(J s BOTANY 01 rm LIVING PLANT 



and therefore confined to aerial and green tissues, protein synthesis 



■anally independent of light, and < A n probably take place in any 



living cell in the presence of suitable raw materials. It is likely, how- 



that the leaf cells are the most important sites of protein synthesis, 



it i- here thai sugars are most abundant. We have little definite 



information with regard to the intermediate stages in the synthesis 



proteins within the plant, though it can be assumed that the process 

 consi sentially in the interaction of nitrogenous compound- with 



the sugars of photosynthesis or their derivatives. A common view 

 has been that by interaction of ammonia, derived from the inorganic 

 nitrogen absorbed from the soil, with organic acids arising from 

 sugars a whole ran^e of amino-acids is produced in the plant. These 



ids may then become linked up to form protein under the in- 

 fluence of the enzymes mentioned above, — the proteases, — which 

 .it other times catalyse the splitting of proteins. This view is, how- 

 ever, rendered rather uncertain by experimental evidence which 

 indicate- that substances known as Amides (especially Asparagine) 

 may figure prominently in protein synthesis. Energy is required 

 for the synthesis of proteins. In this case the energy is not derived 

 directly from the sun's rays, but only indirectly through the process 

 of respiration as discussed below (p. 133). 



Enzymes. 



A number of enzymes have already been mentioned. These agents 

 play a most important part in plant metabolism, since they enable 

 the protoplasm to carry out at high speed chemical transformations 

 which otherwise would not occur at all, or only very slowly. Enzymes 

 are in many respects comparable to the Catalysts of chemistry, and 

 they are in effect protoplasmic catalysts. As an example of ordinary 

 catalysis we have the very rapid combination of hydrogen and 

 oxygen to form water in the presence of finely divided platinum, the 

 latter acting as a catalyst. The combination of the gases is extremely 

 slow in it- absence. An important feature of catalysis, including 

 enzymic catalysis, is that the catalyst remains unchanged at the end 

 of the reaction and can be used over and over again. 



The chemical nature of enzymes is uncertain, though it is agreed 

 that they are complex substances, probably present in a colloidal 

 form in the cell. It is to this colloidal nature, and the resulting high 

 adsorptive capacity, that the catalytic properties of enzymes are in 

 part attributed [Chapter III.). It was formerly believed that they 



