Morphologically intact structures have been isolated from anaerobically grown yeast cells which have many of the properties of yeast mitochondria. The structures are about 0.5 µ in diameter and contain malate dehydrogenase, succinate dehydrogenase, oligomycin-sensitive ATPase, and DNA of buoyant density 1.683 g/cc, characteristic of yeast mitochondria. The morphology of the structures is critically dependent on their lipid composition. When isolated from cells grown anaerobically in the presence of supplements of unsaturated fatty acid and ergosterol, their unsaturated fatty acid content is similar to that of mitochondria from aerobically grown cells. These lipid-complete structures consist pre-dominantly of double-membrane vesicles enclosing a dense matrix which contains a folded inner membrane system bordering electron-transparent regions which are somewhat different from the cristae of functional mitochondria. In contrast, the structures from cells grown without lipid supplements are much simpler in morphology; they have a dense granular matrix surrounded by a double membrane but have no obvious folded inner membrane system within the matrix. The lipid-depleted structures are very fragile and are only isolated in intact form from protoplasts that have been prefixed with glutaraldehyde

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