ANABOLISM AND KATABOLISM — GROWTH 79 



the molecule of sugar, the first stage in the formation 

 of the protein molecule that goes to form the structure 

 of protoplasm, is often called " carbon assimilation." 



Anabolism and Katabolism. — All the constructive 

 chemical processes which take place in the protoplasm 

 are classed together as anabolic ' processes or anabolism, 

 as opposed to the destructive or katabolic 2 processes 

 {katabolism). Thus the formation of sugar from carbon 

 dioxide and water in a green plant is an anabolic process, 

 the breaking down of sugar, and, in the animal body, 

 the breaking down of proteins into urea and uric acid, 

 are katabolic processes. Assimilation in the strict 

 sense, i.e. the incorporation of foodstuff in the proto- 

 plasm, is the ultimate anabolic process. 



(3) Growth. — In its simplest form growth follows 

 directly from assimilation, from the incorporation of 

 new material which increases the mass of the protoplasm. 

 Unless this new protoplasm is destroyed as quickly 

 as or more quickly than it is formed, increase in bulk 

 must result. In amoeba the animal simply increases in 

 size till it reaches a certain limit, when division takes 

 place, and the two daughter amoebse go on feeding and 

 growing independently. In more complex organisms 

 of definite shape growth takes very various forms in 

 different parts of the body, but it always leads to 

 permanent increase in the bulk of the organism — a 

 temporary inflation of part of the body, for example, 

 is not a process of growth. 



The higher plants differ widely from the higher 

 animals, in showing localised growth, in most cases 

 at the ends of the branches of the shoot and of the 

 root, leading to increase in length of the branches, 

 and this may be more or less continuous throughout 

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