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3.2 Moving to the Mitochondrion E X P E C TAT I O N S Describe the energy transformations that occur in aerobic cellular respiration. Explain the role of enzymes in metabolic reactions in the mitochondrion. Interpret qualitative data in a laboratory investigation of enzyme activity in the Krebs cycle. Explain the process of anerobic cellular metabolism. Research and describe how cellular processes are used in the food industry. Whereas glycolysis occurs in the cell cytosol, the remaining reactions of aerobic cellular respiration take place inside the mitochondria. As you have learned in previous studies, mitochondria are small organelles found in eukaryotic cells. A mitochondrion is composed of an outer and inner membrane, separated by an intermembrane space, as shown in Figure 3.7. The inner membrane folds as shelf-like cristae and contains the matrix, an enzyme-rich fluid. The cristae and the matrix are the sites where ATP synthesis occurs. More mitochondria are found in cells that require more energy, such as muscle and liver cells. The two mitochondrial membranes have important differences in their biochemical composition. The outer membrane contains a transport protein called porin that makes it permeable to all molecules of 10 000 u (atomic mass units) or less. It also contains enzymes that help convert fatty acids to molecules that can pass into the interior of the mitochondrion to be broken down further. The inner membrane contains a high proportion of cardiolipin, a phospholipid that makes this membrane especially impermeable to ions. This relative impermeability, as you will discover when you learn about chemiosmosis, is very important in the production of ATP. The intermembrane space is a fluid-filled area containing enzymes that use ATP. During the synthesis of ATP, the intermembrane space serves as a hydrogen ion (H+ ) reservoir, storing the hydrogen ions that will be used for ATP synthesis. Finally, the inner mitochondrial compartment, comprised of the matrix and the cristae, remains relatively isolated from the outer mitochondrial membranes. The inner membrane contains the multienzyme complexes and the electron carriers of the electron transport chain. During aerobic cellular respiration, the transfer of electrons through this chain creates a high hydrogen ion (H+ ) cristae matrix outer membrane intermembrane space 200 nm inner membrane Figure 3.7 Structure of the mitochondrion Chapter 3 Cellular Energy • MHR 69 concentration in the intermembrane space. This process creates a positively charged gradient in which there is a greater concentration of hydrogen ions on one side of the membrane than the other. The passage of these protons to the inner mitochondrial compartment drives the synthesis of ATP. to escape. This compromises the electron transport chain, which is indirectly involved in ATP production. Aerobic respiration stops. Cytochrome c, a key component of the electron transport chain, is released into the cytosol. Here, cytochrome c may combine with ATP and a protein, forming an apoptosome, or apoptosis activator. Endonuclease and protease enzymes in the cytosol are activated and break down the nucleus and the rest of the cell. BIO FACT Eukaryotic cells are believed to have arisen from a relationship, called endosymbiosis, between two prokaryotic cells, or bacteria. In this relationship, one cell, the endosymbiont, lived inside the other cell, called the host cell. The endosymbiont provided the host cell with a surplus of ATP molecules, while the host cell provided some metabolic functions for the endosymbiont. The endosymbiont eventually took the role of the mitochondrion found in eukaryotic cells of today. ELECTRONIC LEARNING PARTNER To learn more about how oxygen and nutrient levels can affect ATP production in mitochondria, go to your Electronic Learning Partner now. The Transition Reaction Mitochondria are the main source of ATP molecules in cells. When the mitochondria are not functioning properly, a depletion of ATP molecules can occur in tissues, such as muscle, brain, and heart. These tissues require large amounts of energy to function properly. Insufficient amounts of ATP can result in cell damage and even cell death. Eventually, entire body systems can fail, and the health of the organism may be severely compromised. The intact membranes of the mitochondria are important to the overall health of the cell. Mitochondria are believed to be key activators for apoptosis, or programmed cell death. When the mitochondrion produces ATP, the membranes are polarized by the high concentration of H+ ions in the intermembrane space. One of the early steps towards cell death is depolarization, the loss of this concentration gradient. Pores in both the outer membrane and inner membrane cause ions cytosol The transition reaction, which occurs in the matrix of the mitochondrion, is the first step in the process of aerobic cellular respiration. This process continues as long as sufficient levels of oxygen are available in the mitochondrion. If little or no oxygen is available, pyruvate in the cytosol can be oxidized through one of two fermentation processes. These processes will be described later in this section. Pyruvate crosses the mitochondrion’s outer membrane and then enters the matrix by way of a transport protein in the inner membrane (see Figure 3.8). Once in the