6 - The Cell 



Popularly the word "life" is sometimes 

 used in a different sense: to designate a mys- 

 terious immaterial "something" that uniquely 

 resides in living bodies, causing their activi- 

 ties. Science, however, has not been able to 

 find the slightest evidence to confirm the ex- 

 istence of anything corresponding to this 

 idea. Nor is such a concept useful. To say 

 that a living body moves, responds, or grows 

 "because it is alive" or because "it possesses 

 life" is like saying that a motor runs "because 

 it is motile" or "because it possesses motility." 

 In science such so-called explanations are not 

 admissible. The only scientifically valid kind 

 of explanation consists in finding and describ- 

 ing an actual mechanism, in which the spe- 

 cial composition, arrangement, and inter- 

 action of the component parts logically ac- 

 count for the observed activities. 



In accordance with this important criterion 

 of science, the aim of biology is to explain 

 life — the activities of living bodies — in terms 

 of the composition and structure of these 

 bodies: what materials are present, how the 

 component materials are uniquely organized 

 in the living body, and how these compo- 

 nents interact to generate the activities that 

 are recognized as life. Just as the chemist or 

 the physicist probes into the visible and sub- 

 visible structure of nonliving matter in seek- 

 ing to understand the mechanism of its be- 

 havior, so the biologist, using essentially the 

 same methods, investigates the ultimate struc- 

 ture of living matter, which is called proto- 

 plasm. 



Protoplasm. The phenomena of life never 

 find complete expression except in associa- 

 tion with a particular kind of matter, namely 

 protoplasm. 



Typically protoplasm is a colorless, trans- 

 lucent, gelatinous fluid, which composes the 

 living part of every living thing. All other 

 parts of any living body — bone, cartilage, 

 wood, etc. — are produced by the protoplas- 

 mic parts. Protoplasm, therefore, must be 

 regarded as the physical basis of life, or, more 

 simply, it may be referred to as living matter. 

 In the ultimate composition and structure of 



protoplasm, the answers to life's unique rid- 

 dles must be sought. 



Chemical Structure. Protoplasm is not a 

 single homogeneous substance. Rather, it is 

 a complexly organized system in which many 

 substances are present. Some of these sub- 

 stances, such as water and mineral salts, are 

 also abundant in nonliving nature. But pro- 

 toplasm is especially characterized by its 

 rich variety of organic substances, especially 

 proteins, which are found nowhere in nature 

 except as components or products of proto- 

 plasm. The manifold chemical compounds of 

 the protoplasm are constantly reacting and 

 interacting. This ebb and flow of chemical 

 activity, which is called metabolism, gen- 

 erates energy and provides for the synthesis 

 of more organic compounds, needed as 

 growth occurs. Moreover, the complex physi- 

 cal and chemical structure of protoplasm is 

 not stable. It tends to disintegrate and be- 

 come disorganized unless energy is constantly 

 available for reconstruction. Just as an air- 

 plane cannot maintain altitude unless energy 

 is available from the combustion of fuel in 

 the motors, so the protoplasmic structure 

 undergoes degradation unless energy is forth- 

 coming from metabolism. The ultimate 

 sources of metabolic energy may vary con- 

 siderably in different forms of life (Chap. 9), 

 but energy for immediate use is provided by 

 a set of basic metabolic reactions that seem 

 to be common to all protoplasm (Chap. 8). 



Modern biologists now recognize that the 

 physics and chemistry of protoplasm repre- 

 sent vitally important areas, and many re- 

 search workers are active in these fields. 

 Considerable attention will therefore be 

 given to protoplasmic structure, particularlv 

 in Chapters 4 and 5. 



Cellular Structure. It is generally recognized 

 that protoplasm seldom, if ever, occurs in 

 the form of any large continuous mass. 

 Rather, it is subdivided into small unit 

 masses, called cells, which usually are micro- 

 scopic in dimension. Moreover, the proto- 

 plasm of every typical cell consists of two 

 complementary and mutually dependent 



