Life Processes and 

 Organization 



Life is the most complex phenomenon known to 

 man. This complexity is indicated by the fact that 

 no completely satisfactory definition of life is possible. 

 Yet this chapter proposes to characterize life. Un- 

 fortunately, the characterization to be given is in a 

 form that makes living appear very different from 

 nonliving. No attempt is made to show that nonliv- 

 ing things have the same basic processes as do liv- 

 ing creatures. Also the question of whether viruses 

 are living or nonliving is not developed. Therefore, 

 the present chapter serves only to introduce the lower 

 part of the life spectrum. 



THE NATURE OF LIFE 



Living creatures have certain structures and func- 

 tions that appear to distinguish them from nonliving 

 things, but none of these basic features is unique to 

 life. On the other hand, most organisms display 

 greater development of certain characteristics than 

 do inanimate objects. An examination of those 

 characteristics follows. 



ORGANIZATION 



Living creatures possess complex structural or- 

 ganization that is associated with even more complex 

 functional relationships. Some of this structure is 



simple in the sense that it is protoplasmic or uni- 

 cellular, that is, not composed of many cells, but most 

 living organization does involve cells (multicellular). 

 In multicellular life, groups of cells are intimately 

 joined to perform most life processes. These cells are 

 composed of chemical compounds whose elements are 

 more than 95 per cent carbon, hydrogen, oxygen, 

 and nitrogen. Naturally these same elements are 

 found in nonliving things, but only living creatures 

 generally have such a large proportion of their mat- 

 ter formed from these elements. Also, the organic 

 compounds include the largest and most complex 

 chemicals known; inorganic compounds, in contrast, 

 are mostly small and simple. 



Another point of living organization is form and 

 size. Most life is of definite size and shape and any 

 nonliving things are not; however, even inanimate 

 objects are regular enough for their physical aspects 

 to be valuable in identification. 



LIFE SPECTRUM 



The life spectrum, the range of biological organiza- 

 tion, here is arbitrarily assumed to exist and to be 

 composed of ten levels of complexity, protoplasm, ''sin- 

 ^/f f^Z/^rf" (unicellular or acellular), cells, tissues, organs, 

 organ systems, populations, communities, ecosystems, 

 and the biosphere (Figure 6.1). The first six levels are 

 progressively more complex structural bases ol or- 



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