matrix, the pyruvate dehydrogenase complex (a complex of enzymes) aids the process of oxidative decarboxylation. NAD+ removes two electrons, oxidizing pyruvate. Carbon dioxide is removed, leaving a two-carbon acetyl group that combines with coenzyme A to form acetyl-CoA. Coenzyme A is a compound that A One carbon atom and two oxygen atoms are removed from pyruvate as a CO2 molecule. mitochondrion transport protein NAD+ O NADH + H+ − B C O C O CH3 pyruvate outer membrane 70 S CoA C O CH3 A C CO2 Coenzyme A C The acetyl group of the acetate ion is transferred to coenzyme A, forming acetyl CoA. acetyl CoA matrix inner membrane inner membrane space MHR • Unit 1 Metabolic Processes B The remaining two-carbon fragment is oxidized to form an acetate ion. Electrons from this reaction are picked up by NAD+ , which is reduced to form NADH. Figure 3.8 In the transition reaction, pyruvate is converted to acetyl-CoA. This reaction marks the junction between glycolysis and the Krebs cycle. contains a sulfur-based functional group. This electronegative sulfur-based functional group binds to a carbon in the acetyl group to make the reactive acetyl-CoA. This product, acetyl-CoA, is a key reactant in the next step in aerobic respiration, the Krebs cycle. At this point, you have seen that carbohydrates provide the energy for ATP production. Lipids and proteins can be broken down to acetyl-CoA in the cell, and produce ATP in the mitochondrion. If ATP levels are high, acetyl-CoA can be directed into other metabolic pathways, such as the production of fatty pyruvate NAD CO2 + A The cycle starts when twocarbon acetate and four-carbon oxaloacetate react to form a six-carbon molecule, citric acid. NADH C C S CoA acetyl-CoA C CoA C E Dehydrogenation of malate forms a third NADH, and the cycle begins again. C C C 9 C C oxaloacetate (C4) H+ + NADH C C 1 aconitate (intermediate) NAD+ 8 C C C citrate (C6) C 2 C isocitrate (C6 ) C malate (C4) NAD+ 3 7 C C C C C C C H2O NADH + H+ CO2 B Oxidative decarboxylation forms NADH and CO2 . fumarate (C4) C α-ketoglutarate (C5) C C FADH2 D Oxidation of succinate forms FADH2. FAD 6 C CO2 NAD+ 4 C succinate (C4) NADH + H+ 5 C C succinyl-CoA (C4) C C C A second oxidative decarboxylation forms another NADH and CO2 . C C C C GTP GDP C C ADP S CoA ATP Figure 3.9 The Krebs cycle, which includes nine (numbered) reactions, oxidizes acetyl-CoA. Can you identify where substrate-level phosphorylation takes place? Chapter 3 Cellular Energy • MHR 71 acids that are required to produce lipids. The energy stored in these molecules can be released later, as needed. You can find a more detailed description of these other metabolic pathways at the end of this section. As the product of the transition reaction from glycolysis, acetyl-CoA enters the Krebs cycle to produce ATP. The Krebs Cycle The Krebs cycle, named after scientist Hans Krebs, is a cyclical metabolic pathway that oxidizes acetyl-CoA to carbon dioxide and water, forming a molecule of ATP (see Figure 3.9 on page 71). In Investigation addition to producing one ATP molecule, the series of redox reactions that form the Krebs cycle involve the transfer of electrons. This electron transfer leads to the formation of three NADH molecules and one FADH2 molecule. The Krebs cycle involves a total of nine reactions. First, an enzyme removes the acetyl group from acetyl-CoA and combines it with a four-carbon oxaloacetate molecule to produce a six-carbon citrate molecule. With the acetyl group removed, coenzyme A is released to participate in another reaction in the matrix. At this point, a series of oxidation-reduction reactions begins that will result in the formation of SKILL FOCUS 3 • A Predicting Enzyme Activity in the Krebs Cycle The enzyme succinic dehydrogenase catalyzes the oxidation of the four-carbon molecule succinic acid to fumaric acid. This is an essential step in the Krebs cycle. In this investigation, you will use a solution of methylene blue to observe the action of succinic dehydrogenase. Fresh beef heart tissue will serve as a source of succinic dehydrogenase. Pre-lab Questions What is the purpose of oxidizing succinic acid in the Krebs cycle? How does methylene blue indicate succinic dehydrogenase activity? Problem Where is succinic acid oxidized in a cell? In other words, where does succinic dehydrogenase activity occur? Prediction Predict how you could observe succinic dehydrogenase activity. Performing and recording Analyzing and interpreting Communicating results Materials 500 mL beaker 3 test tubes scalpel or sharp knife 3 medicine droppers hot plate thermometer grease pencil colour chart blue coloured pencils (several shades) 0.5 mol/L succinic acid methylene blue (0.01%) distilled water mineral oil two pieces of beef heart (each about 2–3 cm3 ) Procedure 1. Set up a hot water bath in the 500 mL beaker, and maintain it at 37°C. 2. Using the grease pencil, number each test tube. CAUTION: The indicator methylene blue is a dye. Avoid any contact with skin, eyes, or clothes. Flush spills on your skin immediately with copious amounts of water and inform your teacher. Exercise care when handling hot objects. Handle thermometers with care. Follow your teacher’s instructions on the safe use of scalpels. Dispose of all chemicals according to your teacher’s instructions and wash your hands before leaving the laboratory. 