The forces that power the molecular machine 1

Biological Physics is in essence a book about statistical physics, but written for biologists rather than physicists. Inevitably, the style is more chatty and less mathematical than might be found in a conventional text on the same subject.

Quite possibly it lacks some rigour as a result, but it does make the subject accessible, though probably more so to physicists than to biologists. Most biologists of my acquaintance not only greatly prefer biology to physics but they also have a strong disliking for mathematics. And although Philip Nelson avoids lengthy mathematical derivations, maths is integral to the subject and therefore unavoidable. Still, for any biologist wanting to learn about the physics of random biological processes this is probably as good a place as any to start.

What distinguishes this book from other statistical physics texts is the clear links established between biological processes at the molecular and cellular level and the physics of random phenomena.

The physics is standard - the thermo-statistical view was established by the early 20th century - but its applicability to biology is more often hinted at than described in the sort of detail found here, where applications are described alongside the statistical thermodynamics so that the physics and biology are fully integrated.

Nelson opens the book with a wide-ranging look at the concept of energy, and in particular at the first and second laws of thermodynamics. These deal respectively with energy conservation and the entropic principle and form the foundations of the subject.

Subsequent chapters describe cellular structure, diffusion, random walks, viscosity and flow processes before returning to a detailed discussion of entropy in chapter six. This chapter is almost entirely concerned with physics and includes such subjects as the Maxwell-Boltzmann speed distribution and ideal gases, concepts that might seem remote, possibly even abstract, to a biologist. Even here, though, some attempt is made to apply the ideas through the thermodynamics of single molecule folding. This integrated approach is quite interesting and the way that the material is presented makes this an excellent introductory text even for physics students.

The lack of rigour is most apparent in chapter seven, "Entropic forces at work". Originally I had imagined that this would be a metaphor for the approach to equilibrium through the equivalent processes of minimising the free energy or maximising the entropy, but there is nothing especially entropic about pressure, electrostatic forces, or the forces involved in single molecule stretching. I regard this as a small flaw in what is otherwise a very good book.

Later chapters are devoted to the behaviour of molecules, membranes and molecular machines and draw heavily on the foundations laid in the first half of the book.

Nelson has made the seemingly arcane topics of moving pistons and colliding gas molecules seem relevant to biology. Perhaps I'm an optimist, but I'd like to think that any biologist patient enough to work through the physics in this book will find his or her understanding of complex biological processes enhanced.

Who is it for? Additional "Track-2" sections contain more advanced material for senior physics majors and graduate students.

Changes since last edition: New end-of-chapter problems.

Presentation: Well structured, clearly laid out and with plenty of diagrams.

Would you recommend it? Yes.

David Sands is a lecturer in physics at Hull University.