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Cellular Respiration Section 3 Cellular Energy Objectives ● Summarize how glucose is broken down in the first stage of cellular respiration. ● Describe how ATP is made in the second stage of cellular respiration. 4B ● Identify the role of fermentation in the second stage of cellular respiration. 4B ● Evaluate the importance of oxygen in aerobic respiration. 4B Most of the foods we eat contain usable energy. Much of the energy in a hamburger, for example, is stored in proteins, carbohydrates, and fats. But before you can use that energy, it is transferred to ATP. Like in most organisms, your cells transfer the energy in organic compounds, especially glucose, to ATP through a process called cellular respiration. Oxygen in the air you breathe makes the production of ATP more efficient, although some ATP is made without oxygen. Metabolic processes that require oxygen are called aerobic (ehr OH bihk). Metabolic processes that do not require oxygen are called anaerobic (AN ehr oh bihk), meaning “without air.” The Stages of Cellular Respiration Cellular respiration is the process cells use to harvest the energy in organic compounds, particularly glucose. The breakdown of glucose during cellular respiration can be summarized by the following equation: Key Terms aerobic anaerobic glycolysis NADH Krebs cycle FADH2 fermentation enzymes C6H12O6 ! 6O2 ⎯⎯⎯⎯→ 6CO2 ! 6H2O ! energy glucose oxygen gas carbon dioxide water ATP As Figure 10 shows, cellular respiration occurs in two stages: Stage 1 Glucose is converted to pyruvate (PIE roo vayt), producing a small amount of ATP and NADH. Figure 10 Cellular respiration Cellular respiration occurs in two stages. NAD+ NADH 1. First, glucose is broken down to pyruvate. Anaerobic (without O2) Ethanol and CO2, or lactate 2. Then, either aerobic respiration or anaerobic processes occur. 104 Stage 2 When oxygen is present, pyruvate and NADH are used to make a large amount of ATP. This process is called aerobic respiration. Aerobic respiration Glucose occurs in the mitochondria of eukaryotic cells and in the cell membrane of prokaryotic cells. When oxygen is not present, pyruvate is converted to either ADP lactate (LAK tayt) or ethanol (ethyl alcoATP hol) and carbon dioxide. The equation above does not show Stage 1 how cellular respiration occurs. It simply Aerobic Pyruvate shows that the complete enzyme-assisted (with O2) breakdown of a glucose molecule uses six oxygen molecules and forms six Stage 2 carbon dioxide molecules, six water molecules, and ATP. Aerobic respiration produces most of the ATP made by cells. Mitochondrion Intermediate products of aerobic respiration form the organic compounds that ATP help build and maintain cells. CHAPTER 5 Photosynthesis and Cellular Respiration Copyright © by Holt, Rinehart and Winston. All rights reserved. Stage One: Breakdown of Glucose The primary fuel for cellular respiration is glucose, which is formed when carbohydrates such as starch and sucrose are broken down. If too few carbohydrates are available to meet an organism’s glucose needs, other molecules, such as fats, can be broken down to make ATP. In fact, one gram of fat contains more energy than two grams of carbohydrates. Proteins and nucleic acids can also be used to make ATP, but they are usually used for building important cell parts. Glycolysis In the first stage of cellular respiration, glucose is broken down in the cytoplasm during a process called glycolysis (glie KAHL uh sihs). Glycolysis is an enzyme-assisted anaerobic process that breaks down one six-carbon molecule of glucose to two threecarbon pyruvate ions. Recall that a molecule that has lost or gained one or more electrons is called an ion. Pyruvate is the ion of a three-carbon organic acid called pyruvic acid. The pyruvate produced during glycolysis still contains some of the energy that was stored in the glucose molecule. As glucose is broken down, some of its hydrogen atoms are transferred to an electron acceptor called NAD!. This forms an electron carrier called NADH . For cellular respiration to continue, the electrons carried by NADH are eventually donated to other organic compounds. This recycles NAD!, making it available to accept more electrons. Glycolysis is summarized in Figure 11. Step Step IO