72 MHR • Unit 1 Metabolic Processes 3. Using the scalpel, cut away any fat tissue from the beef muscle. Cut away two small pieces of tissue and set aside. 4. In test tube 1, add a piece of beef heart, 4 drops of succinic acid, 8 drops of methylene blue, and 8 drops of distilled water. 5. In test tube 2, add 4 drops of succinic acid, 8 drops of methylene blue, and sufficient distilled water to equal the volume of the mixture in test tube 1. two carbon dioxide molecules and one ATP molecule (produced by substrate-level phosphorylation). During the cycle, energetic electrons reduce NAD+ and FAD, which combine with H+ ions to form NADH and FADH2 . Remember that an additional NADH was produced in the transition reaction. As you learned in Chapter 2, FAD is a coenzyme involved in redox reactions. Figure 3.9 shows the various reactions involved in the Krebs cycle. Four NADH molecules and one FADH2 molecule are produced for each molecule of pyruvate that enters the mitochondrion for aerobic respiration. Because two molecules of pyruvate enter the matrix for each molecule of glucose oxidized, 6. In test tube 3, add a piece of beef heart, 8 drops of methylene blue, and sufficient distilled water to equal the volume of the mixture in test tube 1. 7. Gently swirl the contents of each test tube. Then, quickly but carefully, use a medicine dropper to add about a 2 cm layer of mineral oil to each test tube. To do this, tip the test tube slightly to one side and allow the mineral oil to flow down the inside of the test tube. eight NADH and two FADH2 are produced for each molecule of glucose. Of these, six NADH and two FADH2 , along with two ATP molecules, result from the reactions of the Krebs cycle. At this point, oxygen has not been used in the reactions described. Oxygen plays a crucial role in the electron transport chain, during oxidative phosphorylation. NADH and FADH2 molecules that have been formed by redox reactions in the Krebs cycle will donate electrons to the electron transport chain. Energy from these electrons fuels ATP synthesis by aerobic respiration. At the end of the cycle, the glucose molecule that entered glycolysis has been completely 2. Methylene blue is a dye (indicator) that changes from blue to colourless when it is reduced by other substances in a chemical reaction. Describe the chemical reaction that was responsible for changing the colour of the methylene blue in the test samples. 3. What other controls could be incorporated into this procedure? 4. Why was it necessary to add a layer of oil to the surface of each mixture? Conclude and Apply 5. What cell structures (organelles) contain succinic dehydrogenase? Why would you expect to find high concentrations of these cell organelles in mammalian heart tissue? 6. Would this procedure work with other types of animal tissue? Explain briefly. 8. Place the test tubes in the water bath. Record the initial colour of the mixture in each test tube. Use a colour chart or make a series of categories, such as dark blue, medium blue, light blue, colourless. You can also make a series of sketches using coloured pencils. If possible, take a series of photographs of the samples to augment your written observations. 9. Observe the colour of the mixture in each test tube every 5 min. for about 30 min. Use a data chart to record your observations. Post-lab Questions 7. How could you modify this procedure to obtain more accurate evidence of succinic dehydrogenase activity? Exploring Further 8. Repeat the investigation using the additional controls you outlined in answer to question 3. How would these controls help you to interpret the results? 9. Does plant tissue produce similar succinic dehydrogenase activity? Repeat the procedure using germinating white beans in place of beef heart tissue. What other plant parts might be suitable for this investigation? Explain briefly. 1. Which sample(s) showed the most pronounced change in colour? Why? Chapter 3 Cellular Energy • MHR 73 catabolized. As Table 3.1 shows, by the end of the Kreb’s cycle, energy contained in the original molecule of glucose has been used to form four ATP molecules and 12 electron carriers. 2e− reduction NADH Figure 3.9 shows that succinate, the ionic form of succinic acid, is oxidized to produce one molecule of FADH2 . The enzyme that catalyzes this reaction is succinic dehydrogenase. You will study the action of this enzyme in Investigation 3-A. Table 3.1 NAD+ + H+ The output of energy molecules up to the end of the Krebs Cycle A NADH and FADH2 bring electrons to the electron transport chain. ATP is produced at the ATP synthase complex, but the diagram shows the power of the ADP + Pi proton pumps for chemiosmosis. NADH reductase oxidation H+ + H 2e− ATP 2e− reduction FADH2 B Each pair of electrons from NADH pulls 3 pairs of H+ ions into the intermembrane space to make 3 ATP with ATP synthase. FAD + 2H coenzyme Q 2e− D Each of the electron carriers becomes reduced and then oxidized as the electrons move down the chain. reduction cytochrome b, c1 ADP + Pi oxidation 2e− ATP reduction cytochrome c oxidation E As a pair of electrons is passed from carrier to carrier, proton pumps carry H+ ions into the intermembrane space. 