Life: Its Nature and Origin

By Jerome Alexander | Go to book overview

Chapter 5
What Are Living Units?

Many authorities maintain that it is not possible to draw a line of cleavage between living and non-living entities. The task is complicated by the fact that nature is so rich in examples and so gradual in most changes, that it is hard to fit all the data into the rigid frame of any definition. Nevertheless, we must first try to define what we mean by "living" units or entities.

In its broadest sense a living unit or entity is one that can direct chemical changes by catalysis,* and at the same time reproduce itself by autocatalysis, that is, by directing the formation of units like itself from other, and usually simpler chemical substances. Among the simplest known living units are genes, and some of the bacteriophages and ultrafiltrable viruses. which in size approximate molecular dimensions. The electron microscope indicates that some bacteriophages ("the bacteria of bacteria") and some viruses (e.g., that of psittacosis, or "parrot fever") are tiny organisms.

Figure 7, prepared by Dr. W. M. Stanley (Nobel Prize, 1946) shows that some viruses and bacteriophages approach molecular dimensions. See alsoFigure 8, an electron micrograph of tobacco mosaic virus.

Organisms are known which can synthesize their own organic compounds from inorganic substances. These are called autotrophs. The best known of this group are chlorophyll-containing plants, including algae, which by photosynthesis form organic compounds from carbon dioxide. The autotrophic bacteria may live in the absence of light, are generally microscopic, and are not distinguishable morphologically from other bacteria; but they derive their energy from the oxidation of inorganic substances and utilize it to reduce carbon dioxide to organic compounds. Intermediate between the autotrophic bacteria

____________________
*
Catalysis is discussed in the next chapter. In brief, it is the process whereby a specific particulate unit or surface (the catalyst) continuously brings about chemical union, breakdown, or structural change in other units as a result of very close contact or approach, under suitable conditions.

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Life: Its Nature and Origin
Table of contents

Table of contents

  • Title Page i
  • Preface iii
  • Introduction v
  • Contents x
  • Chapter 1 How Did Life Originate? 1
  • References 8
  • Chapter 2 the Smallest Particles of Matter 9
  • References 38
  • Chapter 3 How Molecules Make Masses 41
  • References 61
  • Chapter 4 the Importance of "Impurities" and Trace Substances 64
  • References 78
  • Chapter 5 What Are Living Units? 79
  • References 89
  • Chapter 6 Catalysis: the Guide of Life 90
  • References 136
  • Chapter 7 Immunology and Self-Saving Catalysts 140
  • References 152
  • Chapter 8 Genetics: the Heritable Transmission of Catalysts 154
  • References 177
  • Chapter 9 the Catalyst Entelechy in Differentiation and Morphogenesis 179
  • References 212
  • Chapter 10 Some Catalytic Aspects of Disease and Drugs 215
  • References 244
  • Chapter 11 Catalysis as the Efficient Cause of Evolution 246
  • References 257
  • Chapter 12 Philosophy, the Guide of Mental Life 259
  • References 275
  • Author's Note 277
  • Author Index 279
  • Subject Index 286
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