Food as Fuel—A Metabolic Overview

1 Food as Fuel—A Metabolic OverviewTopic 17 Food as Fuel—...

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2 Topic 17: Food as Fuel—A Metabolic Overview17.1 Overview of Metabolism 17.2 Metabolically Relevant Nucleotides 17.3 Digestion—From Fuel Molecules to Hydrolysis Products 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites 17.5 The Citric Acid Cycle—Central Processing 17.6 Electron Transport and Oxidative Phosphorylation 17.7 ATP Production 17.8 Other Fuel Choices © 2011 Pearson Education, Inc. Topic 17

3 Introduction Metabolism refers to the chemical reactions occurring in the body during the break down or building up of molecules. A metabolic pathway is a series of steps in the chemical reactions in biological systems. In the body, the energy of chemical reactions is captured in the nucleotide, ATP, which is produced during the metabolism of glucose. © 2011 Pearson Education, Inc. Topic 17

4 17.1 Overview of MetabolismAnimals and humans get energy from the covalent bonds contained in carbohydrates, fats, and proteins. The first stage of metabolism involves the breakdown of large biomolecules into smaller units through hydrolysis reactions. Polysaccharides are hydrolyzed into monosaccharide units, triglycerides are hydrolyzed into glycerol and fatty acids, and proteins are hydrolyzed into amino acids. © 2011 Pearson Education, Inc. Topic 17

5 17.1 Overview of Metabolism, ContinuedSmaller molecules produced during metabolism are absorbed through the intestinal wall into the bloodstream and transported to different cells for use. Once in cells, further breakdown occurs into molecules containing two or three carbons. These smaller molecules are referred to as metabolites, which are chemical intermediates formed by enzyme-catalyzed reactions. © 2011 Pearson Education, Inc. Topic 17

6 17.1 Overview of Metabolism, ContinuedIf there is adequate oxygen in the cells and there is a need for energy, two-carbon metabolites, known as acetyl groups, are broken down to carbon dioxide in the citric acid cycle. © 2011 Pearson Education, Inc. Topic 17

7 17.1 Overview of Metabolism, ContinuedThe citric acid cycle works in conjunction with electron transport and oxidative phosphorylation pathways to produce energy. Energy, in the form of ATP, nicotinamide adenine dinucleotide (NADH), and flavin adenine dinucleotide (FADH2), is produced in these sequence of events. © 2011 Pearson Education, Inc. Topic 17

9 17.1 Overview of Metabolism, ContinuedMetabolism occurs in two parts: catabolism and anabolism. Catabolism refers to chemical reactions where large molecules are broken down into smaller molecules. Anabolism refers to chemical reactions where metabolites combine to form larger molecules. © 2011 Pearson Education, Inc. Topic 17

10 17.1 Overview of Metabolism, ContinuedThe reactions of catabolism produce energy and are oxidative, while those of anabolism require energy and are reductive. Energy from catabolism is stored in ATP. © 2011 Pearson Education, Inc. Topic 17

11 17.1 Overview of Metabolism, ContinuedMetabolic Pathways in the Animal Cell Since metabolic pathways occur in cells, it is important to know major parts of an animal cell. The cell membrane separates the materials inside the cell from the exterior environment. The nucleus contains DNA that controls cell replication and protein synthesis. © 2011 Pearson Education, Inc. Topic 17

12 17.1 Overview of Metabolism, ContinuedThe cytoplasm contains all the materials between the nucleus and cell membrane. The cytosol is the aqueous part of the cytoplasm that contains electrolytes and enzymes. Within the cytoplasm, there are specialized organelles that carry out specific cellular functions. © 2011 Pearson Education, Inc. Topic 17

13 17.1 Overview of Metabolism, ContinuedRibosomes are the sites of protein synthesis. Energy is produced in the mitochondria. A mitochondrion is composed of an outer membrane and an inner membrane, with an intermembrane space between them. The fluid section encased by the inner membrane is referred to as the matrix. Enzymes in the matrix and along the inner membrane catalyze the oxidation of carbohydrates, fats, and amino acids. © 2011 Pearson Education, Inc. Topic 17