2e− reduction cytochrome c oxidase ADP + Pi oxidation ATP 2e− F Oxygen is the final acceptor of the electrons, and together with hydrogen becomes water and joins the general water content of the cell. 2H+ 1 O 2 2 Figure 3.10 Overview of the electron transport chain 74 MHR • Unit 1 Metabolic Processes glycolysis ATP produced Energy molecules 2 ATP 2 NADH (in cytosol) oxidation (decarboxylation) of pyruvate (× 2) 2 NADH Krebs cycle (× 2) 2 ATP 6 NADH 2 FADH2 Total 4 ATP 10 NADH 2 FADH2 + C Each pair of electrons from FADH2 pulls 2 pairs of H+ ions into the intermembrane space to make 2 ATP with ATP synthase. oxidation Metabolic process H2O Chemiosmosis and ATP Production The final stage of energy transformation in aerobic cellular respiration includes the electron transport chain and oxidative phosphorylation of ADP by chemiosmosis. The reduced coenzymes NADH and FADH2 shuttle electrons and H+ ions from the Krebs cycle in the matrix to the electron transport chain embedded on the cristae, the folds of the mitochondrion’s inner membrane. The electron transport chain involves a series of electron carriers and multienzyme complexes. These carriers and complexes oxidize NADH and FADH2 molecules. With their extra electrons removed, the additional H+ ions also leave. As a result, the oxidized molecules NAD+ and FAD can participate in a redox reaction in the matrix, such as the Krebs cycle. The oxidation and reduction of the electron carriers in the electron transport chain releases small amounts of energy. This energy is then used to power proton pumps that pull H+ ions across the inner membrane into the intermembrane space. These H+ ions are now trapped between two membranes, building a concentration gradient between the intermembrane space and the matrix. The movement of these H+ ions through special channels drives the production of ATP. (As you learned in the previous section, this movement of H+ ions through a special protein complex is called chemiosmosis.) Figure 3.10 shows how electron transfer moves H+ ions. Recall that during glycolysis and the Krebs cycle, ATP molecules are produced through substratelevel phosphorylation. In this process, the ADP molecule is phosphorylated. A phosphate group is moved from another substrate (like PEP) to ADP to make ATP. In the electron transport chain, the carriers are reduced (accept electrons) and then oxidized (lose electrons to the next carrier) in a sequence. At each step, some energy is liberated and used to pump H+ (against a concentration gradient) across the inner membrane to the intermembrane space. In addition, some of the liberated energy is lost to the environment as thermal energy. At the end of this process, the spent (low energy) electrons must be removed. Oxygen must be present in the matrix to oxidize the last component of the electron transport chain. When oxygen is combined with available H+ ions in the matrix, water is formed. This allows additional electrons to enter the electron transport chain and release the energy needed to pump more H+ ions into the intermembrane space. ATP is produced when the high concentration of H+ ions diffuses through the channel of the ATP synthase complex that is embedded in the inner membrane, as shown in Figure 3.11. Because of the crucial role played by oxygen, this process of A As electrons (e− ) move through the electron transport chain, hydrogen ions (H+ ) are pumped from the matrix into the intermembrane space. H+ + H B A hydrogen ion gradient is formed, with a higher concentration of ions in the intermembrane space than in the matrix. H+ + + H H proton pump chemiosmosis is also called oxidative phosphorylation. In the next section, you will see that a similar process is used to make ATP in photosynthesis. When NADH is oxidized, electrons enter the electron transport chain. The electrons first transfer to NADH dehydrogenase complex, a multienzyme system that oxidizes NADH. The liberated hydrogen ions are released into the matrix, leaving NAD+ . The electrons are passed along from one carrier to another, as shown in Figure 3.10. Some of these carriers are cytochromes, proteins with a heme group containing an iron atom that can be oxidized or reduced reversibly. The two electrons from the FADH2 molecules, which have less energy than the electrons carried by NADH, enter the electron transport chain at a different point. As a result, the less energetic electrons from FADH2 are responsible for the production of fewer ATP molecules. Energy from the electrons powers the multienzyme complexes, called proton pumps, to move hydrogen ions (H+ ) into the intermembrane space, as shown in Figure 3.11. As a result of this high concentration of H+ ions, the inner membrane of the mitochondrion becomes positively charged. At the same time, negative ions are attracted to the exterior of the outer membrane, helping the movement of protons through the proton pumps. C When hydrogen ions flow back into the matrix down their concentration gradient, ATP is synthesized from ADP + Pi by an ATP synthase complex. H ATP synthase + electron carriers intermembrane space H+ + H H+ H+ inner membrane electron pathway NADH NAD+ FADH2 H+ FAD 4 H+ + O2 H2O H2O ADP + Pi ATP H+ matrix Figure 3.11 Mitochondria synthesize ATP by chemiosmosis. ATP production is based on a gradient of hydrogen ion (H+ ) concentration. The gradient is established by pumping hydrogen ions into the intermembrane space of the mitochondrion. Chapter 3 Cellular Energy • MHR 75 The charge difference creates an electrical gradient, while the concentration difference creates a chemical gradient. The H+ ions can leave the intermembrane space through a special channel in the ATP synthase complex, as