8 PROTOPLASM AND CELLS 



are never absent from living matter, and except for the fact 

 that some of the simpler kinds can be synthetically produced 

 in the laboratory they are never derived from any other matter 

 than that which is living. The building up of proteins is the 

 most important factor in life. 



Proteins are highly complex, and are very varied in their 

 nature; but all react in the same way to certain chemicals. 

 Proteins in food differ from those in the living tissues. During 

 digestion the former are broken up and then reconstructed, 

 only to be finally broken do^\Ti again into carbon dioxide, 

 water, sulphuric acid, urea, and other products which are 

 excreted from the body. Animals convert the protein of their 

 vegetable food into the protein of their own body. But the 

 building up of proteins in plants involves the combination of 

 the soluble nitrogenous food taken uj) by the root with the 

 complex carbon compounds formed by the green leaves in 

 sunlight, and we thus have two circles or cycles, the cycle of 

 carbon, which ^^dll be described in Chapter V under the title 

 of Chlorophyll, and the cycle of nitrogen, which will be 

 described in Chapter VI under the heading of The Nitrogen 

 Cycle. 



The molecules of the proteins are very large, certainly the 

 largest and most complex that are known : and consisting as 

 they do of thousands or even tens of thousands of atoms 

 they afford a large scope for the slight variations which we 

 find in the different classes of proteins. The variety in the 

 arrangement of these numerous atoms in the complicated 

 molecule may also explain the differences which exist between 

 the different species of plants and of animals. The protoplasm 

 of one species of animal or plant differs from the protoplasm of 

 all others. "All flesh is not the same flesh; but there is one 

 kind of flesh of men, another flesh of beasts, another of fishes, 

 and another of birds." 



It may be that differences between plant and plant and 

 animal and animal depend on minute differences in the 

 structure of their proteins, or possibly between the ways in 

 which the protein molecule is built up. There are also minute 

 differences, which are nevertheless perceptible, between the 

 starches of one plant and the starches of another. There are 



