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From: richard ruquist <yann...@yahoo.com>
Date: Mon, Sep 10, 2012 at 11:10 AM
Subject: Fw: the physics arXiv blog
To: "yann...@gmail.com" <yann...@gmail.com>
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*From:* Technology Review Feed - arXiv blog <ho...@arxivblog.com>
*Sent:* Monday, September 10, 2012 8:18 AM
*Subject:* the physics arXiv blog
the physics arXiv blog <http://www.technologyreview.com/>
Physicist Derives Laws of Thermodynamics For Life
Posted: 10 Sep 2012 03:58 AM PDT
The laws of thermodynamics must apply to self-replicating systems. Now one
physicist has worked out how
Here's an interesting thought experiment. Imagine a box filled with a
variety of atoms and molecules in proportions roughly equivalent to the
composition of the prebiotic soup in which life thrives.
How likely is it that these molecules will arrange themselves into
fully-fledged living thing, a bacterium, for example? That's a tough
question but Jeremy England at the Massachusetts Institute of Technology in
Cambridge has worked out how to calculate an answer, at least in theory.
His results make for fascinating reading.
Part of the problem here is that life itself is hard to define. But England
has a way round this. His idea is to examine every combination of states
that are possible in this box and to consult an omniscient microbiologist
about whether each state represents a bacterium or not. In that way, it
ought to be possible, at least in principle, to gain an idea of the
statistical physics involved.
Next, he asks the microbiologist to take another look at the box after a
period that is roughly equivalent to the time it takes for bacteria to
The question then is how likely is it that there will be two bacteria in
Once again, the omniscient microbiologist could look at every possible
state of the box and say whether or not self replication has taken place.
If the box contains two bacterium, it's possible to work how much entropy
has been created in the process and how much heat used.
England throws in some basic laws of thermodynamics and in this way builds
a statistical physics model of self replication, a model that is analogous
to the laws that govern the statistical behaviour of any set of particles
in a box.
By way of comparison, he also looks at the statistics that govern the
reverse process--the spontaneous decomposition of the bacteria into carbon
dioxide, hydrogen and so on.
This sets an important bound on what is thermodynamically possible in this
system: in effect, England derives the second law of thermodynamics for the
system. From this he works out various 'laws' such as the minimum amount of
heat that a single round of cell division ought to produce.
Finally, he puts some numbers into his model, including figures such as the
life time of peptide bonds in biological systems, to find out how much heat
complex systems like E. coli bacteria ought to produce when they replicate.
It turns out that E. coli bacteria are remarkably efficient replicators.
"The organism can convert chemical energy into a new copy of itself so
efficiently that if it were to produce even half as much heat it would be
pushing the limits of what is thermodynamically possible!" he says.
He does a similar calculation for the replication of RNA and DNA molecules.
This suggests that in terms of thermodynamics, replication is much easier
for RNA than DNA.
That's an interesting result given that many biologists have suggested that
the first self-replicating systems in Earth's pre-biotic soup must have
been based on RNA rather than DNA
In the past, biologists have studied the catalytic properties of RNA that
are crucial for living cells and noted that DNA does not share these
properties. So the thinking is that RNA must have come first in the
replicating timeline, with DNA evolving later as life became more complex .
England's work backs up this idea but for completely different reasons--RNA
is thermodynamically better at self replication. A fascinating result.
The work has an important limitation, however. It fails to tackle the
definition of nature of life and instead defers the problem to an
omniscient microbiologist who, it is assumed, can always provide an answer.
There is a tantalising hint that England's approach could one day solve
this problem. By exploring the role of statistical physics in more detail,
it maybe possible to define life in terms of precise thermodynamic limits.
Which is why it'll be worth watching where England takes his idea next.
Ref: arxiv.org/abs/1209.1179: Statistical Physics of Self-Replication
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