shown in Figure 3.11. Chemiosmosis, the movement of these H+ ions, creates an electric current of positively charged particles. This current provides the energy needed to phosphorylate ADP with inorganic phosphate ions in the matrix, forming ATP. The multienzyme ATP synthase complex is not part of the electron transport chain. The electrons moving along the electron transport chain are not directly involved in the production of ATP. These electrons activate the proton pumps that move the H+ ions into the intermembrane space. ATP synthase complex is both a collection of enzymes that phosphorylate ADP, and a tunnel that allows the H+ ions to move from the intermembrane space back into the matrix. This flow of charged particles provides energy that is used by the enzymes to phosphorylate ADP, and release H+ ions back into the matrix for other reactions. These reactions include the formation of water at the end of the electron transport chain. This process produces about 90 percent of the ATP molecules in a cell. Most ATP production takes place during the reactions of the electron transport chain and chemiosmosis, because of the input of NADH and FADH2 molecules (see Figure 3.12). NADH that is produced by glycolysis in the cytosol can donate electrons to the electron transport chain by way of the outer membrane of the mitochondrion. This additional step has a cost, however, and the electrons from the cytosol NADH can only pump enough protons for two ATP. In contrast, the NADH molecules that are produced in the matrix can produce three ATP. WEB LINK www.mcgrawhill.ca/links/biology12 Since Boyer and Walker’s work on ATP synthase, biochemists worldwide have been studying the workings of ATP synthase complex. To find out more about the intricacies of ATP synthase, go to the web site above, and click on Web Links. Prepare an abstract about research into one aspect of the ATP synthase complex. cytoplasm glucose 2 glycolysis ATP 2 NADH electron transport chain 2 pyruvate 2 NADH mitochondrion 2 acetyl-CoA 2 CO2 6 NADH 2 Krebs cycle ATP Substrate-level phosphorylation during glycolysis and the Krebs cycle produces four ATP for each molecule of glucose oxidized. Oxidative phosphorylation from the electron transport chain and chemiosmosis accounts for the remaining 32 ATP molecules. The total number of ATP molecules produced from one molecule of glucose is 36, as shown in Figure 3.12. The concept organizer, shown in Figure 3.13 on the next page, summarizes the 4 ATP important concepts you have learned that are involved in ATP 6 ATP synthesis. Refer to the chapters and sections listed to review these concepts. 2 FADH2 18 ATP 4 ATP 4 CO2 O2 ATP Yield 4 + ATP H2O 32 ATP Figure 3.12 Number of ATP 36 76 MHR • Unit 1 Metabolic Processes ATP molecules produced for each molecule of glucose used in aerobic cellular respiration CONCEPT ORGANIZER ATP Synthesis Enzymes (Chapter 2, section 2.2) Energy (Chapter 2, section 2.1) Coupled reactions (Chapter 2, section 2.3) ATP synthesis Electrons (Chapter 1, section 1.1; Chapter 3, section 3.2) Chemiosmosis (Chapter 3, section 3.2) Enzymes catalyze exothermic chemical reactions, which release energy. Energy from these reactions is coupled to ATP formation. In the electron transport chain, energy from electrons is used to move H+ ions into the intermembrane space. During chemiosmosis, the movement of hydrogen ions (H+ ) across the inner membrane releases energy, which is used to form ATP molecules. Figure 3.13 ATP formation during cellular respiration THINKING LAB Metabolic Rate and Exercise Background Researchers working on the impact of exercise and diet on human metabolic processes have come up with six actions to improve resting or basal metabolic rate (BMR). These six actions increase the efficiency with which cells metabolize food molecules into ATP and anabolize macromolecules for cellular work. 1. Exercise frequently, for a long period of time (between 30 and 60 minutes), and with sufficient intensity. 2. Increase metabolically active tissue (or muscles) through total-body exercise. 3. Eat well between five and six times per day. 4. Eat within 30 to 60 minutes after exercising. 5. Split your exercise routine into two sessions per day, and drink a carbohydrate drink 30 to 60 minutes after each session. 2. How would any of these actions increase the rate at which you metabolize food? Record your findings in a notebook. 3. How could you calculate your basal metabolic rate? 4. If you are able, experiment with any two or more of the above actions to improve your basal metabolic rate. First, determine your basal metabolic rate before you begin your study. Then practise the actions for at least three weeks, recording your exercise and eating regimen daily. Calculate your BMR again and compare the result to your original BMR. Did a three-week period of prescribed exercise and diet increase or decrease your BMR? 6. Concentrate on exercising large muscle groups. You Try It 1. Using the library and/or Internet resources, conduct further research on human basal metabolic rate focussing on the actions listed above. Chapter 3 Cellular Energy • MHR 77 Anaerobic Cellular Respiration When oxygen is not present, there is no electron acceptor to remove electrons at the end of the electron transport chain. As a result, no further oxidation of NADH and FADH2 is possible. No H+ ions can be pumped into the intermembrane space. Without the