Life at High Temperatures
In contrast to the simplistic definition of life as the quality that distinguishes a vital and functional being from a dead body, present-day biological sciences are mechanistically oriented, that is, cells and their inventory are functionally determined
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337
Introduction
Adaptation to Extremes of Physical Conditions . . . . . . . . . . . 338
In contrast to the simplistic definition of life as the quality that distinguishes a vital and functional being from a dead body, present-day biological sciences are mechanistically oriented, that is, cells and their inventory are functionally determined by the nonvitalist principle that living matter is composed of chemical substances obeying the fundamental laws of physics. Any biological function, including ecological adaptation, differentiation, and behavior, can be described in terms of the structures of those substances and the reactions that they undergo. However, one apparent difference between the life sciences, on the one hand, and physics or chemistry, on the other, deserves mentioning: Physics and chemistry study the unchanging properties of matter and energy, while the subject matter of biology (presently known organisms) is evolving, that is, includes only a subset that has managed to produce descendents under the changing physical conditions of the biosphere. Within the framework of biology and physical biochemistry, life refers to cellular organisms whose characteristics are (1) the capacity for metabolism (energy transformation), (2) growth, (3) response to stimuli, and (4) reproduction. Their constituent building blocks comprise a relatively small number of complex biomolecules (proteins, nucleic acids, carbohydrates, and lipids), with the first two serving as substrates for the process of evolution. Evolution occurs because natural selection favors, among all the combinations available, those individuals whose characteristics increase their reproduction in a particular environment. At this point, the physical or chemical conditions of the environment come into the play. As a consequence of the stochastic mechanism of ‘‘successful adaptation’’ to changing environmental conditions, all organisms are phylogenetically related to one another; in addition, they share most of the basic biochemical processes involved in replication, transcription, and translation and in the basic reactions governing metabolic and energy-transfer pathways. Thus, fundamental biochemical and biophysical problems may be studied in whatever organism is practical or convenient. In the context of this chapter, the specific properties of biomolecules from thermophilic microorganisms may provide us with a deeper understanding of general mechanisms underlying differences in the stability of proteins, nucleic acids, and lipids, as well as in their metabolic turnover.
Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 General Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339 Temperature Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 Hydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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