22 ANIMAL BIOLOGY 



living complex and, as we shall see later when considering the 

 chemical processes in animals and plants, is largely responsible 

 for their commanding position as "the chemical nucleus or pivot 

 around which revolve a multitude of reactions characteristic of 

 biological phenomena." Study of the relationship of nitrogen to 

 the other chemical elements of proteins has revealed that the 

 protein molecule is a huge complex of linked amino acids - - an 

 amino acid being an organic acid in which one hydrogen atom is 

 replaced by the amino group, NH 2 . The amino acids are the nitrog- 

 enous units with which organisms deal physiologically, rather than 

 the proteins themselves. An animal, for example, with various 

 proteins available in its food, chemically disrupts them into their 

 amino acid constituents, and then takes an amino acid here and 

 another there and synthesizes the specific proteins it demands. 

 And further, if individual amino acids are supplied, the animal 

 employs them. 



Although there are less than two dozen amino acids, the num- 

 ber of proteins is legion. Furthermore, besides the so-called 

 simple proteins composed solely of amino acids, there are many 

 which comprise in addition other radicals, such as the hemo- 

 globins which contain hematin and the nucleoproteins with nucleic 

 acid. It appears that the specific structure of an organism depends 

 upon the chemical specificity of its proteins, but for our purposes 

 it is sufficient to recognize that the presence of proteins and the 

 power of forming them is a prime chemical characteristic of living 

 matter. Apparently these huge, complex molecules with their 

 high lability and, therefore, tendency to chemical change are 

 fundamentally associated with the plasticity and responsiveness 

 of protoplasm. 



Carbohydrates consist of various combinations of carbon, 

 hydrogen, and oxygen, the latter elements typically being present 

 in the proportion found in water (H 2 0). Though more simple in 

 chemical structure than proteins, they range in complexity from the 

 simple sugars, or monosaccharides such as glucose and fructose, 

 to polysaccharides such as starch, glycogen, and cellulose. 



Fats are composed of the same elements as the carbohydrates, 

 but in quite different arrangements. The proportion of oxygen is 

 always less, and therefore they are more oxidizable and richer in 

 potential energy. Fats represent the union of an acid (fatty acid) and 

 glycerol. Examples are butter and oils of plant or animal origin. 