oxidative coenzymes NAD+ and FAD, the Krebs cycle stops. The transition reaction, which requires NAD+ , also stops. Pyruvate can enter the mitochondrion, but it cannot be processed. The Biology Magazine concentration of pyruvate inside the mitochondrion increases until it matches the concentration in the cytosol. As a result, there is no net movement of pyruvate into the mitochondrion. This leads to a high level of pyruvate in the cytosol, where one of two possible fermentation pathways may be followed. These pathways are examples of anaerobic cellular respiration, the production of ATP in the cell without the use of oxygen. Figure 3.14 compares aerobic and anaerobic cellular respiration. TECHNOLOGY • SOCIETY • ENVIRONMENT Nutrition and Cellular Energy then also enter the Krebs cycle in the mitochondria to produce ATP. Most people know that a healthy, balanced diet is important for good health. Dieticians recommend that the average person should consume a balanced diet in which 60 to 65 percent of daily energy requirements come from carbohydrates, 30 to 35 percent from fat, and the remainder from protein. What role does each type of food play in the production of cellular energy? Sources of dietary fats include butter, cream, margarine, and oil, and any food item prepared with these ingredients. Fats are also found in meats and nuts. Health professionals usually recommend choosing unsaturated fats such as olive or canola oil over saturated fats such as butter or margarine. Carbohydrates Protein Carbohydrates are essential to cellular respiration. They break down to form simple sugars such as glucose, which is the primary reactant for glycolysis. Glucose undergoes glycolysis to produce pyruvic acid, which combines with coenzyme A (CoA) to form acetyl-CoA. Acetyl-CoA enters the Krebs cycle to produce ATP molecules. Once in the body, protein is broken down into amino acids. The amino acids contribute to the production of cellular energy via several routes. Excess carbohydrates are converted into glycogen, a large molecule that is stored in muscle and liver cells. When the body requires energy, the glycogen can be broken down into molecules of glucose to begin glycolysis. Amino acids can also be converted into lipids and stored in the body’s fatty tissue. Good sources of dietary carbohydrates include pasta, rice, whole grains, and bread. Dieticians usually advise choosing sources of complex carbohydrates (such as brown rice or whole-wheat bread) over sources of refined carbohydrates (such as sugar, white rice, or white bread) whenever possible. Complex carbohydrates contain vitamins and dietary fibre, which are missing from refined carbohydrates. Amino acids can be converted into glucose or glycogen to enter glycolysis, eventually entering the Kreb’s cycle as pyruvic acid. Amino acids are used for numerous other functions within the body, including the synthesis of new tissue proteins, nucleic acids, hormones, and antibodies. Dietary protein is found in meat, including red meat, poultry, and seafoods. Protein is also found in eggs and milk products. You can get protein from plants in the form of beans, peas, and nuts. Cereals and pasta also contain certain kinds of protein. Fats Fats belong to the group of substances known as lipids. In the body, lipids are hydrolyzed into glycerol and fatty acids. Glycerol is converted to PGAL and then enters glycolysis. The pyruvic acid produced can then enter the Krebs cycle in the mitochondria to produce ATP. Fatty acids may be stored as tissue fat or be converted to a two-carbon fragment that can enter the mitochondrion to form acetyl CoA. The acetyl CoA can 78 MHR • Unit 1 Metabolic Processes Dietary carbohydrates, fats, and protein come from a wide variety of foods. Figure 3.15, on page 80, shows the two possible fermentation pathways. In each case, NADH molecules in the cytosol (formed by glycolysis II) are oxidized to form NAD+ , allowing glycolysis to continue. Figure 3.14 Following the formation of pyruvate from glucose, pyruvate may enter either aerobic cellular respiration or anerobic cellular respiration. Manipulating Nutrient Intake People who want to lose weight or reduce body fat sometimes turn to restrictive low-fat or low-carbohydrate diets. These types of diets may lead to weight loss in the short term, but often have detrimental effects on the body’s metabolic function. The Effects of Extreme Low-Fat Diets For example, some people try to control their weight by severely restricting their fat intake, instead ingesting large amounts of pasta, bread, and other carbohydrates Someone who severely restricts their fat intake will not necessarily prevent further accumulation of fat on their body. This is particularly true when someone who is not eating enough fat compensates by overeating carbohydrates. Glucose from carbohydrates can be converted to fatty acids via acetyl Co-A during the transition reaction. Glucose can also be converted to glycerol through glyceraldegyde-3-phosphate, as shown below. In short, if you ingest food that contains more energy than you can use, your body will store it as fat, regardless of whether the food is carbohydrate, protein, or fat. glucose glyceraldehyde-3-phosphate glycerol fats pyruvate fatty acids acetyl-CoA Excess glucose is used to synthesize fats. The Effects of Extreme Low-Carbohydrate Diets On the opposite end of the scale, extreme lowcarbohydrate diets, which