Anaerobic respiration happens when there isn’t enough oxygen present for aerobic respiration to continue. Different types of organism use different pathways. For both pathways we will learn about today, the yield of ATP is much lower than in aerobic respiration because the only ATP produced comes from glycolysis (2ATP per glucose molecule). In this article we will look at two anaerobic respiration pathways, and finish off by looking at respiratory quotients.
Anaerobic respiration in animals
In animals, anaerobic respiration begins with glycolysis – the partial breakdown of glucose to pyruvate in the cytoplasm. In aerobic respiration, this pyruvate would have entered the mitochondria for the link reaction. However in anaerobic respiration, the pyruvate remains in the cytoplasm and is reduced to lactate (lactic acid). This is sometimes called lactate fermentation. Reduced NAD provides hydrogen for this reaction and is oxidised back to NAD. This is very important so that NAD is regenerated for use in glycolysis. A supply of NAD allows glycolysis to continue and produce 2ATP for each molecule of glucose.
As lactate builds up in cells, the pH becomes more acidic, which can only be tolerated for a short amount of time. It is the build up of lactate that causes our muscles to feel tired and sore during intense exercise. The lactate must be broken down. It is transported to the liver via the blood, and the liver oxidised it into pyruvate which is then converted to glucose for use in respiration or storage as glycogen. However, oxygen is needed for lactate to be oxidised to pyruvate. This is why breathing rate and heart rate remain high after exercise – the oxygen debt must be repaid.
Anaerobic respiration in plants and yeast
Again, the first step is glycolysis to produce pyruvate. But the pyruvate now goes down a different pathway. It is decarboxylated (loses CO2) to ethanal, then converted to ethanol. This is alcoholic fermentation. Conversion of ethanal to ethanol regenerates NAD from reduced NAD by transferring hydrogen, which allows glycolysis to continue and produce a small amount of ATP. The CO2 and ethanol produced by yeast have commercial uses in baking and the alcoholic drinks industry.
Respiratory quotients (RQs)
We’re back to talking about aerobic respiration now, and looking at how molecules other than glucose can be involved.
Although glucose is the classic substrate for respiration, other molecules can act as respiratory substrates and enter respiration. For example, lipids and proteins can enter at the Krebs cycle. Different substrates result in different amounts of ATP being produced, and therefore different amounts of energy being released. It all depends on how much hydrogen a molecule contains per unit of mass. In respiration, the coenzymes NAD and FAD accept hydrogen to become reduced, and the reduced NAD and reduced FAD are used to produce ATP in oxidative phosphorylation. So the more hydrogen a molecule has, the more ATP can be produced from it. Because of this, lipids have a higher energy value than proteins, and proteins have a higher energy value than carbohydrates. A gram of lipid has over twice the amount of energy as a gram of carbohydrate.
A respiratory quotient (RQ) is a value that can be calculated to find out which respiratory substrate an organism is using for respiration, and whether it is using anaerobic respiration. This is possible because it takes more oxygen to oxidise molecules with a higher energy value because of what we discussed above.
RQ is calculated using this formula:
Lipids have an RQ of about 0.7, proteins 0.9, and carbohydrates 1. An RQ greater than 1 suggests anaerobic respiration is being used.
Summary
We have now been through all of the stages of aerobic and anaerobic respiration! Here is a quick summary of today’s article:
- Lactate fermentation (animals) produces lactate and regenerates NAD.
- Alcoholic fermentation (yeast and plants) produces CO2 and ethanol, and regenerates NAD.
- Anaerobic respiration has a lower ATP yield than aerobic respiration because glycolysis only produces 2ATP per glucose molecule.
- Lipids, proteins, and carbohydrates can all be used as respiratory substrates.
- A respiratory quotient (RQ) can show which respiratory substrate is being used.
