# Fermentation --- **Fermentation** is a common form of anerobic (meaning without using oxygen) [[cellular respiration]]. Like [[aerobic respiration|aerobes]] fermentors break down simple carbohydrates to produce [[adenosine triphosphate]], but it uses an organic molceule (such as [[pyruvate]]) as it's final electron acceptor. It is much less efficient than aerobic respiration (i.e. it produces less ATP per glucose molecule), but it is far simpler and it does not have to contend with the dangerous oxygen molecules. In humans,some our cells (like [[skeletal muscle]]) are able to switch to lactic acid fermentation if we the only reason why regular cells will swap to fermentation if they are unable to get enough oxygen. Fermentation (like everything) starts with [[glycolysis]]. In order to make the (meagre) 2 [[adenosine triphosphate]] it depends on reducing [[NAD|NAD+]] into [[NAD|NADH]]. That NAD+ has to come from somwehere or the whole process grinds to a halt, similar to what will happen if there is not final electron acceptor in the [[ETC (mitochondria)|ETC]]. In fermentaiton, the NADH is recycled back into NAD+ by harnesing the breakdown of the other byproduct of glycolysis, [[pyruvate]]. This allows the NAD+ to cycle back into the glycolysis process and allow it to continue to produce pyruvate and ATP. ## Types of Fermentation There are many different types of fermentation that different cells can do, which are named after the waste byproduct that they produce, but the significant difference is in what they use as their *final electron acceptor*. Type (e.g. the byproduct) | Electron receptor :-------------------:|:------------------: [[lactate]] | [[pyruvate]] [[alcohol\|ethanol]] | acetaldehyde butanediol | acetion mixed acid | isomerized pyruvate - **Lactic acid fermentation** is fermentation that uses straight up [[pyruvate]] as a final electron acceptor. The byproduct of this form of fermentation is [[lactate]], or "lactic acid". Humans use this form of fermentation in our [[muscle tissue]] when we can't pump enough oxygen there, such as during intense exercise. [[Bacteria]] can also do this type of fermentation, which we use to help us to do things like curdle milk to make yogurt. ![[lactic acid fermentation.png]] - **Ethanol fermentation** is fermentation that uses acetaldehyde as a final electron acceptor. Acetaldehyde is a modified form of [[pyruvate]] where one carbon has been removed as CO$_2$. When that acetaldehyde is reduced, it becomes ethanol. We use bacteria who can do ethanol fermentation for things like bread or beer. ![[alcohol fermentation.png]] - **Butanediol fermentation** is fermentation that uses acetoin as a final electron acceptor. Acetoin is made by removing one CO$_2$ each from two different [[pyruvate|pyruvates]] and then combining those two molecules into a four carbon molecule. When acetion is reduced it becomes butanediol. - **Mixed acid fermentation** is fermentation that uses [[isomer|isomerized]] version of [[pyruvate]] as the final electron acceptor. Once that has been reduced, it becomes a bunch of different types of acids and some CO$_2$s. ___