What Is Life?
What are the minimal descriptors of life – when do we decide that a collection of chemicals has crossed the line to that of inhering life processes?
The minimal essential features are compartmentalization, self-reproducibility, incorporation of energy controls, and adaptability to the environment. The essential features of biochemical systems include compartmentalization, genetic programming, and enzyme systems that both adapt to the environment (including feedback controls) and utilize or defy energy (generation of products in excess of non-enzymatic equilibria). In other words, if we put the most basic chemical constituents into a flask, we would not have life processes unless we also had the essential features listed above.
Compartmentalization within lipid-bilayer membranes permits seclusional selectivity and chemical control of reactants and products. Compartmentalization also permits the localization of energy storage/utilization, and the control of translation of genetic instruction. Further, membrane-bound systems control internal responses to the external environment. Self-reproducibility – regeneration – utilizes genetic programming. Molecular programming likely first utilized RNA, and later evolved DNA as a more stable system for storage of instructions.
PLoS Biology: What Is Life--and How Do We Search for It in Other Worlds?: " Koshland (2002) lists seven features of life: (1) program (DNA), (2) improvisation (response to environment), (3) compartmentalization, (4) energy, (5) regeneration, (6) adaptability, and (7) seclusion (chemical control and selectivity). A simpler definition is that life is a material system that undergoes reproduction, mutation, and natural selection. Cleland and Chyba (2002) have suggested that life might be like water, hard to define phenomenologically, but easy to define at the fundamental level. But life is like fire, not water--it is a process, not a pure substance. Such definitions are grist for philosophical discussion, but they neither inform biological research nor provide a basis for the search for life on other worlds."
The minimal essential features are compartmentalization, self-reproducibility, incorporation of energy controls, and adaptability to the environment. The essential features of biochemical systems include compartmentalization, genetic programming, and enzyme systems that both adapt to the environment (including feedback controls) and utilize or defy energy (generation of products in excess of non-enzymatic equilibria). In other words, if we put the most basic chemical constituents into a flask, we would not have life processes unless we also had the essential features listed above.
Compartmentalization within lipid-bilayer membranes permits seclusional selectivity and chemical control of reactants and products. Compartmentalization also permits the localization of energy storage/utilization, and the control of translation of genetic instruction. Further, membrane-bound systems control internal responses to the external environment. Self-reproducibility – regeneration – utilizes genetic programming. Molecular programming likely first utilized RNA, and later evolved DNA as a more stable system for storage of instructions.
PLoS Biology: What Is Life--and How Do We Search for It in Other Worlds?: " Koshland (2002) lists seven features of life: (1) program (DNA), (2) improvisation (response to environment), (3) compartmentalization, (4) energy, (5) regeneration, (6) adaptability, and (7) seclusion (chemical control and selectivity). A simpler definition is that life is a material system that undergoes reproduction, mutation, and natural selection. Cleland and Chyba (2002) have suggested that life might be like water, hard to define phenomenologically, but easy to define at the fundamental level. But life is like fire, not water--it is a process, not a pure substance. Such definitions are grist for philosophical discussion, but they neither inform biological research nor provide a basis for the search for life on other worlds."