CHEMICAL STRUCTURE AND METABOLISM 87 



eaters) have mainly protein-splitting enzymes; herbivores (plant eaters) 

 have mainly carbohydrate-splitting ones; omnivorous feeders like ourselves 

 are well supplied with all kinds. In other words, the differences which ex- 

 ist are connected with the adaptation of animals for different diets. 



We have mentioned that proteins are utilized for growth and repair of 

 tissues. Suppose an animal eats more protein than is needed for growth 

 and repair; what happens to the excess? We recall that the protein is split 

 into amino acids. Amino acids not needed for synthesis of new proteins 

 are broken down still further, in higher vertebrates mainly by the liver. 

 The products formed are carbohydrates (simple sugars) and ammonia. 

 This is just the reverse of the process mentioned on page 85 by which 

 amino acids are manufactured in the first place. The carbohydrates de- 

 rived from this process are used as fuel, just as any other carbohydrates 

 are. The ammonia formed is a waste product, and being toxic (poisonous) 

 it must be eliminated from the body as efficiently as possible (see below). 



Fuel for Life's Fires 



Every process connected with the business of living requires the ex- 

 penditure of energy, whether in locomotion of the body as a whole, in 

 the use of muscles in work and play, or in the less obvious activities such 

 as the secretion of glands, the activity of the nervous system, and all the 

 other processes involved in metabolism: digestion, respiration, excretion, 

 and so on. No cell of the body can carry on its activities without using 

 energy. Thus it becomes important to inquire into the means by which the 

 cells obtain the needed energy. 



The sun is the ultimate source of energy for life on this earth. We have 

 seen that green plants are capable, through photosynthesis, of combining 

 carbon dioxide and water to form carbohydrates. The simplest chemical 

 formulation we can write for this reaction is the following: 



6 COo + 6 HoO ^ CeHi.Oe -h 6 Oo 



(For the benefit of readers unfamiliar with even elementary chemistry we 

 translate as follows: "Six molecules of carbon dioxide combine with six 

 molecules of water to form one molecule of glucose plus six molecules of 

 oxygen.*') The oxygen is liberated into the atmosphere; the carbohydrate 

 (glucose) is the product in which we are interested at present. It may be 

 stored in the plant tissues, or, more commonly, molecules of this "simple 

 sugar" may be combined to form more complex sugars or starches or 

 may enter into the formation of amino acids and hence of proteins. 



