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Course: Applied Biology - Class 12 > Unit 1
Lesson 1: Microbes in industrial and household productsLactic acid and alcohol fermentation
Overview of the lactic acid and alcohol fermentation pathways, and their use in making industrial products.
This article provides and overview of the lactic acid and alcohol fermentation pathways.
Key terms
term | meaning |
---|---|
aerobic | in the presence of, or requiring, oxygen |
anaerobic | in the absence of oxygen |
respiration | process that produces energy by breaking down carbon compounds |
glycolysis | pathway that breaks down glucose |
fermentation | anaerobic breakdown of carbon compounds |
What is fermentation?
Have you ever wondered how yeast help make beer? Or how your muscles keep working when you're exercising so hard that they're very low on oxygen?
Both of these processes can happen thanks to alternative glucose breakdown pathways that occur when normal, oxygen-using (aerobic) cellular respiration is not possible—that is, when oxygen isn't around to act as an acceptor at the end of the electron transport chain. These fermentation pathways consist of glycolysis with some extra reactions tacked on at the end. In yeast, these extra reactions make alcohol, while in your muscles, they make lactic acid.
Fermentation is a widespread pathway, but it is not the only way to get energy from fuels anaerobically (in the absence of oxygen). Some living systems instead use an inorganic molecule other than , such as sulfate, as a final electron acceptor for an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaea.
In this article, we'll take a closer look at anaerobic cellular respiration and at the different types of fermentation.
Fermentation vs. cellular respiration
Fermentation and cellular respiration begin the same way, with glycolysis. In fermentation, however, the pyruvate made in glycolysis does not continue through oxidation and the citric acid cycle, and the electron transport chain does not run. Because the electron transport chain isn't functional, the made in glycolysis cannot drop its electrons off there to turn back into
The purpose of the extra reactions in fermentation, then, is to regenerate the electron carrier from the produced in glycolysis. The extra reactions accomplish this by letting drop its electrons off with an organic molecule (such as pyruvate, the end product of glycolysis). This drop-off allows glycolysis to keep running by ensuring a steady supply of .
Lactic acid fermentation
In lactic acid fermentation, transfers its electrons directly to pyruvate, generating lactate as a byproduct. Lactate, which is just the deprotonated form of lactic acid, gives the process its name. The bacteria that make yogurt carry out lactic acid fermentation, as do the red blood cells in your body, which don’t have mitochondria and thus can’t perform cellular respiration.
Muscle cells also carry out lactic acid fermentation, though only when they have too little oxygen for aerobic respiration to continue—for instance, when you’ve been exercising very hard. It was once thought that the accumulation of lactate in muscles was responsible for soreness caused by exercise, but recent research suggests this is probably not the case.
Lactic acid produced in muscle cells is transported through the bloodstream to the liver, where it’s converted back to pyruvate and processed normally in the remaining reactions of cellular respiration.
Alcohol fermentation
Another familiar fermentation process is alcohol fermentation, in which donates its electrons to a derivative of pyruvate, producing ethanol.
Going from pyruvate to ethanol is a two-step process. In the first step, a carboxyl group is removed from pyruvate and released in as carbon dioxide, producing a two-carbon molecule called acetaldehyde. In the second step, passes its electrons to acetaldehyde, regenerating and forming ethanol.
Alcohol fermentation by yeast produces the ethanol found in alcoholic drinks like beer and wine. However, alcohol is toxic to yeasts in large quantities (just as it is to humans), which puts an upper limit on the percentage alcohol in these drinks. Ethanol tolerance of yeast ranges from about percent to percent, depending on the yeast strain and environmental conditions.
Test your understanding
Inside the tanks in the image below, yeasts are busily fermenting grape juice into wine.
Why do winemaking tanks like these need pressure-release valves?
Want to join the conversation?
- what makes teeth rot?(1 vote)
- Teeth rot , is an result of lactice acid too, sugary substances thats been consumed by bacterias present in our mouth that converts it to lactic acid which reacts to enamel thus resulting degradation, but the lactic acid in other product is less concentrated as present in bacteria by products, and long exposure too plays a role in tooth replacements due to carelessness,(1 vote)