B graphic Step Two NADH molecules are produced, and one more phosphate group is transferred to each three-carbon compound. Step In a series of four reactions, each three-carbon compound is converted to a three-carbon pyruvate, producing four ATP molecules in the process. Glycolysis uses two ATP molecules but produces four ATP molecules, yielding a net gain of two ATP molecules. Glycolysis is followed by another set of reactions that use the energy temporarily stored in NADH to make more ATP. Copyright © by Holt, Rinehart and Winston. All rights reserved. Glycolysis Glucose C C C C C C 1 2 ADP 6-carbon compound C C C C C C 2 In a series of three reactions, phosphate groups from two ATP molecules are transferred to a glucose molecule. In two reactions, the resulting six-carbon compound is broken down to two three-carbon compounds, each with a phosphate group. Figure 11 Two 3-carbon compounds C C C C C C 2 NAD+ 3 2 NADH + 2H+ 2 Two 3-carbon compounds C C C C C C 4 ADP 4 Two 3-carbon pyruvates C C C C C C SECTION 3 Cellular Respiration 105 Stage Two: Production of ATP When oxygen is present, pyruvate produced during glycolysis enters a mitochondrion and is converted to a two-carbon compound. This reaction produces one carbon dioxide molecule, one NADH molecule, and one two-carbon acetyl (uh SEET uhl) group. The acetyl group is attached to a molecule called coenzyme A (CoA), forming a compound called acetyl-CoA (uh SEET uhl-koh ay). Krebs Cycle Acetyl-CoA enters a series of enzyme-assisted reactions called the Krebs cycle , summarized in Figure 12. The cycle is named for the biochemist Hans Krebs, who first described the cycle in 1937. O I B p a r g hic Step Acetyl-CoA combines with a four-carbon compound, forming a six-carbon compound and releasing coenzyme A. Step Carbon dioxide, CO2, is released from the six-carbon compound, forming a five-carbon compound. Electrons are transferred to NAD+, making a molecule of NADH. Figure 12 Krebs Cycle The Krebs cycle produces electron carriers that temporarily store chemical energy. 22. CO2 is released 11. Acetyl-CoA combines with a four-carbon compound, forming a six-carbon compound. Acetyl-CoA from the six-carbon compound, leaving a five-carbon compound. CoA C C 6-carbon compound C CO2 NAD+ C C C C C C NADH + H+ 4-carbon compound 5-carbon compound C C C C C C C C C 33. CO2 is released from the five-carbon compound, leaving a four-carbon compound. C CO2 NAD+ NADH + H+ NADH + H+ NAD+ 55. The new four-carbon compound is converted to the four-carbon compound that began the cycle. 4-carbon compound 4-carbon compound C C C C C C C C ADP + P ATP 44. The four-carbon compound FAD is converted to a new four-carbon compound. FADH2 106 CHAPTER 5 Photosynthesis and Cellular Respiration Copyright © by Holt, Rinehart and Winston. All rights reserved. Step Carbon dioxide is released from the five-carbon compound, resulting in a four-carbon compound. A molecule of ATP is made, and a molecule of NADH is also produced. Step The existing four-carbon compound is converted to a new four-carbon compound. Electrons are transferred to an electron acceptor called FAD, making a molecule of FADH2. FADH2 is another type of electron carrier. Step The new four-carbon compound is then converted to the four-carbon compound that began the cycle. Another molecule of NADH is produced. www.scilinks.org Topic: Aerobic Respiration Keyword: HX4004 After the Krebs cycle, NADH and FADH2 now contain much of the energy that was previously stored in glucose and pyruvate. When the Krebs cycle is completed, the four-carbon compound that began the cycle has been recycled, and acetyl-CoA can enter the cycle again. Electron Transport Chain In aerobic respiration, electrons donated by NADH and FADH2 pass through an electron transport chain, as shown in Figure 13. In eukaryotic cells, the electron transport chain is located in the inner membranes of mitochondria. The energy of these electrons is used to pump hydrogen ions out of the inner mitochondrial compartment. Hydrogen ions accumulate in the outer compartment, producing a concentration gradient across the inner membrane. Hydrogen ions diffuse back into the inner compartment through a carrier protein that adds a phosphate group to ADP, making ATP. At the end of the electron transport chain, hydrogen ions and spent electrons combine with oxygen molecules, O2, forming water molecules, H2O. Figure 13 Electron transport chain of aerobic respiration In the inner membranes of mitochondria, electron transport chains (represented by the red lines) make ATP. Outer compartment + H H+ H H+ + H+ H+ 3. ATP is produced as hydrogen ions diffuse into the inner compartment through a channel protein. eATP-producing carrier protein e- H+ H+ NAD+ H+ NADH + H+ Inner compartment 1. The electron transport chain pumps hydrogen ions, H+, out of the inner compartment. Copyright © by Holt, Rinehart and Winston. All rights reserved. 