| Some bacteria use the same compound as an energy source and as a source of building blocks for biosynthesis. To do this, they must balance the rate of ATP production with the rate that they "remove" precursors for biosynthesis.
The animation on the left represents a facultative anaerobe catabolizing glucose (represented by the red blobs, of course). Under aerobic conditions, the molecule passes through glycolysis, then enters the citric acid cycle, where it is completely oxidized. The electrons removed during glycolysis and the citric acid cycle (as NADH) are passsed through an electron transport chain to create a protonmotive force which generates ATP.
However, under anaerobic conditions, the citric acid cycle cannot be utilized, (because it generates too much NADH), and thus, this cell resorts to a fermentation. (Click on the button to show what happens under anaerobic conditions). Much less ATP is produced from glucose during glycolysis, and the cell tries to compensate by "eating" glucose faster. Notice that the rate that precursors are removed for biosynthesis does not increase.
This shift from slow aerobic to rapid anaerobic consumption of glucose was first noted by Pasteur. This shift also happens anytime you are unable to provide oxygen to your own mitochondria- they consume glucose faster in an attempt to produce ATP via the less efficient fermentation to lactate, and lactic acid accumulates in your muscles.