What is life? If you could ask an early eighteenth-century scientist this question, he or she would probably mention vital forces-mysterious, metaphysical energies that inhabit “organic” matter, and which keep organisms alive and functional. is concept is very old, preceding the philosophers of ancient Greece, perhaps even Egypt. Yet, despite the numerous scienti–c revelations of the last millennia in physics, chemistry, and physiology, such ideas advocating the metaphysical uniqueness of living matter enjoyed wide acceptance until about 150 years ago. e change came in the beginning of the nineteenth century, with the gradual spreading of mechanistic theories regarding Nature and physiology. ese philosophies posited that all life-related phenomena can be explained by the same physical and chemical principles that rule the inanimate world. An important breakthrough of this approach was achieved by Louis Pasteur (1822-1895), who demonstrated that the chemical process of converting sugar to alcohol (i.e., fermentation) was a result of the growth of microorganisms. In doing so, Pasteur made the link between life processes and chemical reactions. is was followed by studies of scientists like Marcellin Berthelot and Eduard Buchner (Figure 1.1), who demonstrated that fermentation, as well as other life-related processes, could be reproduced in the absence of the microorganism, by using substances extracted from it.* Although the chemical nature of these substances was at –rst unclear, they were later found (in all cases) to be proteins. ese proteins acted as catalysts, i.e., they accelerated chemical reactions within cells and tissues without changing their nature. is led to a major turning point in scienti–c thinking; life was no longer considered to be a result of mysterious and vague phenomena acting on organisms, but instead the consequence of numerous chemical processes made possible thanks to proteins. Indeed, this notion became the cornerstone of modern biochemistry and molecular biology. In addition to protein catalysts, which were named “enzymes,” many other noncatalytic yet functionally important proteins have been found since then. Perhaps the most
well-known example for the latter is hemoglobin, an animal protein functioning in the transport of oxygen from the lungs to body organs and tissues, as well as in the transport of CO2, a metabolic waste product, back to the lungs. e genetic revolution that started in the second half of the twentieth century with the deciphering of DNA structure, as well as the genetic code, has shown proteins to be more than just “molecular machines” active within cells and tissues; they are also the primary products of genes, responsible (among other things) for the expression of genetic information.