Melvin CalvinMelvin Calvin was an American biochemist who won the Nobel Prize in 1961 for discovering (in 1950) one of the many autocatalytic "hypercycles" that are fundamental to metabolism in living things. The so-called "Calvin cycle" is a part of the photosynthesis in plants that converts carbon dioxide to glucose. Manfred Eigen made "hypercycles" famous and won the 1967 Nobel Prize for explaining (in 1950) the fast chemical reactions they involve. In 1932 Hans Krebs discovered the first hypercycle, the urea cycle which converts ammonia (NH3) to urea (NH2 - O - NH2). Urea in turn is used as fertilizer for plants. The hypercycle in humans that is parallel to the Calvin cycle in plants is the citric-acid cycle. It was also discovered by Krebs in 1950 and often called the Krebs cycle. Using the radioactive isotope carbon-14, Calvin and colleagues followed the path that carbon follows in photosynthesis, starting with CO2 from the atmosphere and ending up as carbohydrates that are the principal food for animals. The cycle uses two energy-rich ATP molecules to do the conversion of inorganic atmospheric carbon dioxide to food, perhaps very important in slowing future climate change. In 1969, Calvin published his book Chemical Evolution, Molecular Evolution Towards the Origin of Living Systems on the Earth and Elsewhere. Although he was reluctant to offer a definition of life and its beginnings, Calvin gives us the essentials as he saw them in the 1960's.
At this point I do not seek to define a ‘living system’ in abstract terms that might be applicable to any collection of matter: for example, for use in extraterrestrial exploration, or perhaps in defining the nature of man. I shall defer these discussions until we have explored the specific problem of terrestrial life as we know it, namely, the elements of which it is constructed and the manner in which these elements are organized and function. That function in a first approximation seems to be the directed use of energy to create order from a disordered, or less ordered, environment: in biological terms, growth and differentiation. We could also use chemical or thermodynamical terms, such as the principle in irreversible thermodynamics calling for a ‘minimum rate of entropy increase’, which might have a requirement for a maximum ‘biomass’ as a corollary. But I think this would lead to some confusion, and I shall limit myself to the more familiar biological terms. Further, function seems to be to generate and transmit the ‘programme’ for growth and differentiation to another system, that is, reproduction. Finally, function includes change in the ‘programme’ in response to a changing environment; the correlative biological terms would be mutation and selection. The general types of materials of construction (shown in Table 5.2) are fairly well known. There are four major biopolymers: the proteins, nucleic acids, polysaccharides, and lipids. In addition there is a wide variety of small molecules that function in energy manipulations (flavins, chlorophylls, haem, ATP, coenzyme Q, etc.) and in material manipulation (coenzyme~A, vitamin B12), and there are also signal transmission materials (hormones, pheromones), and the like. These materials are organized into highly ordered structures at all levels, starting with the simple molecules. The patterns of the cellular and the sub-cellular structures and the details of the substrate structures are already well known. The polymer molecules, including enzymes, are organized into the ribosomes and mitochondria, chromosomes, quantasomes, etc. From this point up to the higher organelles of the cell (cytoplasmic structures, nuclei, membranes), the organization can be examined by the aid of such instruments as the electron microscope. I am assuming, then, that the materials of which the living cell is composed, and the level of organization that will be recognized, are known to the reader. We shall review the question of self-organizing systems later.Normal | Teacher | Scholar