Chapter 8
Chemical forces and self-assembly
The ant has made himself illustrious
Through constant industry industrious.
So What?
Would you be calm and placid
If you were full of formic acid?- Ogden Nash, 1935
Chapter 7 showed how simple free-energy transduction machines, like the osmotic pressure cell
(Figure 1.3 on page 11) or the heat engine (Figure 6.5 on page 190), generate mechanical forces
out of concentration or temperature differences. But while living creatures do make use of these
sources of free energy, their most important energy storage mechanisms involvechemicalenergy.
This chapter will establish chemical energy as just another form of free energy, mutually convertible
with all the other forms. We will do this by developing further the idea that every molecule carries
adefinite stored potential energy, and by adding that energy into the first term of the fundamental
formula for free energy,F=E−TS.Wewill then see how chemical energy drives the self-assembly
responsible for the creation of bilayer membranes and cytoskeletal filaments.
The Focus Question for this chapter is:
Biological question:How can a molecular machine, sitting in the middle of a well-mixed solution,
extract useful work? Doesn’t it need to sit at theboundarybetween chambers of different temper-
ature, pressure or concentration, like a heat engine, turbine, or osmotic cell?
Physical idea: Even a well-mixed solution can contain many different molecular species, at far-
from-equilibrium concentrations. The deviation from equilibrium generates a chemical force.
8.1 Chemical potential
Cells do not run on temperature gradients. Instead they eat food and excrete waste. Moreover,
the “useful work” done by a molecular machine may not be mechanical work, but rather may be
chemical synthesis.
©c2000 Philip C. Nelson