396 LIFE : OUTLINES OF GENERAL BIOLOGY 



simpler lactic acid, and this in turn to carbon dioxide and water. 

 So, in a very different connection, the digestive juices simplify the 

 complex carbohydrates to sugars with sLx carbon atoms (glucose) ; 

 they break up the insoluble fats into two soluble portions (fatty 

 acids and glycerol) ; and they split the protein food into the simpler 

 amino-acids. The chief reason for the simplifications that go on in 

 the alimentary canal is that the lining cells are not able to absorb 

 large molecules or colloidal particles. These splittings belong to the 

 class of reactions called hydrolyses: when the complex molecule is 

 split in two, a molecule of water is also divided into H and 0-H, 

 and the "open ends", so to speak, of the broken molecule are 

 "plugged" with the ions or loose atoms derived from the water. 

 Many substances can be hydrolysed in the laboratory with the aid 

 of strong acids or alkalies; but in the body the action of enzymes is 

 all-important. 



Protein molecules are so complex that the number of possible 

 different proteins is almost infinite, and every species of organism 

 has its own proteins. F^oreign proteins, from a different species, 

 introduced into the blood, may in certain cases have a serious effect. 

 This, then, is another reason why the proteins are simplified to 

 amino-acids in the intestine before being absorbed into the blood. 



When the amino-acids are distributed to the cells they are again 

 built up into proteins suitable for and characteristic of this par- 

 ticular organism. This is an example of the reverse process to break- 

 ing-dowTi, it is a building-up or synthesis of complex molecules from 

 simple ones; another example already studied is the formation of 

 sugar from lactic acid. Two points about syntheses may be noticed: 

 first, they are, as one would expect, usually accompanied by the 

 reappearance of the water used up in the simplification process; 

 second, they are usually expensive processes — they do not take 

 place of themselves, but require supplies of energy. Thus the lactic 

 acid concerned in muscular activity is transformed to sugar by 

 sacrificing a fraction to furnish energy. 



But an exception to this second point deserves some notice. Many 

 chemical reactions are under certain conditions incomplete. When a 

 fatty acid and an alcohol are brought together, they combine to 

 form an ester; but the reaction stops before all the fatty acid and 

 all the alcohol have been used up; only a certain percentage of the 

 total possible amount of ester is formed. In the same way the ester 

 can be split up or hydrolysed to fatty acid and alcohol, but not 

 completely; a certain amount of ester will remain. The reaction is a 

 reversible one, it proceeds in both directions; but at a certain con- 

 centration of ester, of acid, and of alcohol, an equilibrium is estab- 

 lished between the two processes, and there is no further change in 

 the concentration of the various substances unless the conditions 

 of the experiment are altered in some way. The ester-forming 



