or, it may be that the types of chemical reactions that will 



