1. Protoplasts of Saccharomyces cerevisiae N.C.Y.C. 366 were prepared by incubating washed exponential-phase cells in buffered mannitol (0·8m) containing 10mm-magnesium chloride and snail gut juice (about 8mg. of protein/ml. of reaction mixture). Protoplast membranes were obtained by bursting protoplasts in ice-cold phosphate buffer (pH7·0) containing 10mm-magnesium chloride. 2. Protoplast membranes accounted for 13–20% of the dry weight of the yeast cell. They contained on a weight basis about 39% of lipid, 49% of protein, 6% of sterol (assayed spectrophotometrically) and traces of RNA and carbohydrate (glucan+mannan). 3. The principal fatty acids in membrane lipids were C16:0, C16:1 and C18:1 acids. Whole cells contained a slightly greater proportion of C16:0 and a somewhat smaller proportion of C18:1 acids. Membrane and whole-cell lipids included monoglycerides, diglycerides, triglycerides, sterols, sterol esters, phosphatidylcholine, lysophosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol+phosphatidylserine. Phosphorus analyses on phospholipid fractions from membranes and whole cells showed that membranes contained proportionately more phosphatidylethanolamine and phosphatidylinositol+phosphatidylserine than whole cells, which in turn were richer in phosphatidylcholine. Phospholipid fractions from membranes and whole cells had similar fatty acid compositions. 4. Membranes and whole cells contained two major and three minor sterol components. Gas–liquid chromatography, mass spectrometry and u.v. and i.r. spectra indicated that the major components were probably Δ5,7,22,24(28)-ergostatetraen-3β-ol and zymosterol. The minor sterol components in whole cells were probably episterol (or fecosterol), ergosterol and a C29 di-unsaturated sterol. 5. Defatted whole cells contained slightly more glutamate and ornithine and slightly less leucine and isoleucine than membranes. Otherwise, no major differences were detected in the amino acid compositions of defatted whole cells and membranes.

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