Membrane permeability is fundamental to all cell biology and subcellular biology. The cell exists as a closed unit. Import and export depend upon a number of sophisticated mechanisms, such as active transport, endocytosis, exocytosis, and passive diffusion. These systems are critical for the normal housekeeping physiological functions. However, access to the cell is also taken advantage of by toxic microbes (such as cholera or ptomaine) and when designing drugs.

Ernest Overton, one of the pioneers in lipid membrane research, put forward the first comprehensive theory of lipid...