A closer look at what happens after glycolysis if oxygen is not available
‘Big Picture: Exercise, Energy and Movement’ (2012)
Aerobic respiration isn’t always the most appropriate method of energy production. In some cases, the demand for energy is greater than the supply of oxygen available, and some organisms are just incapable of using oxygen to respire. In these cases, the solution is anaerobic respiration.
This process, of extracting energy from food without oxygen or mitochondria, evolved long ago when there was no oxygen in the Earth’s atmosphere. It remains in operation alongside aerobic respiration in organisms that are evolutionarily more recent.
Anaerobic respiration depends on final electron acceptors other than oxygen. The process differs a little in plants and animals, with lactate fermentation occurring in animals and alcohol fermentation in plants.
During strenuous activity, muscles will undergo anaerobic respiration if the body can’t provide them with enough oxygen. This process works by reoxidising reduced NAD (ie hydrogen is removed from it), so that this NAD can then be used again in glycolysis to create adenosine triphosphate (ATP).
- Reduced NAD (NADH + H+) is oxidised to NAD, as a consequence of the enzyme lactate dehydrogenase helping to convert pyruvate into lactate.
- Lactate then moves into the blood and, once sufficient oxygen is available, is converted into other molecules (such as pyruvate and glucose) in the liver, kidney and muscles.
Plants can also respire anaerobically, which can be extremely useful when their roots become completely waterlogged (and thus unable to access oxygen). Plants anaerobically respire by ethanol fermentation in the same way that yeast respires when making alcohol and bread.
- Pyruvate is decarboxylated (loses a CO2 molecule) to produce ethanal (with the help of the enzyme pyruvate decarboxylase).
- Reduced NAD (NADH + H+) is oxidised, with ethanal accepting the hydrogen atoms that are freed, reducing to ethanol (with the help of the enzyme ethanol dehydrogenase).
- NAD is then free to return to the glycolysis reactions to be reduced again – producing more ATP – and to continue as a hydrogen carrier between glycolysis and fermentation.
aglet/Flickr CC BY NC
- Big Picture: Cellular respiration poster [PDF]
- S-cool: Anaerobic respiration
- Mrs Miller’s Blog: Anaerobic respiration