16 17.2 Metabolically Relevant NucleotidesNucleotides have important cellular functions such as energy exchangers and coenzymes. Nucleotides have two forms: A high-energy form A low-energy form Recall that nucleotides consist of a nitrogenous base, a phosphate, and a five-carbon sugar. Many contain a vitamin within their structure. © 2011 Pearson Education, Inc. Topic 17

17 17.2 Metabolically Relevant Nucleotides, ContinuedATP/ADP ATP is referred to as the energy currency of the cell. ATP undergoes hydrolysis to ADP, during which energy is released. Released energy can be coupled to drive a chemical reaction that requires energy during anabolism. © 2011 Pearson Education, Inc. Topic 17

20 17.2 Metabolically Relevant Nucleotides, ContinuedNADH/NAD+ and FADH2/FAD NAD+ and FAD are energy transferring compounds whose high-energy form is reduced (hydrogen added) and whose low-energy form is oxidized (hydrogen removed). NADH is the reduced form of NAD+, and FADH2 is the reduced form of FAD. The active end of each molecule, the end being oxidized or reduced, contains a vitamin component. © 2011 Pearson Education, Inc. Topic 17

22 17.2 Metabolically Relevant Nucleotides, ContinuedAcetyl Coenzyme A and Coenzyme A Acetyl coenzyme A is the high-energy form, and coenzyme A is the low-energy form, of this important energy exchanger. Energy is released from acetyl coenzyme A when the carbon–sulfur bond in the thioester functional group is hydrolyzed. CoA contains adenosine, three phosphates, and a vitamin-derived portion. © 2011 Pearson Education, Inc. Topic 17

24 17.3 Digestion—From Fuel Molecules to Hydrolysis ProductsWhen we consume food, it is broken down in a process known as digestion. In digestion, large biomolecules are broken down into smaller hydrolysis products. The hydrolyzed products are delivered to the cells for further catabolism. © 2011 Pearson Education, Inc. Topic 17

25 17.3 Digestion—From Fuel Molecules to Hydrolysis Products, ContinuedCarbohydrates Starch (amylose and amylopectin) digestion begins in the mouth by the enzyme -amylase secreted in saliva. Alpha-amylase hydrolyzes some of the -glycosidic bonds in starch molecules, producing some glucose, the disaccharide maltose, and oligosaccharides. © 2011 Pearson Education, Inc. Topic 17

26 17.3 Digestion—From Fuel Molecules to Hydrolysis Products, ContinuedComplete digestion of starch molecules to monosaccharides takes place in the small intestine. © 2011 Pearson Education, Inc. Topic 17

27 17.3 Digestion—From Fuel Molecules to Hydrolysis Products, ContinuedOther carbohydrates, like cellulose, cannot be digested because of the lack of the enzyme needed to hydrolyze the -glycosidic bond. Indigestible fibers of cellulose are referred to as insoluble fiber and are important for a healthy digestive tract. Insoluble fiber stimulates the large intestine to excrete waste. © 2011 Pearson Education, Inc. Topic 17

28 17.3 Digestion—From Fuel Molecules to Hydrolysis Products, ContinuedFats Dietary fats are nonpolar and require bile, excreted from the gall bladder, to assist digestion. Bile contains soap-like molecules called bile salts. They contain a polar and nonpolar part. Bile salts break up the larger nonpolar fat globules into smaller droplets called micelles. © 2011 Pearson Education, Inc. Topic 17

29 17.3 Digestion—From Fuel Molecules to Hydrolysis Products, ContinuedThis process of is known as emulsification. The smaller micelles move closer to the intestinal wall so cholesterol can be absorbed and triglycerides can be hydrolyzed by pancreatic lipase to free fatty acids and monoglycerides, which are then absorbed. Once across the intestinal wall, free fatty acids and monoglycerides are reassembled into triglycerides, and cholesterol forms a cholesterol ester with a free fatty acid. © 2011 Pearson Education, Inc. Topic 17

30 17.3 Digestion—From Fuel Molecules to Hydrolysis Products, ContinuedBoth are packaged with protein forming a lipoprotein called a chylomicron. Chylomicrons transport triglycerides through the bloodstream to the tissues where they are used for energy production or stored in the cells. The process of fatty acid digestion is shown on the next slide. © 2011 Pearson Education, Inc. Topic 17