tend also to be low in nutrients glucose pyruvate oxygen present oxygen absent aerobic cellular respiration (reactions inside mitochondria) anerobic cellular respiration (reactions outside mitochondria) overall, can also cause metabolic problems. People on these diets are forced to derive their energy primarily from stored fats. Breaking down lipids to form acetyl-CoA for the Krebs cycle, however, produces byproducts that can be harmful in large amounts. These byproducts are compounds with ketone groups. Some of these compounds are acids. The condition caused by the build-up of ketones is called ketosis, and is characterized by the smell of acetone on the breath. If the condition progresses, the acidic ketones can lower the blood pH to dangerous levels. Brain damage or even death can result. This extreme condition is called ketoacidosis. Most experts agree that the best way to stay healthy and maintain a healthy weight is to eat a broad variety of foods that provide the right balance and quantity of carbohydrates, protein, and fats. Follow-Up 1. What percentage of your daily food intake comes from carbohydrates, lipids, and proteins? Keep a food log for several days to monitor your intake of carbohydrates, proteins, and fats. Should you make any adjustments to your diet to provide a healthier mix of dietary energy sources? 2. To help people to follow a healthy, balanced diet, organizations and governments issue dietary guidelines. Do some research on the Internet to find out about several different dietary guidelines produced for the general public. For example, you might choose to compare the Canada Food Guide and the Federal Dietary Guidelines (U.S.). (a) How do the guidelines compare with respect to recommended nutrient sources and the percentage of each type of nutrient? (b) Which set of guidelines is easier to understand and follow? (c) Make recommendations on how you would improve or expand upon each set of guidelines. Chapter 3 Cellular Energy • MHR 79 Alcohol Fermentation In alcohol fermentation, two reactions of fermentation convert pyruvate to ethanol. The first reaction reduces pyruvate to CO2 and acetaldehyde. In the second reaction, NADH reduces the acetaldehyde to ethanol (see Figure 3.15). Two NAD+ are formed and participate in glycolysis II. This process ensures that glycolysis can continue. Yeast cells, shown in Figure 3.16, are eukaryotes that carry out alcohol fermentation. This process is used in the manufacture of wine, for example. The glucose C6 2 ATP 2 ADP 2 PGAL C3 P 2 Pi 2 NAD + 2 NADH 2 PGAP P P C3 4 ADP 4 ATP 2 2 NADH pyruvate Lactic Acid Fermentation Lactic acid fermentation occurs in certain fungi, bacteria, and muscle cells. For example, when human muscle cells undergo strenuous activity they may experience a depletion of oxygen. When there is insufficient oxygen for muscle cells to undergo aerobic cellular respiration, the cells will continue to produce ATP through fermentation. During this process, no CO2 is produced. Instead, lactic acid is produced. During strenuous activity, lactic acid levels increase in muscle cells. In the short term, lactic acid buildup can cause muscle fatigue and pain resulting from muscle cramps. The excess lactic acid is eventually removed from the cells by the circulatory system and muscle pain begins to ease. The lactic acid is cleansed from the bloodstream by the liver. There is a physiological advantage to producing lactic acid when energy output exceeds oxygen intake. Through lactic acid fermentation, muscles can continue to function in the absence of oxygen. Table 3.2 compares the number of ATP molecules produced through anerobic and aerobic cellular respiration. Table 3.2 C3 2 NAD + CO2 or formation of ethanol from pyruvate occurs naturally if grapes are allowed to ferment on the vine or ferment in winemaking vats. When the ethanol concentration reaches a certain level, 12–16 percent depending on the variety of yeast being used, the toxic effects of the ethanol kill the yeast cells. Lactic acid and ethanol fermentation produce the same number of ATP molecules. These processes are far less efficient in ATP synthesis than is aerobic cellular respiration. Cellular respiration acetaldehyde Anerobic 2 lactic acid (C3 ) 2 ethanol (C2 ) Figure 3.15 Possible pathways of pyruvate metabolism Lactic acid glucose glycolysis pyruvate fermentation Ethanol glucose glycolysis Aerobic glucose glycolysis pyruvate fermentation 2 lactic acid + CO2 + 2 ATP 2 ethanol + 2 ATP pyruvate Krebs cycle and electron transport chain CO2 + water + 36 ATP Figure 3.16 These yeast cells can survive in an oxygen-free environment. How do yeast cells produce ATP molecules? 