Chapter 5

Life in the slow lane: the low

Reynolds-number world

Nobody is silly enough to think that an elephant will only fall

under gravity if its genes tell it to do so, but the same underlying

error can easily be made in less obvious circumstances. So [we

must] distinguish between how much behavior, and what part,

has a genetic origin, and how much comes solely because an

organism lives in the physical universe and is therefore bound by

physical laws. – Ian Stewart,Life’s Other Secret

Before our final assault on the citadel of statistical physics in Chapter 6, this chapter will show
how the ideas we have already developed give some simple but powerful conclusions about cellular,
subcellular, and physiological processes, as well as helping us understand some important laboratory
techniques. One key example will be the propulsion of bacteria by their flagella (see Figure 2.3 on
page 32b).
Section 4.4.1 described how diffusion dominates transport of molecules in the nanoworld. Diffu-
sion is a “dissipative” process: It tends to erase ordered arrangements of molecules. Similarly, this
chapter will outline how viscous friction dominatesmechanicsin the nanoworld. Friction, too, is
dissipative: It tends to erase orderedmotion,converting it to thermal energy. The physical concept
of symmetry will help us to understand and unify the sometimes surprising ramifications of this
statement.
The Focus Question for this chapter is:
Biological question:Why do bacteria swim differently from fish?
Physical idea:The equations of motion appropriate to the nanoworld behave differently undertime
reversalfrom those of the macroworld.
©c2000 Philip C. Nelson

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