GRANULO‐FIBRILLAR AND GLOBULAR SUBSTRUCTURE IN UNIT MEMBRANES *

The unit membrane concept of the molecular pattern of organization of biological membranes that has evolved during the last ten years mainly from studies of developing peripheral myelinated nerve fibers has received wide acceptance.I3 According to this concept. cell membranes are characterized by the general pattern of organization indicated in the diagram in FIGURE 1. The concept states that a membrane consists of a single continuous bilayer of lipid with the nonpolar carbon chains at the center of the membrane and the polar ends pointed outwards. The polar surfaces of the lipid bilayer are each covered by monomolecular films of non-lipid. The outside surface of the membrane is chemically different in some important way from the inside surface. The membrane is thus a chemically asymmetric structure, as indicated in the diagram. The concept says nothing about the particular molecular species present in either the lipid or the non-lipid layers. It deals only with the general pattern of organization of the constituent molecules. It sets a general limit on the thickness of membranes, although this is not rigid since the overall thickness tends to vary considerably as the molecular species making up the pattern vary from membrane to membrane. The important point in this connection is the limitation of the number of lipid monolayers to two. In recent years, there has been increasing interest in the problem of defining substructure within the unit membrane pattern. There have also been efforts to relate certain apparent appendages of membranes to substructure of the membrane i t ~ e l f . ~ , ~ Some of the attempts at definition of substructure have been abortive, but it now appears that a kind of substructure is present that may be of general significance. It is the purpose of this paper to present certain new evidence relating to this topic so as to bring it up to date. No effort will be made to review the entire subject exhaustively, since this has been done elsewhere in other recent review articles.G8 One of the principal problems in dealing with membrane substructure that has not been adequately appreciated in the past is the extreme delicacy of cell membranes. This renders such techniques as the phosphotungstic acid (PTA) negative staining method of limited usefulness, because it is necessary in the procedure to dry down membranes on a substrate in the presence of PTA. In general, membranes have not been found to stand this, and they usually have been converted into some other structure that may display various inconstant patterns in the dry state. Some progress has been made in the case of chloroplast membranes, not by the negative staining method, but by metallic replication by shadow-casting dried membranesg and by the freeze-etching method.1° The studies of chloroplast membranes by Parkg in particular have resulted in apparently meaningful information. He has demonstrated a subunit pattern repeating at a period of about 150A in a regular array. Each of the subunits have also been found to contain a smaller unit of about 90A diameter. This finding appears to be significant, although the drying of the material no doubt introduces artifacts. The 90A subunit