Anaerobic Respiration


Why Fermentation?

What happens if there is not enough oxygen for cellular respiration? Glycolysis can still occur, but it is followed by a different pathway called fermentation.

Processes that do not require oxygen are “anaerobic” processes. Fermentation is an anaerobic process. Cells that use fermentation to generate very little ATP compared to cellular respiration.

There are several forms of fermentation. Two of these forms are lactic acid fermentation and alcoholic fermentation.

Alcoholic Fermentation

Alcoholic fermentation begins with glycolysis. However, since the pyruvic acid cannot move on to the Krebs Cycle due to the lack of oxygen, it is converted to ethyl alcohol and carbon dioxide by using the NADH produced during glycolysis. Two ATP result. This allows glycolysis to continue producing small amounts of ATP.


Alcoholic fermentation occurs in some plants and unicellular organisms such as yeast and bacteria. This process occurs when making bread, beer, and wine.

 Stop and Think: When making bread, you would use yeast, a unicellular organism that undergoes alcoholic fermentation.

– Why do we put yeast in bread?

– If ethyl alcohol is produced by yeast, how is it that bread is not an “alcoholic” food?

Lactic Acid Fermentation

When there is no oxygen available after glycolysis, the pyruvate molecules leaving glycolysis are converted into a waste product called lactic acid using the NADH produced in glycolysis. Lactic acid is a 3-carbon molecule, so we can see that CO2 is not released during lactic acid fermentation.


Some bacteria can use lactic acid fermentation. We use bacteria that undergo lactic acid fermentation to produce yogurt and sauerkraut. Humans may resort to lactic acid fermentation, as you will read below.

Energy and Exercise

Cells normally contain small amounts of ATP produced during glycolysis and cellular respiration.

At the start of a race, muscle cells only contain enough ATP for a few seconds of activity. When running a long race, muscle cells are producing most of their ATP by lactic acid fermentation.

  • Have you ever been running a race and had a muscle cramp? Lactic acid buildup in your muscles could be the cause. Sometimes, an animal’s lungs or bloodstream cannot get enough oxygen to the muscles of the body to make enough ATP. At these times, your muscle cells use lactic acid fermentation to make ATP without using oxygen.   Lactic acid builds up and causes muscle soreness and burning (a cramp). Taking in O2 breaks down lactic acid.When the race is over, the only way to get rid of the extra lactic acid is to acquire more oxygen. This is known as an “oxygen debt” because oxygen must be “paid back” to the cells to remove the lactic acid. The debt is paid as you continue to breathe deeply for several moments after the activity stops.

For longer races, cellular respiration is the only way to generate a continuing supply of ATP. It releases energy more slowly than fermentation.

Comparing Aerobic and Anaerobic Respiration

Aerobic respiration (with oxygen) can produce 36 ATP molecules from each glucose molecule. Anaerobic respiration (without oxygen) only allows production of 2 ATP molecules from each glucose molecule. Therefore, aerobic respiration is significantly more efficient than anaerobic respiration.

Metabolic Pathways Review

Click through the interactive review to practice and compare the processes of cellular respiration and fermentation.