If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Lactic 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

termmeaning
aerobicin the presence of, or requiring, oxygen
anaerobicin the absence of oxygen
respirationprocess that produces energy by breaking down carbon compounds
glycolysispathway that breaks down glucose
fermentationanaerobic 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 O2, 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 NADH made in glycolysis cannot drop its electrons off there to turn back into NAD+
The purpose of the extra reactions in fermentation, then, is to regenerate the electron carrier NAD+ from the NADH produced in glycolysis. The extra reactions accomplish this by letting NADH 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 NAD+.

Lactic acid fermentation

In lactic acid fermentation, NADH 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.
Diagram of lactic acid fermentation. Lactic acid fermentation has two steps: glycolysis and NADH regeneration.
During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.
During NADH regeneration, the two NADH donate electrons and hydrogen atoms to the two pyruvate molecules, producing two lactate molecules and regenerating NAD+.
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 NADH 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, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.
Diagram of alcohol fermentation. Alcohol fermentation has two steps: glycolysis and NADH regeneration.
During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.
During NADH regeneration, the two pyruvate molecules are first converted to two acetaldehyde molecules, releasing two carbon dioxide molecules in the process. The two NADH then donate electrons and hydrogen atoms to the two acetaldehyde molecules, producing two ethanol molecules and regenerating NAD+.
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 5 percent to 21 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.
Image of tanks used for wine production by fermentation of grapes. The tanks are quipped with pressure-release valves.
Image credit: "Metabolism without oxygen: Figure 3" by OpenStax College, Biology, CC BY 3.0
Why do winemaking tanks like these need pressure-release valves?
Choose 1 answer:


Want to join the conversation?