Isolation of the Plasma Membrane

Plasma membranes of plants are purified by using either sucrose gradients or aqueous two-phase system consisting of Dextran T 500 and polyethyleneglycol 3350. The sucrose gradient separation takes advantage of the different sizes and densities of the various membranes in the cell, whereas the two-phase polymer system takes advantage of the different surface charges of membranes. This chapter describes the sucrose gradients and the aqueous two-phase system. The phase-separation procedure purifies one membrane at a time––that is, plasma membranes are purified in the upper phase while all other membranes are transferred to the lower phase. The sucrose gradient, on the other hand, can be prepared such that different membranes can be enriched at different sucrose interfaces at the same time. The procedure of choice for purifying the plasma membrane––that is, sucrose gradient or phase partitioning––will therefore depend upon the intended use of these membranes.