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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 their short race because they 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 their muscle cells to power their activity.

    Sprinter at starting block
    Figure \(\PageIndex{1}\) (CC BY 2.0; tableatny via Wikimedia Commons)

    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 major stages one intermediate stage: glycolysis, oxidation of pyruvate, the Krebs cycle, and electron transport. The latter two stages require oxygen, making cellular respiration an aerobic process. There are also other ways of making ATP from glucose without oxygen, such as anaerobic respiration and fermentation, of making ATP from glucose without oxygen. Our cells do not perform anaerobic respiration. Therefore, will only focus on fermentation in this section. 

    fermentation types, lactic acid and alcoholic
    Figure \(\PageIndex{2}\). The picture shows Aerobic and Anaerobic respiration. There are two types of Anaerobic respiration, 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

    Fermentation starts with glycolysis, but it does not involve the latter two stages of aerobic cellular respiration (the Krebs cycle and oxidative phosphorylation). During glycolysis, two NAD+ electron carriers are reduced to two NADH molecules and 2 net ATPs are produced. The NADH must be oxidized back so that glycolysis can continue and cells can continue making 2 ATPs. The cells cannot make more than 2 ATP in fermentation because oxidative phosphorylation does not happen due to a lack of oxygen. There are two types of fermentation, called alcoholic fermentation and lactic acid fermentation. While our cells can only perform 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 The process by which this happens is summarized in Figure \(\PageIndex{2}\). The two pyruvate molecules are shown in this diagram come from the splitting of glucose through glycolysis. This process also produces 2 molecules of ATP. Continued breakdown of pyruvate produces acetaldehyde, carbon dioxide, and eventually ethanol. Alcoholic fermentation requires the electrons from NADH and results in the generation of NAD+. 

    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 via Wikimedia Commons)

    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 Figure \(\PageIndex{2}\). Again, two pyruvate and two ATP molecules result from glycolysis. Reduction of pyruvate using the electrons carried by NADH produces lactate (i.e. lactic acid). While this is similar to alcoholic fermentation, note that there is no carbon dioxide produced in this process. 

    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: lactic acid build-up can cause muscle fatigue and a burning sensation in muscles. The soreness is thought to be due to microscopic damage to the muscle fibers. 

    Reality: The statement about lactic acid causing the burn in the muscle has no solid experimental proof. Alternate hypotheses suggest that through the production of lactic acid, the internal pH of the muscle decreases, triggering contraction in muscle due to the activation of motor neurons. 

    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.
    • One 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: