Why is oxygen toxic for obligate anaerobes?
Why do aerobic organisms need oxygen? In aerobic organisms, oxygen is required as a terminal electron receptor. What does this mean? Aerobic metabolism (glycolysis and the TCA cycle) generates reduced molecules (NADH and succinate). These are reoxidised by membrane-bound complexes. In eukaryotes, these complexes are found in the membranes of mitochondria. In prokaryotes, functionally-equivalent complexes are located in the plasma membrane. During the reoxidation reactions, electrons pass through the respiratory chain and protons are pumped across the membrane. This generates a proton motive force which is used to synthesise ATP. So what happens to those electrons that are removed from NADH/succinate? They need a sink – and that sink is oxygen, which is reduced to water by the cytochrome oxidase complex. Take a look at this diagram from the KEGG database of metabolic pathways. It summarises the process that I described – oxidative phosphorylation. You can see oxygen being reduced to wat
