Many drinks and the rising of bread.

Many organisms are capable of undergoing anaerobic respiration. In some cases, it’s a ‘backup plan’ when oxygen is absent, and in other cases, they thrive that way. Here we will investigate what anaerobic respiration is, the equation and examples of it at work.

Definition of Anaerobic Respiration

Can you guess what muscle soreness, beer and compost have in common? They all share a process called anaerobic respiration.

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Most organisms have evolved to use oxygen in their metabolic process, whether as a component of respiration or as a product of photosynthesis. But there are many conditions on Earth in which oxygen is absent, and yet, living things still find a way to use function and use energy. How do they pull it off? They bypass the part of respiration that involves oxygen, which results in different byproducts of the process.

Anaerobic respiration is the metabolic process in which oxygen is absent, and only the stage of glycolysis is completed. This process occurs mostly in microorganisms, but it can also be a temporary response to anoxic, or oxygen-less, conditions in the cells of multi-cellular organisms – even us!

Process of Anaerobic Respiration

Glycolysis is the first stage of respiration, in which a glucose molecule is broken down into two pyruvate molecules, releasing electrons in the process and generating two molecules of ATP, the energy ‘currency’ of the cell, and the desired outcome of respiration. During aerobic respiration, when oxygen is present, those pyruvate molecules go through two other stages that serve to release more electrons, later used to power the generation of a very high amount of ATP.But when oxygen is absent, as in the case with anaerobic respiration, those last two stages are bypassed. Instead, pyruvate is converted into a different byproduct through a fermentation process, and carbon dioxide is released as well. Two ATP molecules are generated in the process.

While this is not as many ATP molecules generated during aerobic respiration, it is enough to get by. Enzymes are often used by the cell to help the process along, such as the zymases used in ethanol fermentation. The process is essentially: glucose + enzymes = carbon dioxide + ethanol / lactic acid.

Examples of Anaerobic Respiration

Alcohol fermentation, or ethanol fermentation, occurs when single-celled organisms break down pyruvate into ethanol, releasing carbon dioxide in the process.

This process is utilized in the making of bread, wine, beer and ethanol fuels. Single-celled eukaryotes called yeasts are the organisms involved in this process. The carbon dioxide released causes the fizzy bubbles in drinks and the rising of bread.

Lactic acid fermentation occurs when the pyruvates are converted to lactic acid. This can happen when bacteria are used to make yogurt from milk and also in the muscle cells of animals as a result of hard exercise. When you sprint, for example, your aerobic respiration system cannot keep up with the pace, but you can continue anyway because your muscle cells can switch over to this process instead of operating aerobically.

It doesn’t last very long, though. Lactic acid builds up in muscles and is responsible for the soreness that you feel after working out.Decomposition of organic matter is when bacteria breaks down organic matter under oxygen-less conditions, usually generating substantial heat and odor in the process. This happens in composting, for example.

Lesson Summary

Living things can find pretty amazing ways to survive, and anaerobic respiration is one example of this. Anaerobic respiration is the metabolic process in which oxygen is absent, and only the stage of glycolysis is completed.

This occurs in microorganisms, but is also a temporary response to oxygen-less conditions in the cells of multicellular organisms, such as humans. Some examples of anaerobic respiration include alcohol fermentation, lactic acid fermentation and in decomposition of organic matter. The equation is: glucose + enzymes = carbon dioxide + ethanol / lactic acid.Though it does not produce as much energy as aerobic respiration, it gets the job done. The next time you benefit from the short burst of energy your muscles give you while sprinting, you can thank the alternative pathway of anaerobic respiration that many cells are capable of.


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