The Fine Structure of Membranes and Intercellular Communication in Insects

The cell membrane has been the subject of intensive investigation and recent review (8,21,83,90,93). Despite obvious compositional and func­ tional differences between membranes of different organisms or even between different membrane systems within any given cell, the notion has emerged that most membranes are built on a fundamentally similar plan (Figure 1). In general, the concepts arising from studies of membrane structure can be summarized as follows: 1. Lipid bilayers (17) -lamellar lipid phases-provide the basis for structural and generalized self-assembly properties of membranes. 2. Lipid bilayers provide a structural basis for generalized membrane permeability properties, partially the function of the membrane as a physi­ cal permeability barrier and its relative permeability to lipid soluble sub­ stances. 3. Proteins are associatcd with the hydrophilic surfaces of the bilayer and may also be incorporated into the hydrophobic regions of the bilayer. The membrane interior consists of the hydrophobic regions of the lipid bilayer, in­ terrupted at intervals by such proteinaceous intercalations. 4. Proteins may be organized into specific arrays within the membrane to produce localized regions of functional membrane differentiation. The physi­ cal techniques used to study membranes are mainly averaging techniques and do not imply that localized portions of the membrane could not be specially structured. In particular, recent work with labeling and freeze-etching tech­ niques has emphasized the mosaic character of the membrane. 5. Proteinaceous components convey much of the functional information for a membrane including ionic permeability, antigenic properties, and enzy­ matic activities. 6. Sugars are found asymmetrically distributed on most membranes. They are generally present on the external surface of the plasma membrane where they may be prime determiners of such regulatory activities as cell recognition, growth, and transport. 7. Membranes are generally � 75 A thick. They can be resolved in thin