Metabolism 5: Anaerobic Respiration

What happens without oxygen?

Without oxygen, cells require alternate electron acceptors to allow oxidation of NADH and FADH2. Unfortunately, this means less energy is generated than aerobic respiration and oxygen is no longer the terminal electron acceptor. Alternate electron acceptors include carbon dioxide, sulfates and the fermentation of organic molecules in eukaryotes.


Anaerobic respiration and fermentation are two terms used interchangeably, the first referring to lactic acid production and the latter referring to alcohol fermentation using yeasts.


The process of energy production in anaerobic respiration is not completely different to aerobic respiration in that glycolysis still occurs, where one glucose molecule is catabolised to produce 2 pyruvate molecules. Recall that pyruvate is required, as it can enter the mitochondria. However, instead of entering the citric acid cycle, pyruvate enters fermentation which produces either ethanol (alcohol) or lactic acid. This fermentation allows for our cells to oxidise NADH to NAD+ in the absence of oxygen.


In alcohol fermentation, pyruvate is converted to acetaldehyde and then ethanol. This reaction facilitates the oxidation of NADH to NAD+. The NAD+ is then reused or recycled in glycolysis to help produce 2 ATP.


In lactic acid fermentation, pyruvate is converted to lactate or lactic acid, which facilitates NADH oxidation to NAD+.


Thus, NAD+ is the oxidised form and NADH is the reduced form.