A spin-labeled fatty acid was incorporated in vivo into the polar lipids of Mycoplasma laidlawii membranes. The electron paramagnetic resonance signal from either intact cells or their extracted lipids reflected the fatty acid composition of the Mycoplasma membranes. Comparison of signals from intact cells, gramicidin-treated cells, heat-treated cells, and extracted lipids indicates that a major portion of the membrane lipids is in a semiviscous hydrocarbon environment. The results also show that the spin label in the intact membrane is slightly but significantly less mobile than it is in protein-free lipid extracts made from these membranes. Correlated electron microscope examinations using the freeze-etch technique reveal particulate components in the hydrophobic region of the membrane. The mobility of the lipids in the intact cell membrane may be influenced by their association with these particles.

Cell Membrane↗

PubMed1970

The septate junction: a structural basis for intercellular coupling.

Electron microscopy, freeze-etching, and optical diffraction show how the structure of the septate junction may provide the basis for the low-resistance pathway between the electrically coupled cells in mussel gill epithelia. Conventional electron microscopy suggests that the septa are pleated sheets that differentiate from and are structurally continuous with the junctional cell membranes. Freeze-etching exposes geometrically arranged rows of 85-A particles within the junctional cell membranes. Diffraction evidence shows that these membrane particles and the alternate vertices of the intercellular septal sheets are congruent and therefore superposable. Together, the membrane particles and septal sheets provide a channel that extends from the cytoplasm of one cell through the septate junction to the cytoplasm of the adjacent cell.

Animals↗

PubMed1970

Membrane splitting in freeze-ethching. Covalently bound ferritin as a membrane marker.

The freeze-etch technique was used to observe red blood cell ghosts labeled on both surfaces with covalently bound ferritin. Ferritin molecules were never observed on fracture faces, thus indicating that fracture does not show membrane-surface detail. Subliming away the surrounding ice did expose the ferritin on the membrane surface. These results were consistent with the concept that membranes split during the fracture process of freeze-etching.

Animals↗

PubMed1969

Changes in the plasma membrane of Escherichia coli during magnesium starvation.

The effect of Mg(++) starvation on the structure of the Escherichia coli cell membrane was studied with the freeze-etch technique. Special attention was paid to changes within the plane of the membrane, which in normal exponentially growing cells has a netlike arrangement of particles 2 to 6 nm in diameter. During Mg(++) starvation, a paracrystalline particle pattern appeared on the plasma membrane, and large areas devoid of particles were seen. Although these changes are reproducibly associated with Mg(++) starvation of the bacteria, no decrease in the Mg(++) content of the cell envelope per se was detected, even after 24 hr of Mg(++) deprivation. The structural changes caused by Mg(++) deprivation appeared to involve specific and permanent alterations in membrane development. The absence of other nutrients or divalent cations did not induce similar alterations.

Cell Membrane↗

PubMed1968

Fracture faces in frozen outer segments from the guinea pig retina.

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Animals↗

PubMed1967

Fracture planes in an ice-bilayer model membrane system.

Experiments with transferred stearate layers were performed to determine the location of fracture planes in frozen ice-lipid systems. Bilayers and multilayers of carbon-14-labeled stearate were frozen in contact with an aqueous phase and then fractured. The distribution of radioactivity on both sides of the fracture showed that the stearate layers were cleaved apart predominantly in the plane of their hydrocarbon tails. Because bilayers split in this manner, it was possible to measure time-dependent exchange of label between the layers. Exchange occurred with a half-time of 50 minutes in the presence of calcium and 25 minutes in the absence of calcium. Since stearate bilayers and multilayers are models of hydrophobically stabilized structures, the strong influence of their hydrophobic region on the fracture plane provides an explanation of how the freeze-etch technique of electron microscopy can expose inner, hydrophobic faces of cell membranes.

Carbon Isotopes↗

PubMed1967