36 



LIFE: ITS BEGINNINGS AND NATURE 



may carry on all of the processes that are 

 essential for life they must have a constant 

 source of energy (Fig. 2-13). They must 

 also have building materials with which to 

 construct more of their own substance if 

 they are to grow or to replace parts that 

 are worn out. The source of this energy 

 and this building material in animals comes 

 from the breaking down of large molecules 

 into smaller ones and the delivery of these 

 substances in utilizable form to all of the 

 cells of the animal body. Animals must have 

 food which usually exists as complex, in- 

 soluble molecules. These are broken dowii 

 in the digestive tract and delivered to the 

 cells where they can be used as needed. 

 This is nutrition, which is confined to the 

 animate world only; it is closely linked with 

 metabolism. 



4. Metabolism. Once the utilizable build- 

 ing materials and the energy-giving mole- 

 cules are delivered to the cells, they un- 

 dergo further changes depending on how 

 they are used. If they are further degraded 

 with large quantities of energy released, the 

 process is referred to as catabolism, or de- 

 structive metabolism; if they are used in 

 building new or repairing old parts, the 

 process is called anabolism, or constructive 

 metabolism. This constant building up and 

 tearing down, storing and consuming, is 

 referred to as metabolism, the crux of life 

 itself. 



During the dynamic process of metabo- 

 lism oxygen is constantly utilized to release 

 energy, with the resulting production of 

 carbon dioxide; this is called respiration. 

 The removal of the waste products of me- 

 tabolism, such as carbon dioxide and nitrog- 

 enous wastes, is called excretion. These 

 two processes are intimately linked with 

 metabolism. All living things are charac- 

 terized by metabolism, a process which is 

 without duplication in the inanimate world. 



5. Growth. When the constructive forces 

 exceed the destructive forces in an organ- 

 ism, it increases in size, or, in other words, 

 it grows (Fig. 2-14). This is typical of aU 



organisms, particularly during their early 

 life. However, a stage is reached where 

 there is a balance between these two forces, 

 and this is referred to as maturity; at 

 the time of maturity the organism merely 

 holds its own, becoming neither larger nor 

 smaller. As life continues, the anabolic 

 processes fail to keep pace with the catabo- 

 lic processes, and the whole bulk of the or- 

 ganism loses ground and finally dies. This 

 involves the processes of aging, the nature 

 of which is understood only vaguely. 



Growth of living things is quite different 

 from that of inanimate bodies. A crystal 

 may grow in size by the addition of other 

 similar crystals to its own bulk, but the 

 pattern and the method of executing it are 

 quite unlike that of a living cell. The crys- 

 tal merely adds other crystals to its own 

 external mass (accretion), like a mason 

 adds bricks to a wall, whereas the cell takes 

 its building materials within, and there 

 makes them an integral part of its own 

 structure ( intussusception ) . 



6. Reproduction. As a result of growth 

 and increase in size, the or2;anism is able 

 to duplicate itself (Fig. 2-15). It may do 

 this by simple fission, that is, by dividing 

 into two equal parts, or it may produce 

 special reproductive cells, eggs and sperms, 

 which subsequently unite to grow into a 

 new organism similar in most respects to 

 the parents. Duplication by the first process 

 is called asexual reproduction, and by the 

 latter, sexual reproduction. Reproduction 

 by either of these methods is not shared 

 by the inanimate world. 



One of the remarkable things that stems 

 from reproduction is the continuity of pat- 

 tern from generation to generation. Off- 

 spring are endowed with structural and 

 physiological characteristics that are exact 

 duplicates of those found in the parents. 

 The pup, colt, or child is expected to 

 possess bodily form, and even personality 

 traits, similar to those of its parents. This 

 knowledge is so commonplace that it was 

 taken for granted for centuries. The trans- 



