The plasma membrane forms the living barrier between the cell and its surroundings. For this reason it has a wide range of important functions related to the regulation of the composition of the cell interior and to com munication with the cell exterior. The plasma membrane has therefore attracted a lot of research interest. Until the early 1970''s it was only pos sible to study the plasma membrane in situ, its structure e. g. by electron microscopy and its function e. g. by uptake of radioactively labeled com pounds into the intact cell or tissue. The first isolation of plant protoplasts by enzymatic digestion of the cell wall in the early 1970''s was an important step forward in that it provided direct access to the outer surface of the plasma membrane. More importantly, T. K. Hodges and R. J. Leonard in 1972 published the description of a method by which a fraction enriched in plasma membranes could be isolated from plant tissues using sucrose gradient centrifugation. As a result, the 1970''s saw a leap forward in our understanding of the structurc and function of the plasma membrane. In 1981, S. Widell and C. Larsson published the first of a series of papers in which plasma membrane vesicles of high yield and purity were isolated from a wide range of plant tissues using aqueous polymer two-phase parti tioning.
