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~ CHAPTER XI 
THE CONSTRUCTION OF PROTEINS 
The simple compounds containing nitrogen which we have 
seen to be absorbed by the roots of green plants, are as 
unavailable for direct nutrition as the carbon dioxide taken 
in from the air. The nitrogenous organic material which 
is actually assimilated by the protoplasts is thought to be 
some form of protein. With very few exceptions, and these 
occurring only among micro-organisms, gelatin and similar 
bodies cannot be made to support vegetable living sub- 
stance, though they can be made use of by animals to 
supplement, but not to replace, their protein supplies. 
In studying the story of the construction of proteins 
from the nitrates and ammonia-compounds taken into the 
plant, we meet with even greater difficulties than those which 
are presented by the photosynthesis of carbohydrates. These 
difficulties are connected with the stages which occur in 
the course of the construction, with the mechanism which 
is concerned in the transformation, and with the condi- 
tions under which the building up of protein takes place. 
At the outset of the study we find ourselves in almost com- 
plete ignorance as to the chemical nature of protein. We 
know that it is the most complex material found in the plant 
with the exception of the livmg substance itself, but we 
know hardly anything about its molecular structure or the 
arrangement or grouping of its constituent atoms. De- 
structive analysis has revealed its percentage composition 
within certain limits, although, as there are many kinds of 
