Article objectives

  • To distinguish between obligate aerobes, obligate anaerobes, and facultative anaerobes.
  • To explain that, in the absence of oxygen fermentation reactions must regenerate NAD+ in order for glycolysis to continue making ATP.
  • To discuss how your muscles continue to work for you even when your respiratory and cardiovascular system can no longer keep up a continuous supply of oxygen.
  • To describe how bacteria, including those we employ to make yogurt, make ATP in the absence of oxygen.
  • To compare and contrast alcoholic and lactic acid fermentation pathways.
  • To outline the process used to produce fuel from corn.
  • To explain how we employ anaerobic organisms to make bread, beer, and wine.
  • To compare the energy efficiency of aerobic cellular respiration to that of fermentation.
  • To list the advantages of anaerobic over aerobic respiration.
  • To explain why vertebrate muscles use both aerobic and anaerobic pathways to make ATP.
  • After the photosynthetic “oxygen catastrophe” challenged life between 2.5 and 3 billion years ago, evolution rebounded with biochemical pathways to harness and protect against oxygen’s power. Today, most organisms use \(O_2\) in aerobic respiration to produce ATP. Almost all animals, most fungi, and some bacteria are obligate aerobes, which require oxygen. Some plants and fungi and many bacteria retain the ability to make ATP without oxygen. These facultative anaerobes use ancient anaerobic pathways when oxygen is limited. A few bacteria remain as obligate anaerobes, which die in the presence of oxygen and depend on only the first (anaerobic) stage of cellular respiration.

    Aerobic and anaerobic pathways diverge after glycolysis splits glucose into two molecules of pyruvate:

    $$C_6 H_{12} O_6 + 2NAD^+ + 2P_i + 2ADP \longrightarrow 2\text{pyruvate} + 2NADH + 2ATP$$

    Pyruvate still contains a great deal of chemical energy. If oxygen is present, pyruvate enters the mitochondria for complete breakdown by the Krebs Cycle and electron transport chain. If oxygen is not present, cells must transform pyruvate to regenerate NAD+ in order to continue making ATP. Two different pathways accomplish this with rather famous products: lactic acid and ethyl alcohol (Figure 1). Making ATP in the absence of oxygen by glycolysis alone is known as fermentation. Therefore, these two pathways are called lactic acid fermentation and alcoholic fermentation. If you lack interest in organisms, such as yeast and bacteria, which have “stuck with” the anaerobic tradition, the products of these chemical reactions may still intrigue you. Fermentation makes bread, yogurt, beer, wine, and some new biofuels. In addition, some of your body’s cells are facultative anaerobes, retaining one of these ancient pathways for short-term, emergency use.

    Figure 1: Anaerobic and aerobic respiration share the glycolysis pathway. If oxygen is not present, fermentation may take place, producing lactic acid or ethyl alcohol and carbon dioxide. Products of fermentation still contain chemical energy, and are used widely to make foods and fuels.

    Lactic Acid Fermentation: Muscle Cells and Yogurt

    For chicken or turkey dinners, do you prefer light meat or dark? Do you consider yourself a sprinter, or a distance runner? (Figure 2)

    Figure 2: Light meat or dark? Sprinting or endurance? Muscle cells know two ways of making ATP – aerobic and anaerobic respiration.

    Are Drumsticks and Athletic Prowess Related

    Yes! Muscle color reflects its specialization for aerobic or anaerobic metabolism. Although humans are obligate aerobes, our muscle cells have not given up on ancient pathways which allow them to keep producing ATP quickly when oxygen runs low. The difference is more pronounced in chickens and grouse (Figure 3), which stand around all day on their legs. For long periods of time, they carry out aerobic respiration in their “specializedfor- endurance” red muscles. If you have ever hunted grouse, you know that these birds “flush” with great speed over short distances. Such “sprinting” flight depends on anaerobic respiration in the white cells of breast and wing muscle. No human muscle is all red or all white, but chances are, if you excel at running short distances or at weight lifting, you have more white glycolytic fibers in your leg muscles. If you run marathons, you probably have more red oxidative fibers.

    You probably were not aware that muscle cells “ferment.” Lactic acid fermentation is the type of anaerobic respiration carried out by yogurt bacteria (Lactobacillus and others) and by your own muscle cells when you work them hard and fast. Converting pyruvate to 3-carbon lactic acid (see Figure below) regenerates NAD+ so that glycolysis can continue to make ATP in low-oxygen conditions.