32 17.3 Digestion—From Fuel Molecules to Hydrolysis Products, ContinuedProteins Protein digestion begins in the stomach. Proteins are denatured by the acidic digestive juices and are partially digested by pepsin. Complete digestion of proteins occurs in the small intestine by the enzymes trypsin and chymotrypsin. Free amino acids are absorbed into the bloodstream for delivery to the tissues. © 2011 Pearson Education, Inc. Topic 17

34 17.4 Glycolysis—From Hydrolysis Products to Common MetabolitesGlycolysis is the chemical pathway for the catabolism of glucose, the main source of fuel for the body. When there is insufficient glucose entering the body, glucose is made in the body from noncarbohydrate molecules through a process called gluconeogenesis. Glucose enters the cell with the aid of a transporter protein. © 2011 Pearson Education, Inc. Topic 17

35 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedGlycolysis occurs in the cytosol where glucose is broken down into two three-carbon molecules of pyruvate. Molecules like fructose, amino acids, and free fatty acids can be used as fuel by the cells. These molecules enter glycolysis later in the pathway or by chemical conversion to one of the common metabolites. © 2011 Pearson Education, Inc. Topic 17

36 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedThe Chemical Reactions in Glycolysis Energy must be transferred in small amounts to minimize the heat released during the process of glycolysis. Reactions that produce energy are coupled with reactions that require energy. In glycolysis, energy is transferred through phosphate groups undergoing condensation and hydrolysis reactions. © 2011 Pearson Education, Inc. Topic 17

37 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedThere are 10 chemical reactions in glycolysis to produce two molecules of pyruvate. The first five reactions require the investment of energy from two molecules of ATP, which are used to add a phosphate group to glucose and to fructose-6-phosphate. Two sugar phosphate molecules are formed in reaction 5. © 2011 Pearson Education, Inc. Topic 17

39 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedReactions 6–10 generate two high-energy NADH molecules and four ATP molecules. Two molecules of pyruvate are formed in reaction 10. © 2011 Pearson Education, Inc. Topic 17

45 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedThe net result of 10 reactions in glycolysis is that one molecule of glucose is converted to two molecules of pyruvate. The net energy output is two NADH and two ATP (4 ATP produced in reactions 6 through 10 minus 2 ATP used in reactions 1 through 5). © 2011 Pearson Education, Inc. Topic 17

46 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedRegulation of Glycolysis Metabolic pathways have one or more steps that regulate the flow of reactants to final product. Glycolysis is regulated at reaction 3 of the pathway. Phosphofructokinase, the enzyme formed in reaction 3, is regulated by ATP. ATP acts as an inhibitor to this enzyme. If the cells have enough ATP, glycolysis slows down. © 2011 Pearson Education, Inc. Topic 17

47 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedThe Fates of Pyruvate Pyruvate can be broken down further to produce more energy. The availability of oxygen determines how this breakdown will occur. In aerobic conditions (ample oxygen), pyruvate is oxidized further to acetyl coenzyme A. In anaerobic conditions (lacking oxygen), pyruvate is reduced to lactate. © 2011 Pearson Education, Inc. Topic 17

48 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedAerobic Conditions The carboxylate functional group of pyruvate is liberated as CO2, producing a two-carbon acetyl group and NADH. The acetyl group reacts with coenzyme A to produce acetyl coenzyme A. A high-energy thioester functional group is formed when acetyl coenzyme A is formed. © 2011 Pearson Education, Inc. Topic 17

49 This reaction occurs in the mitochondria.17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, Continued Aerobic Conditions, Continued This reaction occurs in the mitochondria. © 2011 Pearson Education, Inc. Topic 17

50 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedAnaerobic Conditions During strenuous exercise, oxygen is in short supply. The middle carbonyl in pyruvate is reduced to an alcohol group, and lactate is formed. NADH supplies the hydrogen needed in the reaction. NAD+ is produced. No energy is produced, so NAD+ is funneled back into glycolysis to oxidize more glucose. © 2011 Pearson Education, Inc. Topic 17

52 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedYeast converts pyruvate to ethanol under anaerobic conditions. This process is known as fermentation. Yeast is used in the preparation of alcoholic beverages. © 2011 Pearson Education, Inc. Topic 17

