The Difference Between Fermentation and Anaerobic Respiration

Both provide energy for living things, but the processes are different

Beer fermenting in a huge stainless steel container in a brewery

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All living things must have constant sources of energy to continue performing even the most basic life functions. Whether that energy comes straight from the sun through photosynthesis or through eating plants or animals, the energy must be consumed and then changed into a usable form such as adenosine triphosphate (ATP).

Many mechanisms can convert the original energy source into ATP. The most efficient way is through aerobic respiration, which requires oxygen. This method gives the most ATP per energy input. However, if oxygen isn't available, the organism must still convert the energy using other means. Such processes that happen without oxygen are called anaerobic. Fermentation is a common way for living things to make ATP without oxygen. Does this make fermentation the same thing as anaerobic respiration?

The short answer is no. Even though they have similar parts and neither uses oxygen, there are differences between fermentation and anaerobic respiration. In fact, anaerobic respiration is much more like aerobic respiration than it is like fermentation.

Fermentation

Most science classes discuss fermentation only as an alternative to aerobic respiration. Aerobic respiration begins with a process called glycolysis, in which a carbohydrate such as glucose is broken down and, after losing some electrons, forms a molecule called pyruvate. If there's a sufficient supply of oxygen, or sometimes other types of electron acceptors, the pyruvate moves to the next part of aerobic respiration. The process of glycolysis makes a net gain of 2 ATP.

Fermentation is essentially the same process. The carbohydrate is broken down, but instead of making pyruvate, the final product is a different molecule depending on the type of fermentation. Fermentation is most often triggered by a lack of sufficient amounts of oxygen to continue running the aerobic respiration chain. Humans undergo lactic acid fermentation. Instead of finishing with pyruvate, lactic acid is created. Distance runners are familiar with lactic acid, which can build up in the muscles and cause cramping.

Other organisms can undergo alcoholic fermentation, where the result is neither pyruvate nor lactic acid. In this case, the organism makes ethyl alcohol. Other types of fermentation are less common, but all yield different products depending on the organism undergoing fermentation. Since fermentation doesn't use the electron transport chain, it isn't considered a type of respiration.

Anaerobic Respiration

Even though fermentation happens without oxygen, it isn't the same as anaerobic respiration. Anaerobic respiration begins the same way as aerobic respiration and fermentation. The first step is still glycolysis, and it still creates 2 ATP from one carbohydrate molecule. However, instead of ending with glycolysis, as fermentation does, anaerobic respiration creates pyruvate and then continues on the same path as aerobic respiration.

After making a molecule called acetyl coenzyme A, it continues to the citric acid cycle. More electron carriers are made and then everything ends up at the electron transport chain. The electron carriers deposit the electrons at the beginning of the chain and then, through a process called chemiosmosis, produce many ATP. For the electron transport chain to continue working, there must be a final electron acceptor. If that acceptor is oxygen, the process is considered aerobic respiration. However, some types of organisms, including many types of bacteria and other microorganisms, can use different final electron acceptors. These include nitrate ions, sulfate ions, or even carbon dioxide. 

Scientists believe that fermentation and anaerobic respiration are older processes than aerobic respiration. Lack of oxygen in the early Earth's atmosphere made aerobic respiration impossible. Through evolution, eukaryotes acquired the ability to use the oxygen "waste" from photosynthesis to create aerobic respiration.