Fig. 6. (A) Relative amounts of cellular ATP over time. Control measurements (filled squares) are displayed as the ratio of cellular ATP in cells placed at pH 2 and pH 7. There is no significant difference in ATP between the cells for at least 1 h. After 105 and 165 min, there is a 17% and 10% increase in the ATP ratio, respectively, which is due to a decrease in ATP levels in the cells at pH 7. Open squares, ratio of the relative amount of ATP in azide-treated cells. Filled circles, normalization of the relative amount of cellular ATP in azide-treated and control cells. There is a significant decrease in cellular ATP in azide-treated cells in pH 2 medium at 5, 25, 45 and 65 min. At 105 and 165 min, ATP levels have recovered in the cells growing in pH 2 so that there is no longer a significant difference between the cells growing at pH 2 and pH 7. (B) Rate of change of the relative amounts of ATP in azide-treated cells at pH 2 and pH 7. The difference is plotted as the percent of the total cellular ATP pool at the middle time point between the two measuring points in A (filled circles). Relatively higher ATP consumption occurs for nearly 40 min, after which time the cells in pH 2 medium produce relatively more ATP. At 135 min the rate of ATP change is nearly the same in cells growing at pH 2 and pH 7. The greatest rate of consumption was 0.027% of the total cellular ATP pool per second. This was the average for the first 5 min of measurement in A (filled circles), plotted here at 2.5 min.