53 17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, ContinuedFructose and Glycolysis Fructose is converted to fructose-6-phosphate in the muscles and enters glycolysis at reaction 3. In the liver, it is converted to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate, used in reaction 5. These two products provide an excess of reactants for glycolysis, creating excess pyruvate and acetyl CoA that is not needed and is ultimately converted to fat. © 2011 Pearson Education, Inc. Topic 17

54 17.5 The Citric Acid Cycle—Central ProcessingWhen there is sufficient oxygen, glucose, fats, and amino acids enter the citric acid cycle. The citric acid cycle is a series of reactions that degrade the two-carbon acetyl groups from acetyl CoA into carbon dioxide while generating the high-energy molecules NADH and FADH2. The citric acid cycle is also known as the Krebs cycle or the tricarboxylic acid cycle. © 2011 Pearson Education, Inc. Topic 17

55 17.5 The Citric Acid Cycle—Central Processing, ContinuedThe first reaction in the cycle involves the condensation between acetyl CoA and a four-carbon molecule, oxaloacetate, which produces a six-carbon molecule, citrate. Hence, the name citric acid cycle. Citrate loses two carbons as CO2 to form succinyl CoA. The breaking of the carbon–carbon bonds transfers energy to produce NADH from NAD+. © 2011 Pearson Education, Inc. Topic 17

56 17.5 The Citric Acid Cycle—Central Processing, ContinuedSuccinyl CoA, in a series of reactions, regenerates oxaloacetate, which starts the cycle again. NADH, FADH2, and GTP are produced in these series of reactions. © 2011 Pearson Education, Inc. Topic 17

57 17.5 The Citric Acid Cycle—Central Processing, ContinuedReactions of the Citric Acid Cycle There are eight enzyme-catalyzed reactions in the citric acid cycle. The citric acid cycle occurs in the mitochondrial matrix. © 2011 Pearson Education, Inc. Topic 17

59 17.5 The Citric Acid Cycle—Central Processing, ContinuedCitric Acid Cycle Summary One turn of the citric acid cycle produces three NADH, one FADH2, one GTP (forms ATP), two CO2, and one CoA. The reactants in the cycle are regenerated, so the net reaction is: © 2011 Pearson Education, Inc. Topic 17

60 17.6 Electron Transport and Oxidative PhosphorylationA single molecule of glucose entering glycolysis and the citric acid cycle will produce two ATPs during glycolysis and two ATPs in the cycle. So, where is all the energy produced? It is produced in the mitochondria by a process called oxidative phosphorylation. The high-energy forms of the nucleotides NADH and FADH2 are produced in glycolysis and the citric acid cycle. © 2011 Pearson Education, Inc. Topic 17

61 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedElectrons and hydrogens are transferred from the high-energy forms of the nucleotides through the inner mitochondrial membrane and combine with oxygen to produce water. Energy from this process drives the reaction ADP + Pi to form ATP. This process of producing ATP from reduced nucleotides is called oxidative phosphorylation. © 2011 Pearson Education, Inc. Topic 17

62 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedMitochondria are the ATP factories of the cell. Reduced nucleotides are produced in the mitochondria, and their energy upon oxidation is used to produce ATP. The citric acid cycle occurs in the mitochondrial matrix, and the reduced nucleotides begin their journey through the inner membrane here. © 2011 Pearson Education, Inc. Topic 17

63 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedComplexes I and IV, a set of enzyme complexes, are located in the inner membrane of mitochondria. Electron carriers located in these complexes carry the electrons and protons of NADH and FADH2 through the inner mitochondrial membrane. Electron carriers, coenzyme Q and cytochrome c, are not membrane-bound, so they serve to shuttle electrons between complexes. © 2011 Pearson Education, Inc. Topic 17

65 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedComplex I NADH Dehydrogenase NADH enters here. Two electrons and two protons are transferred to coenzyme Q. NAD+ is regenerated and returns to the citric acid cycle. NADH + H+ + Q → NAD+ + QH2 Complex II Succinate Dehydrogenase FADH2 enters at this complex. Two electrons and two protons from FADH2 are also transferred to a coenzyme Q to form QH2. FADH2 + Q → FAD + QH2 © 2011 Pearson Education, Inc. Topic 17