80 MHR • Unit 1 Metabolic Processes Probeware Try the investigation: Aerobic or Anaerobic: Can You Tell When the Switch Occurs? BIO FACT Athletes train to improve the ability of their muscle cells to accommodate lactic acid buildup. They do this by using breathing techniques and “sprinting” exercises that serve to saturate muscle cells with lactic acid. Such exercises involve working muscles at a maximum heart rate for about 20 minutes, then resting until the body has recovered and removed the lactic acid from the blood. Other Metabolic Pathways Glycolysis, the transition reaction, the Krebs cycle, and oxidative phosphorylation are the key steps in aerobic cellular respiration. However, it is important to consider that intermediate compounds produced in aerobic cellular respiration may be shunted sideways into other reactions that are not part of aerobic cellular respiration. For instance, carbohydrate metabolism in the mitochondrion, including the oxidation of glucose by aerobic cellular respiration, produces many molecules important in cellular processes (see Figure 3.17). Anabolic reactions take the carbon-based molecules that result from cellular metabolism and build them into such complex macromolecules as steroid hormones, lipids, porphyrins (e.g. hemoglobin), and proteins. Although glucose molecules are the main substrate in cellular respiration, ATP can be manufactured through the breakdown of proteins and fats as well. The breakdown of proteins in this type of reaction is called deamination which involves the initial removal of an −NH2 group from the protein. In fats, β-oxidation removes twocarbon acetate units from the carboxyl end of longchain fatty acids. This reaction occurs in the outer mitochondrial membrane where enzymes catalyze the Coenzyme A reaction to produce acetyl-CoA. This is the same acetyl-CoA that is produced by the transition reaction of pyruvate. The acetyl-CoA enters the Krebs cycle in exactly the same manner. Some organisms, such as anaerobic bacteria, make enough ATP using fermentation to survive. Eukaryotes, however, tend to rely on mitochondria to produce energy. The mitochondria are more efficient, using the product of glycolysis — carbohydrate glucose-6- P glycerol GAP alanine protein lipid pyruvate fatty acids acetyl-CoA cholesterol aspartate oxaloacetate steroid hormones citrate Krebs cycle succinate α-ketoglutarate porphyrins (e.g. hemoglobin) protein glutamate pyrimidine nucleotides Figure 3.17 Molecules formed in nucleic acids conjunction with aerobic cellular respiration Chapter 3 Cellular Energy • MHR 81 pyruvate — to make ATP. The organisms that use aerobic cellular respiration also rely, to a certain extent, on the anaerobic process of lactic acid fermentation. The fermentation of lactic acid can continue to provide muscles with energy in ATP molecules when oxygen is not available. In the next section, you will learn that green plants also use aerobic cellular respiration to SECTION REVIEW 9. I A strain of cells undergoes a mutation that increases the permeability of the inner mitochondrial membrane to hydrogen ions. 1. K/U Explain how enzymes within the mitochondrion catalyze metabolic reactions that involve the products of glycolysis. 2. How many turns of the citric acid cycle are required to catabolize one molecule of glucose? (a) What effect would you expect this mutation to have on the process of cellular respiration? 3. Draw a diagram that shows three possible reaction pathways for pyruvate following glycolysis. Identify the main products of each reaction. (b) Assuming the mutant cells can survive, how might the metabolic requirements of these cells differ from those of a non-mutant strain of the same variety? 4. K/U Under what conditions does fermentation take place in an animal muscle cell? Explain. 5. K/U The conversion of pyruvate to lactic acid does not produce any ATP. How, then, does this reaction contribute to the production of energy by a cell? 6. K/U C C Working with a partner or in a small group, use an analogy to explain the role of the mitochondrion in cellular respiration. Outline how various details in the analogy relate to various components of respiration in the mitochondrion. 7. MC In your local grocery store, compare the prices of foods that are rich in fat with those rich in carbohydrates. What general pattern can you see? How could you explain this pattern in terms of cellular metabolism? 8. A baker wishes to make a loaf of bread. According to the recipe, she should first prepare a yeast culture by mixing some dried yeast with warm water and a little sugar. The other ingredients are added to this mixture later. MC (a) Draw a diagram that illustrates the process of respiration taking place in the yeast cells. Why is the yeast necessary for the bread to rise? (b) If a strain of yeast existed that employed lactic acid fermentation, could this yeast be used in place of ordinary baker’s yeast? Explain. 82 convert pyruvate to ATP molecules. However, unlike animals, plants must first manufacture glucose through photosynthesis. Although often considered the reverse of aerobic cellular respiration, photosynthesis involves many different enzymes and metabolic pathways to produce glucose. You will study photosynthesis in some detail in the next section. MHR • Unit 1 Metabolic Processes 10. I The “four-minute mile” is often cited as an example of the limit of physical performance. That is, no matter how much athletes train, there will always be a limit to their endurance. (a) Why does this limit exist? (b) Design an experiment that you could conduct to test your hypothesis. 11. MC Crash diets that focus on highly regimented eating routines often produce yo-yo syndrome, in which weight lost is quickly regained. In many cases, dieters gain back more weight than they lost. How does an understanding of basal metabolic rate explain this? What kind of advice would help someone in this situation? UNIT INVESTIGATION PREP As you have learned, different types of cells can use different types of cellular respiration. What experiment could you perform to distinguish between yeast and muscle cells in an oxygen-free environment?