Wednesday, August 28, 2013

Bacteria Grow Rather Efficiently

Things that live and grow must, like all things, obey the laws of physics, including the Second Law of Thermodynamics, even if living organisms appear highly ordered. In closed systems (ones that are isolated from their surroundings), the degree of disorder (entropy) always increases until they reach thermodynamic equilibrium, at which point entropy change becomes zero. Similarly, the total entropy of the universe is always increasing. A living organism is not a closed system but an open one, since it exchanges energy with its surroundings. If the entropy of an open system is decreasing, then it must increase the entropy of its surroundings by an even greater degree so that the total entropy of the universe increases.

Living organisms increase the entropy of their surroundings over the course of their lives by generating heat (largely through the controlled burning - or oxidation - of glucose, which produces carbon dioxide and heat). The localized decrease in entropy that occurs during biosynthesis is compensated for by the greater increase in entropy of the universe that results from the production of heat through catabolic metabolism. This raises the following question. How much heat must an organism produce in order to live and grow and how does this compare to the minimum that would be required to obey the Second Law of Thermodynamics?

Jeremy L. England addresses this question in a paper in the Journal of Chemical Physics. It turns out that bacterium Escherichia coli produces only about six times the amount of heat than the minimum required to live and divide in accordance with with the Second Law of Thermodynamics. This implies a high though not maximal degree of efficiency, since the experimental value (previously determined by others and not the author of this particular article) is within an order of magnitude of the minimum value calculated by the author of this paper. So E. coli are substantially but not perfectly optimized for efficient growth.

It is possible that, even though E. coli evolved to grow rapidly when the conditions were good, all life requires at least some amount of "waste" for the sake of robustness. Having some excess energetic capacity allows for mobilization of resources needed for appropriate responses when environmental conditions change, even if that capacity is not needed under ideal conditions.


J. Chem. Phys. 139, 121923 (2013); (8 pages)

Statistical physics of self-replication

Jeremy L. England
Department of Physics, Massachusetts Institute of Technology, Building 6C, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA 
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(Received 28 April 2013; accepted 1 August 2013; published online 21 August 2013)

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Self-replication is a capacity common to every species of living thing, and simple physical intuition dictates that such a process must invariably be fueled by the production of entropy. Here, we undertake to make this intuition rigorous and quantitative by deriving a lower bound for the amount of heat that is produced during a process of self-replication in a system coupled to a thermal bath. We find that the minimum value for the physically allowed rate of heat production is determined by the growth rate, internal entropy, and durability of the replicator, and we discuss the implications of this finding for bacterial cell division, as well as for the pre-biotic emergence of self-replicating nucleic acids.

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