66 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedComplex III Coenzyme Q—Cytochrome c Reductase QH2 molecules are reoxidized to Q. Electrons are transferred to cytochrome c, which moves the electrons from complex III to complex IV. Complex IV Cytochrome c Oxidase Electrons are transferred from cytochrome c to combine with hydrogen ions and oxygen to form water. This is the final stop for electrons. 4H+ + 4e- + O2 → 2H2O © 2011 Pearson Education, Inc. Topic 17

67 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedEnergy is produced as a result of electron transport generating a proton (H+) difference, or gradient, on either side of the inner membrane. This is known as the chemiosmotic model. Complexes I, III, and IV span the inner membrane and pump protons from the matrix into the intermembrane space as electrons are shuttled through the complexes. © 2011 Pearson Education, Inc. Topic 17

68 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedAn electrical charge difference, as well as a difference in concentration of protons on either side of the membrane, results in an electrochemical gradient. The proton gradient provides the energy necessary for ATP synthesis. Protons move back to the matrix by passing through a protein complex called complex V, or ATP synthase. © 2011 Pearson Education, Inc. Topic 17

69 17.6 Electron Transport and Oxidative Phosphorylation, ContinuedThe movement of protons from an area of high concentration of protons to an area of fewer protons releases energy that drives ATP synthase. ADP + Pi + energy → ATP © 2011 Pearson Education, Inc. Topic 17

70 17.7 ATP Production How Many ATPs Are Synthesized for Each Reduced Nucleotide Entering Electron Transport? NADH and FADH2 enter the electron transport chain at different complexes, so the amount of energy produced for each is different. Ten H+ are pumped for each NADH, and six H+ are pumped for each FADH2. © 2011 Pearson Education, Inc. Topic 17

72 17.7 ATP Production, ContinuedCounting ATP from One Glucose How much ATP is generated from one molecule of glucose during oxidative catabolism? Glycolysis One molecule of glucose produces two molecules of pyruvate, two molecules of ATP, and two molecules of NADH. The direct shuttling of two NADH into the matrix results in the production of five ATPs. © 2011 Pearson Education, Inc. Topic 17

73 17.7 ATP Production, ContinuedCounting ATP from One Glucose, Continued Oxidation of pyruvate The two pyruvate molecules are oxidized to produce two acetyl CoA, two CO2, and two NADH. The two NADH account for five ATPs. Citric acid cycle Each acetyl CoA produced enters the citric acid cycle. A total of six NADH, two FADH2, and two ATPs are produced. Note the accompanying art at the top of the next slide. © 2011 Pearson Education, Inc. Topic 17

74 17.7 ATP Production, ContinuedCounting ATP from One Glucose, Continued Total ATP from glucose oxidation: By summing the ATPs produced from glycolysis, pyruvate oxidation, and the citric acid cycle, a net number of ATPs/glucose can be estimated. © 2011 Pearson Education, Inc. Topic 17

77 17.8 Other Fuel Choices Glucose is our primary source of fuel. It is stored as glycogen in the liver and muscles. Glycogenolysis (hydrolysis of glycogen) produces glucose quickly when glucose concentrations are low. Once glycogen stores are depleted, glucose is synthesized from noncarbohydrate sources through gluconeogenesis. © 2011 Pearson Education, Inc. Topic 17

78 17.8 Other Fuel Choices, ContinuedEnergy from Fatty Acids When glucose and glycogen are not available, fatty acids can be oxidized to acetyl CoA through a process known as beta oxidation. In beta oxidation, carbons are removed two at a time from an activated fatty acid. An activated fatty acid consists of a fatty acid bonded to a CoA, called a fatty acyl CoA. © 2011 Pearson Education, Inc. Topic 17

79 17.8 Other Fuel Choices, ContinuedRemoval of two carbons during oxidation produces an acetyl CoA and a fatty acyl CoA shortened by two carbons. The cycle is repeated until the original fatty acid is completely degraded to acetyl CoA units. Beta oxidation occurs in the mitochondrial matrix. © 2011 Pearson Education, Inc. Topic 17

80 17.8 Other Fuel Choices, ContinuedThe Beta Oxidation Cycle Reaction 1. Oxidation (Dehydrogenation). One hydrogen from each of the  and  carbons is removed and a double bond is formed. FADH2 is produced. Reaction 2. Hydration. Water is added across the double bond between the  and  carbons. Reaction 3. Oxidation (Dehydrogenation). The alcohol formed is oxidized to a ketone. NADH is produced. © 2011 Pearson Education, Inc. Topic 17

