U MMARYI. By freeze fracturing electron microscopy regular band patterns were visualized on fracture faces of liposomes prepared from dimyristoylphosphatidylcholine, dielaidoylphosphatidylcholine, and I-oleoyl-2-stearoylphosphatidylcholine below their respective transition temperatures. Above these temperatures only smooth fracture faces were observed.2. Liposomes of these phospholipids prepared after addition of more than 2o mole % cholesterol exhibited no band patterns below their transition temperatures.3-Fracture faces of the membrane of Acholeplasma laidlawii B (previously denoted as Mycoplasma laidlawii) below the transition temperature showed structural details that can be attributed to a redistribution of membrane molecules as a consequence of the solidification of the membrane.
A B S T R A C TA basis for the interpretation of the structure of the cell membrane is often looked for in electron microscope investigations on the structure of lipid models. A difficulty in these investigations is our lack of knowledge of the effect of the preparative treatment on the structure studied. This applies especially to the strongly oxidizing fixatives: osmium tetroxide and potassium permanganate. These agents have, moreover, the drawback that they cannot be used to fix fully saturated lipids. On the basis of existing theories concerning complex colloid systems, a fixation method was developed that allows the electron microscope study of the structure of phospholipid models, irrespective of whether they consist of saturated or unsaturated compounds. With this fixation, namely tricomplex flocculation by means of suitable ions, the structure of the lipid molecules is not altered. Moreover, the site and mode of action of this fixation are well known. The first application of this method to the study of isolated beef brain phospholipids is described.
Acid phosphatase is present in two layers of the cell envelope of Saccharomyces cerevisiae. These are separated by another layer, which is free of acid phosphatase. We have evidence that the cell wall is built up in two stages, which are independent. In the first stage, the cell wall is built up during the formation of the bud. Glucanase vesicles are involved in this process. In the second stage, a thick layer is deposited at the inside against the new cell wall. This results in the thick, rigid wall of the mature yeast cell. This latter layer is probably assembled on the outer surface of the plasmalemma.
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