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5.9: Anaerobic Respiration

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    22469
  • Fast and Furious

    The muscles of this sprinter will need a lot of energy to complete her short race because she will be running at top speed. The action won't last long, but it will be very intense. The energy the sprinter needs can't be provided quickly enough by aerobic cellular respiration. Instead, a different process must be used by her muscle cells to power their activity.

    Sprinter at starting block
    Figure \(\PageIndex{1}\) (By tableatny (BXP135624) [CC BY 2.0], via Wikimedia Commons; Sprinter at startingblock.jpg)

    Making ATP Without Oxygen

    The cells of living things power their activities with the energy-carrying molecule ATP (adenosine triphosphate). The cells of most living things make ATP from glucose in the process of cellular respiration. This process occurs in three stages: glycolysis, the Krebs cycle, and electron transport. The latter two stages require oxygen, making cellular respiration anaerobic process. There are also ways of making ATP from glucose without oxygen. These processes are referred to collectively as anaerobic respiration.

    fermentation types
    Figure \(\PageIndex{2}\). The picture shows Aerobic and Anaerobic respiration. There are two types of Anaerobic respirations, Alcoholic and lactic acid. Alcoholic fermentation produces ethanol and NAD+. The NAD+ allows glycolysis to continue making ATP. Lactic acid fermentation produces lactic acid and NAD+. The NAD+ cycles back to allow glycolysis to continue so more ATP is made. Each circle represents a carbon atom. (CC BY-SA 3.0; Vtvu via Wikimedia.org)
     

    Fermentation

    An important way of making ATP without oxygen is fermentation. Fermentation starts with glycolysis, which does not require oxygen, but it does not involve the latter two stages of aerobic cellular respiration (the Krebs cycle and electron transport). There are two types of fermentation, called alcoholic fermentation and lactic acid fermentation. We make use of both types of fermentation using other organisms, but only lactic acid fermentation actually takes place inside the human body.

    Alcoholic Fermentation

    Alcoholic fermentation is carried out by single-celled fungi called yeasts and also by some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine. For example, the biofuel ethanol (a type of alcohol) is produced by the alcoholic fermentation of the glucose in corn or other plants. The process by which this happens is summarized in the diagram below. The two pyruvic acid molecules are shown in the diagram come from the splitting of glucose in the first, or glycolysis, stage of the process. Glycolysis is also when ATP is made. Two molecules of ATP are produced from each molecule of glucose.

    Yeasts in bread dough also use alcoholic fermentation for energy and produce carbon dioxide gas as a waste product. The carbon dioxide that is released causes bubbles in the dough and explains why the dough rises. Do you see the small holes in the bread pictured below? The holes were formed by bubbles of carbon dioxide gas.

    Slices of bread
    Figure \(\PageIndex{3}\): Holes from carbon dioxide gas in bread dough are left behind after the bread bakes. (Public domain; Daniel Sone (Photographer) via Wikimedia Commons; Slices of bread.jpg)

    Lactic Acid Fermentation

    Lactic acid fermentation is carried out by certain bacteria, including the bacteria in yogurt. It is also carried out by your muscle cells when you work them hard and fast. This is how the muscles of the sprinter pictured above get energy for their short-duration but intense activity. The process by which this happens is summarized in the diagram below. Again, the two pyruvic acid molecules shown in the diagram come from the splitting of glucose in the first, or glycolysis, stage of the process. It is also during this stage that two ATP molecules are produced. The rest of the processes produces lactic acid. Note that there is no carbon dioxide waste product in lactic acid fermentation as there is with alcoholic fermentation.

    Did you ever run a race, lift heavy weights, or participate in some other intense activity and notice that your muscles start to feel a burning sensation? This may occur when your muscle cells use lactic acid fermentation to provide ATP for energy. The buildup of lactic acid in the muscles causes the feeling of burning. The painful sensation is useful if it gets you to stop overworking your muscles and allow them a recovery period during which cells can eliminate the lactic acid.

    Pros and Cons of Anaerobic Respiration

    With oxygen, organisms can use aerobic cellular respiration to produce up to 36 molecules of ATP from just one molecule of glucose. Without oxygen, organisms must use anaerobic respiration to produce ATP, and this process produces only two molecules of ATP per molecule of glucose. Although anaerobic respiration produces less ATP, it has the advantage of doing so very quickly. It allows your muscles, for example, to get the energy they need for short bursts of intense activity. Aerobic cellular respiration, in contrast, produces ATP more slowly.

    Feature: Myth vs. Reality

    Myth: No doubt you have participated in strenuous exercise or other intense activity and then felt soreness in your muscles the next day. The soreness may even last for several days following the activity. Many people, even well-seasoned athletes, believe that lactic acid buildup is responsible for this type of delayed muscle soreness.

    Reality: Although lactic acid produced during extreme exertion may result in a burning sensation while the muscles are active, it is not thought to be the cause of muscle soreness afterward. The reason for delayed muscle soreness is not yet well understood, but it may be due to actual injury of the muscles, which causes inflammation, swelling, and pain.

    Summary

    • The cells of most living things produce ATP from glucose by aerobic cellular respiration, which uses oxygen. Some organisms can also or instead produce ATP from glucose by anaerobic respiration, which does not require oxygen.
    • An important way of making ATP without oxygen is fermentation. There are two types of fermentation: alcoholic fermentation and lactic acid fermentation. Both start with glycolysis, the first and anaerobic stage of cellular respiration, in which two molecules of ATP are produced from one molecule of glucose.
    • Alcoholic fermentation is carried out by single-celled organisms including yeasts and some bacteria. We use alcoholic fermentation in these organisms to make biofuels, bread, and wine.
    • Lactic acid fermentation is undertaken by certain bacteria, including the bacteria in yogurt, and also by our muscle cells when they are worked hard and fast.
    • Anaerobic respiration produces far less ATP than does aerobic cellular respiration, but it has the advantage of being much faster. It allows muscles, for example, to get the energy they need for short bursts of intense activity.

    Review

    1. State the main difference between aerobic cellular respiration and anaerobic respiration.
    2. What is fermentation?
    3. Compare and contrast alcoholic and lactic acid fermentation.
    4. Identify the major pro and the major con of anaerobic respiration relative to aerobic cellular respiration.
    5. a. What process is shared between aerobic cellular respiration and anaerobic respiration? Describe the process briefly.

      b. Why is this process able to occur in anaerobic respiration as well as aerobic respiration?

    6. Which type of respiration occurs in the human body?

      A. Aerobic cellular respiration

      B. Alcoholic fermentation

      C. Lactic acid fermentation

      D. Both A and C

    7. True or False. Lactic acid fermentation produces carbon dioxide.

    8. True or False. Types of bacteria can carry out alcoholic fermentation and lactic acid fermentation.

    9. True or False. No ATP is produced by fermentation.

    10. Both lactic acid fermentation and alcoholic fermentation use _________ acid molecules to make their final products.

    11. Which type of respiration is used in the making of bread and wine?

      A. Alcoholic fermentation

      B. Lactic acid fermentation

      C. Aerobic cellular respiration

      D. Prokaryotic respiration

    12. Fermentation is a form of ___________ respiration.

    13. What is the reactant, or starting material, shared by aerobic respiration and both types of fermentation?

    Explore More

    Check out this video to learn more about anaerobic glycolysis:

    While many people think that Brewer's are artisans for their production of beer, in actuality, the true craft and process of beer making is due to anaerobic glycolysis from yeast. Learn more here: