When 1-14C-palmitic acid is used to pulse label logarithmic cultures of Tetrahymena pyriformis, radioactivity appears in lipids of the various membrane types at vastly differing rates. The microsomes and postmicrosomal supernatant attain a high specific radioactivity within 1 min, while the membranes enveloping the cilia require several hours to reach the microsomal level. A similar pattern is obtained when the tracer is sodium 1-14C-acetate or 8,9-3H-hexadecyl glycerol. In all fractions the phosphonolipid incorporates radioactivity from 14C-palmitate much less rapidly than do the other major phospholipids. The patterns of labeling suggest that new lipids are transported from a cytoplasmic site of synthesis to points of membrane fabrication throughout the cell.

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