81 17.8 Other Fuel Choices, ContinuedThe Beta Oxidation Cycle, Continued Reaction 4. Removal of Acetyl CoA. The bond between the alpha and beta carbon is broken. A second CoA is added forming a fatty acyl CoA shortened by two carbons. The cycle continues. The net for one cycle of beta oxidation is: © 2011 Pearson Education, Inc. Topic 17

83 17.8 Other Fuel Choices, ContinuedCycle Repeats and ATP Production Consider the saturated 18-carbon fatty acid, stearic acid. Two carbons will produce an acetyl CoA, so nine acetyl CoA are produced from stearic acid. This will take eight turns of the beta oxidation cycle since the last turn produces two acetyl CoA. © 2011 Pearson Education, Inc. Topic 17

86 17.8 Other Fuel Choices, ContinuedToo Much Acetyl CoA—Ketosis In the absence of glucose, the body breaks down its body fat to continue ATP production. Oxidation of fatty acids cause large amounts of acetyl CoA to accumulate in the liver. When this occurs, two acetyl units condense, forming the four-carbon ketone molecules β-hydroxybutyrate and acetoacetate. Acetone is also produced. © 2011 Pearson Education, Inc. Topic 17

87 17.8 Other Fuel Choices, ContinuedThese three ketones are referred to as ketone bodies. Their formation is termed ketogenesis. Ketosis occurs when an excessive amount of ketone bodies are present in the body. Ketosis is often seen in individuals with diabetes. © 2011 Pearson Education, Inc. Topic 17

88 17.8 Other Fuel Choices, ContinuedEnergy from Amino Acids We get about 10% of our energy from amino acids. More energy is extracted from amino acids during periods of fasting or starvation. When amino acids are our only source of energy, proteins in body tissues are degraded. Nitrogen is produced when amino acids from proteins are metabolized. Amino acids provide nitrogen for anabolism of nitrogen compounds. © 2011 Pearson Education, Inc. Topic 17

89 17.8 Other Fuel Choices, ContinuedEnergy from Amino Acids The -amino group is removed yielding an -keto acid through a process called transamination. Ammonium ions produced during transamination are toxic to the body and are removed as urea through a series of reactions called the urea cycle The -keto acid can be converted into intermediates for other metabolic pathways. © 2011 Pearson Education, Inc. Topic 17

90 17.8 Other Fuel Choices, ContinuedEnergy from Amino Acids Amino acids can replenish the intermediates in the citric acid cycle and therefore, have the ability to produce ATP. The number of carbons in the amino acid determines which intermediates are replenished. © 2011 Pearson Education, Inc. Topic 17

92 Topic Summary 17.1 Overview of MetabolismMetabolism refers to chemical reactions in the body. Catabolism refers to reactions that break down larger molecules into smaller ones. Catabolic reactions produce energy and are oxidative. © 2011 Pearson Education, Inc. Topic 17

93 Topic Summary, Continued17.1 Overview of Metabolism, Continued Anabolism refers to reactions that produce larger molecules from smaller ones. Anabolic reactions require the input of energy and are reductive. Biochemical reactions that require energy are coupled with reactions that produce energy. © 2011 Pearson Education, Inc. Topic 17

94 Topic Summary, Continued17.1 Overview of Metabolism, Continued Metabolic pathways are compartmentalized in different parts of the cell. The mitochondria are the energy-producing factories of the cell. © 2011 Pearson Education, Inc. Topic 17

95 Topic Summary, Continued17.2 Metabolically Relevant Nucleotides Nucleotides are used in nucleic acids. Nucleotide components are also found in the coenzymes of metabolism. ATP is the main energy molecule of the cell and produces energy when it is hydrolyzed to ADP. © 2011 Pearson Education, Inc. Topic 17

96 Topic Summary, Continued17.2 Metabolically Relevant Nucleotides, Continued NADH and FADH2 are important coenzymes that transport hydrogen and electrons in the cell. These reduced forms are high-energy forms, whereas the oxidized forms (NAD+ and FAD) are low-energy forms. Acetyl CoA is the high-energy form of coenzyme A. It has a two-carbon acetyl group attached to CoA through a high-energy thioester bond. © 2011 Pearson Education, Inc. Topic 17

