ABSTRACT
The cellular structure of biologic tissues was not fully recognized until the early nineteenth century, well over 100 years following the introduction of the microscope. The concept of the cell membrane as the structure that, in living cells, separates the cytoplasm from its extracellular surroundings evolved over the same period (see reviews by Kleinzeller [1994, 1996]); these membranes, however, were regarded as no more than passive and semipermeable envelopes shielding cell contents against toxic compounds and pathogenic organisms, and perhaps also reducing the loss of essential metabolites or cytoplasmic proteins.
