A Biological Constant

Membranes, Ions
and Impulses,
by Kenneth Cole

Physics has many famous constants: Planck’s constant, the speed of light, and the gravitational constant, to name a few. Biology has few such constants. Life is so full of variety that almost any parameter can vary between species or tissues. In fact, physicists differ from biologists by their focus on the unity rather than the diversity of life.

There is, however, one parameter that comes close to being a biological constant. All cells are surrounded by a membrane whose thickness and composition varies little among species. Therefore, the capacitance per unit area, C_m, of a membrane is as close to being a biological constant as you are likely to find.

In Section 6.17 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I calculate the capacitance of a lipid bilayer, and find that C_m is about 0.01 farads per square meter. Many of the key papers during the “golden age” of classical biophysics didn’t use standard SI units. Instead of measuring distance in meters, they used centimeters. If you express C_m as per square centimeter, and if you use microfarads instead of farads, you get the easy-to-remember value of C_m = 1 μF/cm². Kenneth Cole wrote in his book Membranes, Ions and Impulses: A Chapter of Classical Biophysics

This figure of about 1 μF/cm² has been so confirmed and refined, extended and approximated for membranes of red cells and almost all other living cells, as to become a biophysical constant.

Are there other biological constants? I suppose some constants governing the structure of key biological molecules, such as the distance between adjacent base pairs of the DNA double helix (0.34 nm), are conserved throughout biology. But these parameters belong more to the realm of biochemistry than biophysics. If you restrict your selection to parameters discussed in IPMB, C_m is one of the few biological constants.

Membranes, Ions and Impulses, by Kenneth Cole.