Chapter 3

The molecular dance

Who will lead me into that still more hidden and dimmer region

where Thought weds Fact, where the mental operation of the

mathematician and the physical action of the molecules are seen

in their true relation? Does not the way pass through the very

den of the metaphysician, strewed with the remains of former

explorers? – James Clerk Maxwell, 1870

The previous chapter made it clear that living cells are full of fantastically ordered structures,
all the way down to the molecular scale. But Chapter 1 proposed that heat is disorganized molec-
ular motion, and tends to destroy order. Does that imply that cells work best at the coldest
temperatures? No, life processesstopat low temperature.
Towork our way out of this paradox, and ultimately own the concept of free energy sketched
in Chapter 1, we must first understand more precisely the sense in which heat is a form of motion.
This chapter will begin to explain and justify that claim. We will see how the idea of random
molecular motion quantitatively explains the ideal gas law (Section 1.5.4), as well as many common
observations, from the evaporation of water to the speeding-up of chemical reactions when we add
heat.
These physical ideas have an immediate biological application: As soon as we appreciate the
nanoworld as a violent place, full of incessant thermal motion, we also realize just how miraculous
it is that the tiny cell nucleus can maintain a huge database—your genome—without serious loss of
information over many generations. Section 3.3 will see how physical reasoning led the founders of
molecular biology to infer the existence of a linear-chain molecule carrying the database, decades
before the actual discovery of DNA.
Here is a question to focus our thoughts:
Biological question:Why is the nanoworld so different from the macroworld?
Physical idea:Everything is (thermally) dancing.

©c2000 Philip C. Nelson

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