FOOD DIGESTION AND RESPIRATION 45 



one state to the other may be brought about more or less readily. 

 The energy obtained when a compound of carbon with hydro- 

 gen is oxidised to C(X and H.,O must arise from changes in 

 the internal structure of the atoms, as already pointed out. 

 We have just seen that O=C H and O=C OH have quite 



different properties, although both contain O C . The carbon 



is united to H in the one case, to OH in the other. The energy 

 content of the former is greater than that of the latter. 



Source of Nitrogen the Amino-Acids 



We may now pass on to the consideration of the simplest 

 compound of nitrogen that will serve for animal nutrition, leaving 

 the case of the plant to be dealt with later. The animal organism 

 cannot utilise any compounds of nitrogen simpler than those 

 known as amino-acids. What is the chemical nature of these 

 substances ? 



The carboxyl group, COOH, is a characteristic of acids. 

 Although its place can be taken by sulphur or phosphorus 

 derivatives of similar nature, the most important organic acids 

 concerned with the functions of living cells are the carboxylic acids. 

 The free bond of the carboxyl group must, of course, be united 

 with some other molecular group, and the most obvious to begin 

 with is hydrogen ; thus we get formic acid, H COOH. Many 

 series of compounds of ascending degree of complexity and size 

 are formed by the successive addition of CH 2 , one of the free bonds 

 of the carbon being used to join on to the original compound, 

 while the other serves to attach further groups. Adding CH., then 

 to formic acid, we get acetic acid, H CH 2 COOH. Continuing 

 the process, we have the numerous straight chain fatty acids : 

 H (CH,X COOH. 



One of the commonest and most important compounds of 

 nitrogen is ammonia (NH 3 ), so that it is not surprising to find that 

 its derivatives form the basis of the source of nitrogen for living 

 matter. It is joined on to an acid, such as acetic acid, in the form 

 NH 2 , taking the place of one of the hydrogen atoms combined 

 with a carbon other than that of the carboxyl : 



H 



H C COOH 



I 

 NH, 



which is amino-acetic acid, or glycine. 



