Uptake and utilization of lipids by Paramecium tetraurelia

Uptake rates of a variety of 14C-labeled fatty acids and complex lipids by Paramecium tetraurelia during 48 h of log-phase growth varied. Fatty acid uptake was maximal during lag phase of growth when phagosome (food vacuole) formation was minimal. Food vacuole formation was shown to be suppressed by the presence of exogenous lipids and by starvation. The rates of uptake of lipids were significantly greater than those of small organic compounds such as amino acids, cyclitols, fatty acid precursors and metabolic intermediates. Significant amounts of radioactivity from 14C-labeled fatty acids were metabolized to 14CO2. The uptake rates of different saturated, straight-chain fatty acids of even carbon numbers were different and were not correlated with chain length, results suggesting that the primary mechanism for uptake of these compounds was neither by bulk transport nor simple diffusion and that carrier-mediated processes could possibly be involved.