what are examples of circumstances in which O2 might be unavailable for electron transport chain?

Answer 1

An anaerobic environment, where there is no O2, or not enough, will shift away from the ETC and TCA cycle and towards glycolysis and the Cori/Lactic acid pathway. Poisoning can also interfere with O2 use in the ETC, such as cyanide poisoning preventing the use of oxygen as a final electron acceptor, or carbon monoxide poisoning, which will produce an anaerobic environment because of the higher affinity for CO than O2 with hemoglobin.