The rate of oxidation of cytochrome following absorption of a short pulse of light from a ruby laser in the photosynthetic bacterium Chromatium has been measured spectrophotometrically. The half-time is about 2 musec at room temperature increasing to 2.3 msec at about 100 degrees K and constant at the latter value to 35 degrees K or below. The temperature dependence above 120 degrees K corresponds to an activation energy of 3.3 kcal/mole; that below 100 degrees K to less than 80 cal/mol: essentially a temperature-independent electron transport reaction. Since the slowness below 100 degrees K indicates the presence of a barrier, the lack of activation energy is taken to mean penetration by quantum-mechanical "tunneling."

For solid-state physicists and engineers the “ultimate in miniaturization” would be to produce devices with structures that are about 8 or 10 nm across—about a tenth of the smallest scale that can currently be produced. (See PHYSICS TODAY, November 1979, page 25.) Biological systems, however, have, in a sense, solved the problems associated with such small microstructures. The fundamental unit of many cell functions, the lipid bilayer membrane (figure 1), is 4 nm thick; in regions where the membrane carries proteins it may be as much as 10 nm thick. Other elements of the cell, such as the microtubules that provide its structural framework, have similar dimensions.

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