Anaerobic Respiration: Definition, Equation & Examples

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Lesson Transcript
Meredith Mikell
Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

Many organisms are capable of undergoing anaerobic respiration. In some cases, it's a 'backup plan' when oxygen is absent, and in other cases, they thrive that way. Here we will investigate what anaerobic respiration is, the equation and examples of it at work.

Definition of Anaerobic Respiration

Can you guess what muscle soreness, beer and compost have in common? They all share a process called anaerobic respiration. Most organisms have evolved to use oxygen in their metabolic process, whether as a component of respiration or as a product of photosynthesis. But there are many conditions on Earth in which oxygen is absent, and yet, living things still find a way to use function and use energy. How do they pull it off? They bypass the part of respiration that involves oxygen, which results in different byproducts of the process.

Anaerobic respiration is the metabolic process in which oxygen is absent, and only the stage of glycolysis is completed. This process occurs mostly in microorganisms, but it can also be a temporary response to anoxic, or oxygen-less, conditions in the cells of multicellular organisms - even us!

Process of Anaerobic Respiration

Glycolysis is the first stage of respiration, in which a glucose molecule is broken down into two pyruvate molecules, releasing electrons in the process and generating two molecules of ATP, the energy 'currency' of the cell, and the desired outcome of respiration. During aerobic respiration, when oxygen is present, those pyruvate molecules go through two other stages that serve to release more electrons, later used to power the generation of a very high amount of ATP.

But when oxygen is absent, as in the case with anaerobic respiration, those last two stages are bypassed. Instead, pyruvate is converted into a different byproduct through a fermentation process, and carbon dioxide is released as well. Two ATP molecules are generated in the process. While this is not as many ATP molecules generated during aerobic respiration, it is enough to get by. Enzymes are often used by the cell to help the process along, such as the zymases used in ethanol fermentation. The process is essentially: glucose + enzymes = carbon dioxide + ethanol / lactic acid.

Examples of Anaerobic Respiration

Alcohol fermentation, or ethanol fermentation, occurs when single-celled organisms break down pyruvate into ethanol, releasing carbon dioxide in the process. This process is utilized in the making of bread, wine, beer and ethanol fuels. Single-celled eukaryotes called yeasts are the organisms involved in this process. The carbon dioxide released causes the fizzy bubbles in drinks and the rising of bread.

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Additional Activities

Life Without Oxygen Crossword Puzzle

Now that you have been through the lesson on Anaerobic Respiration and taken the quiz, check your understanding one more time by completing the crossword. You may find it helpful to review the lesson diagrams of the metabolic reactions.


Use the word bank provided for completing the crossword. Each word will be used. Where an answer consists of two words, do not include the space in the puzzle blanks.

Word Bank

  • ATP
  • NAD
  • TWO



5. product of photosynthesis

6. product of fermentation by bacteria and muscle cells

7. yeasts make this from pyruvate

8. product of glycolysis

9. location in a cell where fermentation occurs

10. a set of enzymes that produce ethanol from glucose

12. releases many electrons from glucose, which later power generation of many ATP molecules

14. location in a cell where later stages of aerobic respiration occur

16. a backup plan when oxygen is absent

17. another word for lacking oxygen


1. first stage of respiration

2. number of ATP generated by anaerobic respiration

3. breakdown of organic matter

4. starting molecule in glycolysis

11. in the absence of oxygen

12. energy currency of cells

13. causes the bubbles in beer

15. is an electron carrier that is regenerated by fermentation


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