Uptake and utilization of small organic molecules by Paramecium tetraurelia

Several methods for the metabolic radiolabeling of proteins in axenically grown Paramecium tetraurelia were examined. Less than 5% of the initial radioactivity from exogenously supplied radiolabeled compounds was incorporated into cellular proteins under all conditions tested. The relatively low uptake rates of 14C-labeled amino acids and other small organic compounds by the cells and the subsequent metabolism of these molecules producing radioactive CO2 under conditions of rapid culture growth suggested that the transport of most or all of these compounds may be limited to bulk transport in solution and possibly accounted for by the volume transported by food vacuoles. Active transport of these molecules against a concentration gradient via carrier-mediated mechanisms was not apparent. The low rate of amino acid uptake explains the inefficient in vivo radiolabeling of cellular proteins by radioactive amino acids.