Anaerobic respiration happens when there isn’t enough oxygen present for aerobic respiration to continue. Different types of organism use different pathways – lactate fermentation or alcoholic fermentation. For both pathways, the amount of ATP produced is much lower than in aerobic respiration because the only ATP produced comes from glycolysis (2ATP per glucose molecule). You could say that there is a lower ATP yield. In this article we will look at the two anaerobic respiration pathways, and finish off by looking at respiratory quotients.
This process happens in mammals normally under conditions of intense exercise. Some bacteria also use lactate fermentation. It 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 converted into lactate (lactic acid). This reaction oxidises reduced NAD back to NAD by transferring hydrogen, so NAD is regenerated for use in glycolysis. This 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. Some of it is transported to the liver via the blood, and the liver converts it into glucose for use in respiration or storage as glycogen. Alternatively, lactate can be converted back to pyruvate in cells, and the pyruvate can enter the mitochondria for aerobic respiration.
This process happens in yeast and some plants. Again, the first step is glycolysis to produce pyruvate. But the pyruvate now goes down a different path. It is decarboxylated (loses CO2) to ethanal, then converted to ethanol. 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 drink 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 or earlier stages. 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.
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 (mammals and some bacteria) produces lactic acid and regenerates NAD.
- Alcoholic fermentation (yeast and some plants) produces CO2 and ethanol, and regenerates NAD.
- Anaerobic respiration has a lower ATP yield than aerobic respiration.
- Lipids, proteins, and carbohydrates can all be used as respiratory substrates.
- A respiratory quotient (RQ) can show which respiratory substrate is being used.