22. Proteins and Nucleic Acids: Information Carriers          Previous PageNext Page
       What are the Criteria for Life?


This is the universal and essential thing that a living creature does, because it is the means by which life continues. Higher organisms undergo a cycle of birth, sexual reproduction, and death. Many lower organisms propagate by fissioning, budding, or subdividing in some way, and experience individual death only by accident. Viruses reproduce, but only with the aid of other kinds of organisms. All living things propagate in some way, and life goes on.

This continuation of life in the family of organisms or in the individual is different from a static enduring. Rocks and minerals endure, and the material within them remains unchanged. A living creature, in contrast, maintains the same form amid a continuous exchange of molecules with its surroundings. Its individual molecules come and go, but its structure and organization persist. It maintains its identity in the midst of a constant flow-through of matter.



Living creatures generally increase in size and complexity with the passage of time. They go through a controlled, predictable life cycle or pattern. This pattern of development is not a product only of simple physical forces (as is the "healing" of a broken crystal), but of programmed, prestored information contained in DNA molecules. The proper analogy is not with a bubbling pot or a growing crystal, but with a programmed digital computer, although the computer analogy is grossly insulting to even the simplest bacterium.


Living organisms take chemical substances and free energy from their environment and modify both for their own particular needs. These processes involve chemical transformations: both spontaneous breakdowns that release free energy, and nonspontaneous syntheses that must be driven by some other free-energy source. For the analogy between cell growth and crystal growth to be valid, one would have to propose that a crystal of calcium carbonate (limestone), if dropped into a calcium chloride solution, could grow by ignoring the chloride ions around it and taking C02 from the atmosphere to make carbonate ions.

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