MEMBRANOUS STRUCTURES IN YEASTS

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Abstract
Summary: 1. Most yeast cells carrying out active respiration have spherical or ellipsoidal mitochondria, with plate‐like cristae.2. Cytoplasmic petite strains of Saccharomyces cerevisiae have aberrant mitochondria, often containing whorled membranes. Mutants with deficiencies in the tricarboxylic acid cycle have mitochondria which appear normal when the cells are grown in low levels of glucose.3. Cells of normal and petite S. cerevisiae grown strictly anaerobically show no recognizable mitochondrial profiles.4. Carbon substrates which can only be respired promote the development of well‐defined mitochondria. In certain facultatively anaerobic yeasts respiration is suppressed by glucose and the mitochondria under these conditions are large, pleomorphic and few in number. Other fermentable carbohydrates do not give this repression.5. A number of antibacterial antibiotics, which inhibit mitochondrial protein synthesis, cause a disorganization of the mitochondrial cristae.6. In yeast cells adapting from anaerobic to aerobic conditions mitochondria appear to develop from proliferations of the endoplasmic reticulum, which become progressively more organized.7. Vacuoles often contain granular material, but in S. cerevisiae the vacuole, which has been described as a lysosome, frequently contains myelin‐like lipid inclusions. The material in these inclusions is apparently derived from spherosomes.8. Endoplasmic reticulum, orientated parallel to the plasmalemma, may be associated with fermentative ability in certain facultatively anaerobic yeasts. Endoplasmic reticulum is also actively involved in the budding process.9. Normally the yeast‐cell plasmalemma shows only minor convolutions, but in chloramphenicol‐grown Rhodotorula glutinis the plasmalemma produces vesicular structures termed ‘paramural bodies’.10. The yeast nuclear membrane has about 200 pores occupying 6–8 % of the total surface area. The nuclear membrane remains intact during mitotic division in yeasts until the daughter nuclei separate.