97 Topic Summary, Continued17.3 Digestion—From Fuel Molecules to Hydrolysis Products Carbohydrates are metabolized into monosaccharides. Monosaccharides can be absorbed into the bloodstream and carried to the cells for metabolism. Dietary fats are emulsified by bile salt, so they can be enzymatically hydrolyzed to monoglycerides for absorption. © 2011 Pearson Education, Inc. Topic 17

98 Topic Summary, Continued17.3 Digestion—From Fuel Molecules to Hydrolysis Products, Continued Triglycerides in the blood are repackaged into lipoproteins called chylomicrons for delivery to cells. Protein digestion begins in the stomach and continues in the small intestine. Proteins are metabolized into amino acids through hydrolysis of peptide bonds. © 2011 Pearson Education, Inc. Topic 17

99 Topic Summary, Continued17.4 Glycolysis—From Hydrolysis Products to Common Metabolites Glycolysis describes the 10 reactions that breaks down glucose to two three-carbon pyruvate molecules. ATP and NADH are produced during glycolysis. Under aerobic conditions, pyruvate is oxidized to acetyl CoA. © 2011 Pearson Education, Inc. Topic 17

100 Topic Summary, Continued17.4 Glycolysis—From Hydrolysis Products to Common Metabolites, Continued Under anaerobic conditions, pyruvate is reduced to lactate and regenerates NAD+ so glycolysis can continue. Fermentation is the process of making ethanol, instead of pyruvate, under anaerobic conditions. Glucose can be stored as glycogen for later use. © 2011 Pearson Education, Inc. Topic 17

101 Topic Summary, Continued17.5 The Citric Acid Cycle—Central Processing The citric acid cycle occurs in the mitochondrial matrix and combines acetyl CoA with oxaloacetate to produce citrate. Citrate undergoes oxidation to produce CO2, GTP, three NADH, and one FADH2. Oxaloacetate is regenerated in the cycle and GTP is converted to ATP. © 2011 Pearson Education, Inc. Topic 17

102 Topic Summary, Continued17.6 Electron Transport and Oxidative Phosphorylation NADH and FADH2 become oxidized, transporting H+ and electrons through a series of complexes in the inner mitochondrial membrane. The final electron acceptor is oxygen. Protons move from the matrix to the inner membrane space, producing a proton gradient. © 2011 Pearson Education, Inc. Topic 17

103 Topic Summary, Continued17.6 Electron Transport and Oxidative Phosphorylation, Continued Protons move back to the matrix through complex V, ATP synthase, which generates ATP. When ATP and proton transport are uncoupled, the energy is released as heat, which assists in maintaining body temperature in a process called thermogenesis. © 2011 Pearson Education, Inc. Topic 17

104 Topic Summary, Continued17.7 ATP Production For every four H+ pumped into the inner membrane space and returned to the matrix, one ATP is produced. Oxidation of one NADH provides 2.5 ATPs. One FADH2 provides 1.5 ATPs. Complete oxidation of one molecule of glucose produces a total of 32 ATPs. © 2011 Pearson Education, Inc. Topic 17

105 Topic Summary, Continued17.8 Other Fuel Choices Fatty acids can be used for fuel when glucose supplies are low. Fatty acids are linked to coenzyme A to form an activated fatty acyl CoA, which is transported to the mitochondria for catabolism in a reaction cycle called  oxidation. Fatty acyl CoA is oxidized producing a new fatty acyl CoA that is two carbons shorter and one molecule of acetyl CoA. © 2011 Pearson Education, Inc. Topic 17

106 Topic Summary, Continued17.8 Other Fuel Choices, Continued Each turn of  oxidation produces one NADH and one FADH2. Ketogenesis is activated in the cell by high levels of acetyl CoA. Ketogenesis produces ketone bodies that can lead to ketosis and acidosis. Amino acids can be used for energy when other fuel sources are not available. © 2011 Pearson Education, Inc. Topic 17

107 Topic Summary, Continued17.8 Other Fuel Choices, Continued Ammonium ions removed from amino acids are removed for the body as urea by the urea cycle. Carbons of amino acids can enter into oxidative catabolism as different intermediates depending on the amino acid. © 2011 Pearson Education, Inc. Topic 17

Food as Fuel—A Metabolic Overview

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