Chapter 11
Machines in membranes
In going on with these Experiments how many pretty Systems
do we build which we soon find ourselves oblig’d to destroy! If
there is no other Use discover’d of Electricity this however is
something considerable, that it mayhelp to make a vain man
humble.–B.Franklin to P. Collinson, 1747Chapter 12 will discuss the question of nerve impulses, the electrical signals running along
nerve fibers that make up the ghostly fabric of thought. Before we can discuss nerve impulses,
however, this penultimate chapter must look at how living cells generate electricity in the first
place. Chapter 4 skirted this question in the discussion of the Nernst formula; we are now ready to
return to this question as a matter of free energy transduction, armed with a general understanding
of molecular machines. We will see how indirect, physical arguments led to the discovery of a
remarkable class of molecular machines, theactive ion pumps,long before the precise biochemical
identity of these devices was known. The story may remind you of how Muller, Delbr ̈uck, and their
colleagues characterized the nature of the genetic molecule, using physical experiments and ideas,
many years before others identified it chemically as DNA (Section 3.3.3). The interplay of physical
and biochemical attack on life-science problems will continue to bear fruit as long as both sets of
researchers know about each others’ work.
The Focus Question for this chapter is:
Biological question:The cytosol’s composition is very different from that of the outside world. Why
doesn’t osmotic flow through the plasma membrane burst (or shrink) the cell?
Physical idea:Active ion pumping by molecular machines can maintain a nonequilibrium, osmoti-
cally regulated state.
11.1 Electro-osmotic effects
11.1.1 Before the ancients
The separation of the sciences into disciplines is just a modern aberration. Historically there was a
lively interplay between the study of bioelectric phenomena and the great project of understanding
©c2000 Philip C. Nelson409