The mitochondria of aerobically grown yeast, in common with those of higher organisms, contain the terminal electron transport, oxidative phosphorylation, and citric acid cycle enzymes of the cell (9). However, unlike higher organisms, the yeast cell is capable of both aerobic and anaerobic growth, and, as was reported some years ago, the b and a cytochromes are not synthesized in the absence of oxygen but are adaptive enzymes formed in response to the presence of oxygen (2, 7).This communication compares some of the cytology and enzymology of the yeast Torulopsis utilis grown both aerobically and anaerobically. It is shown that, indeed, none of the respiratory chain cytochromes normally associated with the mitochondrial matrix are synthesized by T. utilis

Vegetative cells of Saccharomyces cerevisiae were fixed with potassium permanganate followed by uranyl nitrate, embedded in methacrylate, and studied in electron micrographs of thin sections. Details of the structure of the cell wall, cytoplasmic membrane, nucleus, vacuole, and mitochondria are described. Cell membranes, about 70 to 80 A thick, have been resolved into two dense layers, 20 to 25 A thick, separated by a light layer of the same dimensions, which correspond in thickness and appearance to the components of the "unit membrane" as described by Robertson (15). The cell wall is made up of zones of different electron opacity. Underlying the cell wall is the cytoplasmic membrane, a sinuous structure with numerous invaginations. The nucleoplasm, often of uneven electron opacity, is enclosed in a pair of unit membranes in which nuclear pores are apparent. The vacuole, limited by a single unit membrane, is usually irregular in outline and contains some dense material. Rod-shaped mitochondria, 0.4 to 0.6 g in length and 0.2 to 0.3 g in diameter, are smaller in size, but similar in structure to some of those described in plant and animal cells. Attempts to use osmium tetroxide as fixative were unsuccessful, a result similar to that obtained by other workers. It is suggested that yeast cells are impermeable to osmium tetroxide, except when grown under specific conditions.

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