Читать книгу Astrobiology - Charles S. Cockell - Страница 118

The minimum size of a cell is an important measure, since it defines the size below which we would question whether a putative fossil or living cell was a self-replicating living organism. For a cell to grow and reproduce, it must have some minimal machinery to produce proteins needed for division. It must have other molecules and proteins to gather energy and carry out basic functions such as replicating genetic information. Consider the following statement from a United States National Research Council report on the “Size Limits of Very Small Microorganisms”: “Free-living organisms require a minimum of 250–450 proteins along with the genes and ribosomes necessary for their synthesis. A sphere capable of holding this minimal molecular complement would be 250–300 nm in diameter, including its bounding membrane. Given the uncertainties inherent in this estimate, the panel agreed that 250 ± 50 nm constitutes a reasonable lower size limit for life as we know it.” Discuss this statement and whether you agree with the assessment of the minimum number of proteins required for life. How would the minimum size of a cell be different if it required 10 times this number of proteins or 50 times this number?

United States National Research Council (1999). Size Limits of Very Small Microorganisms: Proceedings of a Workshop, 1999. Washington, DC: National Academies Press (available online: http://www.nap.edu/books/0309066344/html).