Fig. 2:

Nomenclature for describing the aspects of membranes revealed by freeze fracture. The membrane comprises a lipid bilayer with intercalated proteins. For the plasma membrane, the half-membrane leaflet adjacent to the extracellular space is termed the E half; that adjacent to the protoplasm is termed the P half. The term fracture face is applied to the interior views of membranes exposed by freeze fracturing, while the term surface is used for the true, natural surfaces of the membrane. The fracture face of the P half is thus termed the P face (or PF), while that of the E half is termed the E face (or EF). The true surfaces of the membrane are correspondingly termed the P surface and the E surface (PS and ES), respectively. The terminology is similarly applied to intracellular membranes. When membrane vesicles are produced from the plasma membrane by endocytosis, the P half of the membrane forms the outer monolayer of the vesicle membrane and the E half forms the inner monolayer. The interior of the vesicle is derived from the extracellular space. Thus, convex fractures of vesicles show the E face, and concave fractures the P face of the vesicle membrane. Corresponding spatial relationships apply to all the membrane systems of the cell. The term "E" designates the interior of all single-membrane organelles and the spaces between inner and outer membranes of all double membrane-bound organelles (nucleus, mitochondria and chloroplasts). The term "P" encompasses cytoplasm, nucleoplasm, the matrix of mitochondria and the stroma of chloroplasts.