Does a facultative anaerobe consume the same amount of glucose in aerobic and anaerobic conditions?

Question

How much ATP does it consume while on anaerobe contiditions or aerobe conditions? Which one is better for the facultative anaerobe?

Answer 1

No, an organism in anaerobic conditions is going to burn much more glucose to produce the same amount of energy it would if it was in normal, aerobic conditions.

This is largely due to the fact that the anaerobic metabolic pathways are less efficient than the aerobic ones, since they bypass many of the energy-producing cycles that require oxygen.

1. Glycolysis, or the breakdown of glucose, is the first step in glucose breakdown. It does not require oxygen, so organisms do it in both aerobic and anaerobic conditions. Glycolysis produces NADH, pyruvate, and some ATP.
2. Most anaerobic organisms have to use some form of fermentation to oxidize that NADH back into NAD+, which allows glycolysis to continue and produce more energy (and more NADH). There are generally three types of fermentation: lactic acid, alcoholic, and mixed fermentation.
3. Because oxygen is not available, anaerobic organisms must use something other than oxygen (O2) as as the terminal electron acceptor if they execute the final step of cellular respiration (the electron transport chain/ETC). This process converts the electrons in NADH into useable energy, oxidizing it back into NAD+ in the proccess (thus, also allowing glycolysis to continue producing energy without a buildup of NADH). Some alternative terminal electron acceptors would be nitrare, sulfate, and carbon dioxide.
4. Almost all anaerobic organisms skip the Citric Acid Cycle (TCA) which indirectly relies on oxygen. Skipping the TCA means they miss another opportunity to produce energy from glucose breakdown.

It's important to note that different species are going to use different kinds of fermentation (lactic acid vs. alcoholic). They will also use different terminal electron acceptors, and each of their approaches vary in efficiency. In fact, many anaerobic species will omit the electron transport chain entirely, relying solely on fermentation to replenish NAD+. This means that their only means of energy production are via glycolysis, producing a net of only 2 ATP per glucose (in contrast to the 30-32 per glucose produced from all the aerobic pathways combined).

In conclusion, due to the variation in what pathways and compounds they use, the amount of glucose each anaerobic microbe needs is going to vary from species to species. However, all of them are going to require much more glucose in anaerobic conditions than if they were in aerobic conditions, due to the lower efficiency of these anaerobic pathways.