THE PHYSICAL BASIS OF LIFE 13 



AMINO ACIDS an amino acid being an organic acid in 

 which one hydrogen atom is replaced by the amino group, 

 NH 2 . But at the present time it is becoming increasingly 

 patent that the amino acids are, as it were, the nitrogenous 

 units with which organisms deal physiologically, rather than 

 the proteins themselves. An animal, for example, with 

 various proteins available in its food, chemically disrupts 

 these 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. So it 

 seems highly probable that the specific structure of an or- 

 ganism depends upon the chemical specificity of its proteins. 



Although the presence of proteins and the power of form- 

 ing them is the chief diagnostic chemical characteristic of 

 living matter, at the present stage of our knowledge it is 

 impossible to define proteins satisfactorily on the basis of 

 chemical or physiological properties. The most we can say 

 is that the biochemist describes proteins as "huge molecules, 

 complex in structure, labile in character, and therefore prone 

 to chemical change " and the latter characteristic un- 

 doubtedly is closely associated with the perennial plasticity 

 and responsiveness of the protoplasmic system itself. 



CARBOHYDRATES consist of various combinations of 

 carbon, hydrogen, and oxygen, the latter elements invariably 

 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 simgle^ugars, or monosac- 

 charids, such as glucose and fructose, to polysaccharids such 

 as dextrins, starches, and cellulose. 



FATS are composed of the same elements as the carbo- 

 hydrates, but in quite different arrangements. The propor- 

 tion of oxygen is always less, and therefore they are more 



