PROTEINS 43 



could not build up a different body. It is probable 

 that differences between species ultimately depend 

 upon comparatively slight differences in structure 

 between some of the proteins present, or between the 

 relative amounts of the different proteins present, or 

 perhaps between the ways in which different protein 

 molecules are built up into more complex protoplasmic 

 structures, of which we actually know practically 

 nothing. Such biochemical differences between even 

 closely allied species have recently been shown to hold 

 for other important organic substances of the body, 

 for instance between the starches of different species 

 of plants and between the haemoglobins (the red pig- 

 ment of the blood) of different species of animals. It 

 has been shown that the starch of every species (out 

 of many hundreds examined) is in some respect different 

 from the starch of every other species ; and the same 

 is true of haemoglobin. The proteins are much more 

 difficult to investigate, but when we consider that the 

 molecules of many of them are much more complicated, 

 it becomes obvious that the possibihties of slight but 

 perfectly definite differences are much more numerous. 



Proteins as a class show certain colour changes 

 when acted upon by certain substances ; for instance 

 a yellow colour, when after boiling with nitric acid 

 ammonia is added ; a violet colour, with a drop of 

 dilute copper sulphate followed by excess of caustic 

 soda. A dilute solution of iodine in potassium iodide, 

 which colours starch blue, stains proteins yellow. In 

 such ways proteins, as a class, can be distinguished 

 from other classes of organic compounds. Into the 

 characters of the different proteins we shall not attempt 

 to enter here. 



Apart from their universal occurrence in living proto- 



