The Difference Between Fermentation and Anaerobic Respiration

Beer fermenting in a glass carboy.
Beer fermenting in a glass carboy. Getty/Matt Nuzzaco

All living things must have a constant source of energy in order to continue performing even the most basic of life functions.  Whether that energy comes straight from the Sun through photosynthesis, or by eating other living plants or animals, the energy must be consumed and then changed into a usable form like Adenosine Triphosphate (ATP).  There are many different mechanisms that can convert the original energy source into ATP.  The most efficient way is through aerobic respiration, which requires oxygen. This method will give the most ATP per input energy source.  However, if no oxygen is available, the organism must still convert the energy using other means.  Processes that happen without oxygen are called anaerobic.  Fermentation is a common way for living things to continue making ATP without oxygen.  Does this make fermentation the same thing as anaerobic respiration?

The short answer is no.  Even though they both do not use oxygen and have similar parts to them, there are some differences between fermentation and anaerobic respiration.  In fact, anaerobic respiration is actually much more like aerobic respiration than it is like fermentation.

Fermentation

Most science classes the majority of students take really only discuss fermentation as an alternative to aerobic respiration.  Aerobic respiration begins with a process called glycolysis.  In glycolysis, a carbohydrate (such as glucose) gets broken down and, after losing some electrons, forms a molecule called pyruvate.  If there is a sufficient supply of oxygen, or sometimes other types of electron acceptors, the pyruvate then goes on to the next part of aerobic respiration.  The process of glycolysis will make a net gain of 2 ATP.

Fermentation is essentially the same process.  The carbohydrate gets 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 instead.  Long distance runners are familiar with lactic acid.  It can build up in the muscles and cause cramping.

Other organisms can undergo alcoholic fermentation where the end product is neither pyruvate nor lactic acid.  This time, the organism makes ethyl alcohol as an end product.  There are also several other types of fermentation that are not as common, but all have different end products depending on the organism that is undergoing the fermentation.  Since fermentation does not use the electron transport chain, it is not considered a type of respiration.

Anaerobic Respiration

Even though fermentation happens without oxygen, it is not 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 just ending with the product of glycolysis like fermentation does, anaerobic respiration will create pyruvate and then continue on the same path as aerobic respiration.

After making a molecule called acetyl coenzyme A, it continues on into 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.  In order for the electron transport chain to continue working, there must be a final electron acceptor.  If the final electron acceptor is oxygen, the process is considered aerobic respiration.  However, some types of organisms, like many types of bacteria and other microorganisms, can use different final electron acceptors.  These include, but are not limited to nitrate ions, sulfate ions, or even carbon dioxide.  

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