Life

Deriving from the reductionism-holism debate, an important issue from the 1930s onward has been the extent to which living systems are ultimately reducible to molecules and chemical reactions. Biochemical definitions of life surfaced in the late nineteenth century with the discovery of enzymes as "living ferments" and became particularly prominent during the heyday of biochemical work (in England and Germany from 1920 to 1939) on enzyme-catalyzed pathways for synthesizing or degrading the major molecules in living systems. Many biochemists, flushed with success in elucidating the multistep pathways for fermentation or oxidation, attempted to define life in terms of enzyme catalysis. They held that what differentiated living from nonliving systems was the rapidity with which enzyme-catalyzed reactions and energy conversions could take place and the precision with which they could be controlled. The cell, one biochemist argued, is nothing more than a bag of enzymes. The biochemical view of life paid little if any attention to cell structure and organization, focusing almost exclusively on metabolic pathways, their interconnections, reaction kinetics, and energetics.

In the decades following the working out of the double helical structure of DNA by James D. Watson and Francis Crick in 1953, the biochemical definition of life was replaced by, or encompassed within, what came to be called the molecular view of life. The molecular view was more comprehensive than the biochemical, including the study of the three-dimensional structure of molecules, such as hemoglobin and myoglobin, and attention to cell structure and its relation to function, using techniques such as electron microscopy, ultracentrifugation, electrophoresis, fluorescence dyeing, and later confocal microscopy. Paradigmatic along these lines were detailed investigation of the structure of hemoglobin, the oxygen-carrying molecule in animal blood, and the discovery of its allosteric changes (positional shifts) in structure as it alternately bonds to and releases oxygen. The molecular and biochemical views of life tended to be highly reductionist, seeing life as merely a manifestation of molecular structure. Nonetheless, the molecular view did emphasize the importance of understanding life in terms of molecular configurations and the ways various molecules interacted chemically in such living processes as respiration, photosynthesis, protein synthesis, cell-to-cell communication, and signal transduction (the way a cell responds internally to receiving a specific message from the outside).

A particularly prominent aspect of the biochemical and molecular views of life has been the field of abiogenesis or the origin of life. Beginning with the work of the Russian biochemist A. I. Oparin (1894–1980) in the 1930s through that of Sidney Fox (1912–2001) from the 1950s to the 1990s, Stanley Miller in the 1950s, and Cyril Ponnamperuma in the 1970s, investigations as to how living systems might have originated on the primitive earth (or other extraterrestrial bodies) have gained considerable attention. Oparin showed that simple globular formulations that he called coacervates (formed from gum arabic and other organic substances in an aqueous medium) could perform simple functions analogous to living cells (movement, fission). Miller's experiments in the early 1950s demonstrated that basic amino acids, sugars, and other organic compounds (formic acid, urea) could be produced from components of what was hypothesized to have been the earth's early atmosphere (ammonia, carbon dioxide, water vapor, and hydrogen), thus giving credence to the view that life could indeed have originated on earth by simple biochemical processes. Later work of Fox and others on how the basic building blocks of organic matter (amino acids, simple sugars, nucleotides, and glycerides) could have become organized into macromolecules and basic cell structures showed that the origin of the next level of organization up from the molecule, the cell and its components, could be studied by experimental means. These investigations gave considerable support to the view that life is truly an expression, though an emergent one, of the basic properties of all matter, as understood through the analysis of atomic and molecular structure.