4H+ + O2 H+ Inner mitochondrial membrane 2H2O H+ 2. At the end of the chain, electrons and hydrogen ions combine with oxygen, forming water. ADP + P ATP SECTION 3 Cellular Respiration 107 Fermentation in the Absence of Oxygen Figure 14 Fermentation. In cheese making, fungi or prokaryotes added to milk carry out lactic acid fermentation on some of the sugar in the milk. What happens when there is not enough oxygen for aerobic respiration to occur? The electron transport chain does not function because oxygen is not available to serve as the final electron acceptor. Electrons are not transferred from NADH, and NAD! therefore cannot be recycled. When oxygen is not present, NAD! is recycled in another way. Under anaerobic conditions, electrons carried by NADH are transferred to pyruvate produced during glycolysis. This process recycles NAD! needed to continue making ATP through glycolysis. The recycling of NAD! using an organic hydrogen acceptor is called fermentation . Prokaryotes carry out more than a dozen kinds of fermentation, all using some form of organic hydrogen acceptor to recycle NAD!. Two important types of fermentation are lactic acid fermentation and alcoholic fermentation. Lactic acid fermentation by some prokaryotes and fungi is used in the production of foods such as yogurt and some cheeses, as shown in Figure 14. Lactic Acid Fermentation In some organisms, a three-carbon pyruvate is converted to a threecarbon lactate through lactic acid fermentation, as shown in Figure 15. Lactate is the ion of an organic acid called lactic acid. For example, during vigorous exercise pyruvate in muscles is converted to lactate when muscle cells must operate without enough oxygen. Fermentation enables glycolysis to continue producing ATP in muscles as long as the glucose supply lasts. Blood removes excess lactate from muscles. Lactate can build up in muscle cells if it is not removed quickly enough, sometimes causing muscle soreness. Figure 15 Two types of fermentation When oxygen is not present, cells recycle NAD+ through fermentation. In lactic acid fermentation, pyruvate is converted to lactate. Glucose Glycolysis C C C C C C In alcoholic fermentation, pyruvate is broken down to ethanol, releasing carbon dioxide, CO2. Pyruvate Glucose C C C C C C C C C Glycolysis Pyruvate C C C C NAD+ NADH + H+ Lactate NAD+ C C Lactic acid fermentation CHAPTER 5 Photosynthesis and Cellular Respiration CO2 2-carbon compound Ethanol C C C 108 NADH + H+ C C Alcoholic fermentation Copyright © by Holt, Rinehart and Winston. All rights reserved. Alcoholic Fermentation In other organisms, the three-carbon pyruvate is broken down to ethanol (ethyl alcohol), a two-carbon compound, through alcoholic fermentation. Carbon dioxide is released during the process. As shown in Figure 15, alcoholic fermentation is a two-step process. First, pyruvate is converted to a two-carbon compound, releasing carbon dioxide. Second, electrons are transferred from a molecule of NADH to the twocarbon compound, producing ethanol. As in lactic acid fermentation, NAD! is recycled, and glycolysis can continue to produce ATP. Alcoholic fermentation by yeast, a fungus, has been used in the preparation of many foods and beverages. Wine and beer contain ethanol made during alcoholic fermentation by yeast. Carbon dioxide released by the yeast causes the rising of bread dough and the carbonation of some alcoholic beverages, such as beer. Ethanol is actually toxic to yeast. At a concentration of about 12 percent ethanol kills yeast. Thus, naturally fermented wine contains about 12 percent ethanol. www.scilinks.org Topic: Fermentation Keyword: HX4080 Muscle Fatigue and Endurance Training A nyone who runs