    Figure 3: Ruffed grouse use anaerobic respiration (lactic acid fermentation) in wing and breast muscles for quick bursts of speed to escape from predators (and hunters!).

    f For Lactobacillus bacteria, the acid resulting from fermentation kills bacterial competitors in buttermilk, yogurt, and some cottage cheese. The benefits extend to humans who enjoy these foods, as well (Figure 4).

    $$C_3 H_3 O_3 (\text{pyruvate}) \; + \; NADH \longrightarrow C_3 H_6 O_3 (\text{lactic acid}) \; + \; NAD^+$$

    You may have noticed this type of fermentation in your own muscles, because muscle fatigue and pain are associated with lactic acid. Keep this in mind, however, as we discuss a second type of fermentation, which produces alcohol. Imagine what would happen as you ran a race if muscle cells conducted alcoholic rather than lactic acid fermentation!

    Figure 4: Lactobacillus bacteria use the same type of anaerobic respiration as our muscle cells. Lactic acid reduces competition from other bacteria, and flavors yogurt, as well!

    Alcoholic Fermentation: A "New" Source of Energy?

    Have you fueled your car with corn? You have, if you bought gas within the city of Portland, Oregon. Portland was the first city to require that all gasoline sold within the city limits contain at least 10% ethanol. By mid-2006, nearly 6 million “flex-fuel” vehicles – which can use gasoline blends up to 85% ethanol (E85 – Figure 5) were traveling US roads. This “new” industry employs an “old” crew of yeast and bacteria to make ethanol by an even older biochemical pathway – alcoholic fermentation. Many people consider “renewable” biofuels such as ethanol a partial solution to the declining availability of “nonrenewable” fossil fuels. Although controversy still surrounds the true efficiency of producing fuel from corn, ethanol is creeping into the world fuel resource picture (Figure 6).

    Figure 5: Ethanol provides up to 85% of the energy needs of new “fuel-flex” cars. Although its energy efficiency is still controversial, ethanol from corn or cellulose appears to be more “renewable” than fossil fuels.

    Figure 6: One of the newest kids on the block, ethanol from corn or cellulose is produced by yeasts through alcoholic fermentation – an anaerobic type of respiration.

    You are probably most familiar with the term ”fermentation” in terms of alcoholic beverages. You may not have considered that the process is actually a chemical reaction certain bacteria and yeasts use to make ATP. Like lactic acid fermentation, alcoholic fermentation processes pyruvate one step further in order to regenerate NAD+ so that glycolysis can continue to make ATP. In this form of anaerobic respiration, pyruvate is broken down into ethyl alcohol and carbon dioxide:

    $$C_3 H_3 O_3 (\text{pyruvate}) \; + \; NADH \longrightarrow C_2 H_5 OH (\text{ethyl alcohol}) \; + \; CO_2 + NAD^+$$

    We have domesticated yeast (Figures 7 and Figure 8) to carry out this type of anaerobic respiration for many commercial purposes. When you make bread, you employ the yeast to make the bread “rise” by producing bubbles of carbon dioxide gas. Why do you suppose that eating bread does not intoxicate you?

    Figure 7: Yeasts are facultative anaerobes, which means that in the absence of oxygen, they use alcoholic fermentation to produce ethyl alcohol and carbon dioxide. Both products are important commercially.

    Figure 8: We employ yeasts to use their anaerobic talents to help bread rise (via bubbles of \(CO_2\)) and grapes ferment (adding ethanol).

    Brewers of beer and wine use yeast to add alcohol to beverages. Traditional varieties of yeast not only make but also limit the quantity of alcohol in these beverages, because above 18% by volume, alcohol becomes toxic to the yeast itself! We have recently developed new strains of yeast which can tolerate up to 25% alcohol by volume. These are used primarily in the production of ethanol fuel.

    Human use of alcoholic fermentation depends on the chemical energy remaining in pyruvate after glycolysis. Transforming pyruvate does not add ATP to that produced in glycolysis, and for anaerobic organisms, this is the end of the ATP-producing line. All types of anaerobic respiration yield only 2 ATP per glucose.

    Aerobic vs. Anaerobic Respiration: A Comparison

    As aerobes in a world of aerobic organisms, we tend to consider aerobic respiration “better” than fermentation. In some ways, it is. However, anaerobic respiration has persisted far longer on this planet, through major changes in atmosphere and life. There must be value in this alternative way of making ATP. We will compare the advantages and disadvantages of these two types of respiration.

