Chapter 1. Anaerobic Respiration - Advanced CHAPTER 1 Anaerobic Respiration - Advanced Learning Objectives • Distinguish between obligate aerobes, obligate anaerobes, and facultative anaerobes. • Explain that, in the absence of oxygen fermentation reactions must regenerate NAD+ in order for glycolysis to continue making ATP. • 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. Why does a bakery smell so good? When bread bakes, yeast releases carbon dioxide gas, forming the small holes in bread. The gas was produced by alcoholic fermentation carried out by yeast. Anaerobic Respiration After the photosynthetic “oxygen catastrophe” challenged life between 2.5 and 3 billion years ago, evolution re- bounded with biochemical pathways to harness and protect against oxygen’s power. Today, most organisms use O2 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. Recall that O2 is the final electron acceptor at the end of the electron transport chain during aerobic respiration. Oxygen is required for oxidative phosphorylation to produce ATP. But in the absence of O2. ATP must still be made. The 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: + C6H12O6 + 2NAD + 2Pi + 2ADP → 2NADH + 2ATP 1 www.ck12.org FIGURE 1.1 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. Pyruvate still contains a great deal of chemical energy. In fact, two pyruvate molecules contain most of the chemical energy from the original glucose molecule. If oxygen is present, pyruvate enters the mitochondria for complete breakdown by the Kreb’s Cycle and electron transport chain. If oxygen is not present, cells must transform pyruvate to regenerate NAD+ in order to continue making ATP. Keep in mind that glycolysis produces a net total of 2 ATP. + Two different pathways accomplish the regeneration of NAD with rather famous products: lactic acid (C3H6O3) and ethyl alcohol (C2H6O) (Figure 1.1). 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. MEDIA Click image to the left or use the URL below. URL: https://www.ck12.org/flx/render/embeddedobject/184610 Gut Fermentation Behind each release of gas is an army of gut bacteria undergoing some crazy biochemistry. These bacteria break down the remains of digested food through fermentation, creating gas in the process. 2 www.ck12.org Chapter 1. Anaerobic Respiration - Advanced MEDIA Click image to the left or use the URL below. URL: https://www.ck12.org/flx/render/embeddedobject/143958 Summary • In the two to three billion years since photosynthesis added oxygen to earth’s atmosphere, life has become mostly aerobic. • Some organisms and types of cells retain the older, anaerobic pathways for making ATP; these pathways comprise anaerobic respiration or fermentation. • Obligate aerobes require oxygen to make ATP. Facultative anaerobes make ATP with oxygen, but if oxygen levels become low, they can use fermentation. Review 1. Define fermentation. 2. What are the two paths of fermentation and how do they differentiate? 3. Describe obligate aerobes, obligate anaerobes, or facultative anaerobes, and give examples. Vocabulary References 1. Bread: Flickr:PhotoDawg; Ethanol gas: Flickr:diaper; Red wine: George Hodan; Yogurt: Flickr:Mom the Barbarian; Body builder: Lin Mei. Bread:http://commons.wikimedia.org/wiki/File:Mixed_bread_loaves.jpg; Ethanol gas: http://www.flickr.com/photos/diaper/3799581840/; Red wine: http://www.publicdomainpictures .net/view-image.php?image=35183&picture=glass-of-red-wine; Yogurt: http://www.flickr.com/photos/momt hebarbarian/2441500/; Body builder: http://www.flickr.com/photos/leomei/2651904068/ . Bread, Ethanol gas, Yogurt, Muscle builder: CC BY 2.0; Red wine: Public Domain 3 .