or exercises for a long period of time soon learns about muscle fatigue. As you continue vigorous exercise, the muscles you are using become fatigued—that is, tired and less able to generate force. The reasons for muscle fatigue are not fully understood, but in most cases the fatigue increases when the production of lactic acid by the exercising muscle increases. Anaerobic Threshold Why does an exercising muscle produce lactic acid? A resting muscle obtains most of its energy from aerobic respiration. A continuously exercising muscle, however, soon depletes its available oxygen. At this point, called the anaerobic threshold, the exercising muscle begins to obtain the ATP needed anaerobically. In the absence of oxygen, glycolysis extracts the required ATP from glycogen in the muscle. Glycogen Copyright © by Holt, Rinehart and Winston. All rights reserved. is a storable form of glucose that acts as an energy reserve. Glycolysis converts the muscle glycogen to pyruvate, which is then fermented to lactic acid. The ability to perform continuous exercise is limited by the body’s stored glycogen. So, physical endurance can increase if glycogen stored in muscles is spared during exercise. Trained athletes such as cyclist Lance Armstrong, shown at right, get a relatively large portion of their energy from aerobic respiration. Thus, their muscle glycogen reserve is depleted more slowly than that in untrained individuals. In fact, the greater the level of physical training, the higher the proportion of energy the body derives from aerobic respiration. Athletic Endurance Endurance-trained athletes generally have more muscle mass than untrained people. But it is Lance Armstrong endurance-trained athletes’ high aerobic capacity—rather than their greater muscle mass—that allows these athletes to exercise more before lactic acid production and glycogen depletion cause muscle fatigue. www.scilinks.org Topic: Anaerobic Threshold Keyword: HX4192 SECTION 3 Cellular Respiration 109 Figure 16 Effect of oxygen on ATP production Most ATP is produced during aerobic respiration. Glucose Glycolysis Fermentation Lactate Without O2 Pyruvate (Net) With O2 2 ATP Krebs cycle Ethanol and CO2 Electron transport chain Anaerobic processes 2 ATP (Up to) 34 ATP Aerobic respiration Production of ATP The total amount of ATP that a cell is able to harvest from each glucose molecule that enters glycolysis depends on the presence or absence of oxygen. As shown in Figure 16, cells use energy most efficiently when oxygen is present. In the first stage of cellular respiration, glucose is broken down to pyruvate during glycolysis. Glycolysis is an anaerobic process, and it results in a net gain of two ATP molecules. In the second stage of cellular respiration, the pyruvate passes through either aerobic respiration or (anaerobic) fermentation. When oxygen is present, aerobic respiration occurs. When oxygen is not present, fermentation occurs instead. The NAD! that gets recycled during fermentation allows glycolysis to continue producing ATP. Thus, a small amount of ATP is produced even during fermentation. Most of a cell’s ATP is made, however, during aerobic respiration. For each molecule of glucose that is broken down, as many as two ATP molecules are made directly during the Krebs cycle, and up to 34 ATP molecules are produced later by the electron transport chain. Section 3 Review List the products of glycolysis. What is the role of each of these products in cellular 4B respiration? Summarize the roles of the Krebs cycle and the electron transport chain during aerobic 4B respiration. Describe the role of fermentation in the second 4B stage of cellular respiration. Critical Thinking Comparing Functions Critical Thinking Inferring Conclusions Excess glucose in your blood is stored in your liver as glycogen. How might your body senses when to convert glucose to glycogen and glyco4B gen back to glucose? TAKS Test Prep When oxygen is present, most of the ATP made in cellular respiration is 9B produced by A aerobic respiration. C alcoholic fermentation. B glycolysis. D lactic acid fermentation. Explain why cellular respiration is more efficient 4B when oxygen is present in cells. 110 CHAPTER 5 Photosynthesis and Cellular Respiration Copyright © by Holt, Rinehart and Winston. All rights reserved.