    A major argument in favor of aerobic over anaerobic respiration is overall energy production. Without oxygen, organisms can only break 6-carbon glucose into two 3-carbon molecules. As we saw earlier, glycolysis releases only enough energy to produce two (net) ATP per molecule of glucose. In contrast, aerobic respiration breaks glucose all the way down to \(CO_2\), producing up to 38 ATP. Membrane transport costs can reduce this theoretical yield, but aerobic respiration consistently produces at least 15 times as much ATP as anaerobic respiration. This vast increase in energy production probably explains why aerobic organisms have come to dominate life on earth. It may also explain how organisms were able to increase in size, adding multicellularity and great diversity.

    However, anaerobic pathways persist, and a few obligate anaerobes have survived over 2 billion years beyond the evolution of aerobic respiration. What are the advantages of fermentation?

    One advantage is available to organisms occupying the few anoxic (lacking oxygen) niches remaining on earth. Oxygen remains the highly reactive, toxic gas which caused the “Oxygen Catastrophe.” Aerobic organisms have merely learned a few tricks – enzymes and antioxidants - to protect themselves. Organisms living in anoxic niches do not run the risk of oxygen exposure, so they do not need to spend energy to build these elaborate chemicals.

    Individual cells which experience anoxic conditions face greater challenges. We mentioned earlier that muscle cells “still remember” anaerobic respiration, using lactic acid fermentation to make ATP in low-oxygen conditions. Brain cells do not “remember”, and consequently cannot make any ATP without oxygen. This explains why death follows for most humans who endure more than four minutes without oxygen.

    Variation in muscle cells gives further insight into some benefits of anaerobic respiration. In vertebrate muscles, lactic acid fermentation allows muscles to produce ATP quickly during short bursts of strenuous activity. Muscle cells specialized for this type of activity show differences in structure as well as chemistry. Red muscle fibers are “dark” because they have a rich blood supply for a steady supply of oxygen, and a protein, myoglobin, which holds extra oxygen. They also contain more mitochondria, the organelle in which the Krebs cycle and electron transport chain conclude aerobic respiration. White muscle cells are “light” because they lack the rich blood supply, have fewer mitochondria, and store glycogen rather than oxygen. When you eat dark meat, you are eating endurance muscle. When you eat white meat, you are eating muscle built for sprinting.

    Each type of muscle fiber has advantages and disadvantages, which reflect their differing biochemical pathways. Aerobic respiration in red muscles produces a great deal of ATP from far less glucose - but slowly, over a long time. Anaerobic respiration in white muscles produces ATP rapidly for quick bursts of speed, but a predator who continues pursuit may eventually catch a white-muscled prey.

    In summary, aerobic and anaerobic respiration each have advantages under specific conditions. Aerobic respiration produces far more ATP, but risks exposure to oxygen toxicity. Anaerobic respiration is less energy-efficient, but allows survival in habitats which lack oxygen. Within the human body, both are important to muscle function. Muscle cells specialized for aerobic respiration provide endurance, and those specialized for lactic acid fermentation support short but intense energy expenditures. Both ways of making ATP play critical roles in life on earth.

    Images courtesy of:

    CK-12 Foundation. http://www.flickr.com/photos/kosherpickle/201168636/
    http://commons.wikimedia.org/wiki/Image:Red_Wine_Glas.jpg
    http://www.flickr.com/photos/momthebarbarian/2441500/
    http://commons.wikimedia.org/wiki/Image:Skeletal_muscle_diagram.jpg.
    CC-BY-SA 2.0,CC-BY-SA 2.5,CC-BY 2.0,GNU-FDL.

    http://www.flickr.com/photos/oskarn/459851554/. Mikol Graves,CC-BY-SA 2.0.

    http://www.flickr.com/photos/65189390@N00/622469923/. CC-BY.

    http://www.flickr.com/photos/momthebarbarian/2441500/. CC-BY.

    http://www.flickr.com/photos/kosherpickle/201168636/. CC-BY 2.0,CC-BY-SA.

    http://sca21.wikia.com/wiki/File:World_renewable_energy_2005.png. Public Domain.

    http://commons.wikimedia.org/wiki/Image:Yeast_cell_english.svg. (a)CC-BY-SA 2.5,(b)GNU-FDL.

    http://commons.wikimedia.org/wiki/Image:Red_Wine_Glas.jpg. (a)GNU-FDL,(b)CC-BY-SA.