Time courses for the uptake of [¹⁴C]isoleucine into isolated P falciparum trophozoites at 20°C. The extracellular concentration of isoleucine was in the range of 0.07 to 0.09 μM. Protein synthesis was inhibited by the addition of cycloheximide (40 μM) and anisomycin (150 μM). The isolated parasites were suspended in control (•), glucose-free (○), or Na⁺-free (⊡) HEPES-buffered media (solutions B, C, and D, respectively) at pH 7.1. A HEPES/MES-buffered solution (solution E) was used to suspend cells at pH 7.3 (▾) and pH 5.5 (▿). The error bars for uptake in glucose-free, Na⁺-free, and pH 7.3 media were of a similar magnitude to the error bars shown for the control data and have been omitted for clarity. The distribution ratio is the concentration of [¹⁴C]isoleucine within the parasite relative to that in the extracellular medium. The data are averaged from 4 separate experiments performed on different days.