Bio 211 CH 9 Cell Respiration PDF

Key Concepts In cells, the endergonic reactions needed for life are paired with exergonic reactions requiring ATP. Cellular respiration produces ATP from molecules with high potential energy—often glucose. Glucose processing has four components: (1) Glycolysis (2) Pyruvat...

Key Concepts In cells, the endergonic reactions needed for life are paired with exergonic reactions requiring ATP. Cellular respiration produces ATP from molecules with high potential energy—often glucose. Glucose processing has four components: (1) Glycolysis (2) Pyruvate processing (3) Krebs cycle (4) Electron transport coupled with oxidative phosphorylation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Key Concepts Fermentation pathways allow glycolysis to continue when the lack of an electron acceptor shuts down electron transport chains. Respiration and fermentation are carefully regulated. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley ATP Has High Potential Energy ATP consists of three phosphate groups, ribose, and adenine. Adenine Phosphate groups Ribose Energy is released when ATP is hydrolyzed. ATP Water ADP Inorganic Energy phosphate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley ATP Has High Potential Energy ATP consists of three phosphate groups, ribose, and adenine. Adenine Phosphate groups Ribose Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley How Does ATP Drive Endergonic Reactions? When a protein is phosphorylated, the exergonic phosphorylation reaction is paired with an endergonic reaction in a process called energetic coupling. In cells, endergonic reactions become exergonic when the substrates or enzymes involved are phosphorylated. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley What Is a Redox Reaction? Reduction–oxidation reactions (redox reactions) are a class of chemical reactions that involve the loss or gain of an electron. Redox reactions drive the formation of ATP. In a redox reaction, the atom that loses one or more electrons is oxidized, and the atom that gains one or more electrons is reduced. Oxidation events are always coupled with a reduction— an electron donor is always paired with a reactant that acts as an electron acceptor. During a redox reaction, electrons can be transferred completely from one atom to another, or they can shift their position in covalent bonds. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley What Happens When Glucose Is Oxidized? Glucose is highly reduced. The carbon atoms of glucose are oxidized to form carbon dioxide, and the oxygen atoms in oxygen are reduced to form water: C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy glucose oxygen carbon water dioxide Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley An Overview of Cellular Respiration Glucose is a key intermediary in cell metabolism. Cells use glucose to build fats, carbohydrates, and other compounds; cells recover glucose by breaking down these molecules. Cellular respiration is a four-step process: (1) Glucose is broken down to pyruvate. (2) Pyruvate is processed to form acetyl-CoA. (3) Acetyl-CoA is oxidized to CO2. (4) Compounds that were reduced in steps 1–3 are oxidized in reactions that usually lead to ATP production. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Processing Glucose: Glycolysis Glycolysis, a series of ten chemical reactions, is the first step in glucose oxidation. In glycolysis, glucose is broken down into two 3-carbon molecules of pyruvate, and the potential energy released is used to phosphorylate ADP to form ATP. In the process, nicotinamide adenine dinucleotide (NAD+) is reduced to NADH, an electron carrier that donates electrons to more oxidized molecules. The reactions of glycolysis are summarized in Figure 9.8a. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley An Overview of Glucose Oxidation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Pyruvate Processing During the second step in glucose oxidation, in the presence of O2 pyruvate undergoes a series of reactions that results in the product molecule acetyl-CoA. During these reactions, another molecule of NADH is synthesized, and one of the carbon atoms in pyruvate is oxidized to CO2. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley The Krebs Cycle During the third step in glucose oxidation, the acetyl-CoA produced by pyruvate processing enters the Krebs cycle. In the Krebs cycle, each acetyl-CoA is oxidized to two molecules of CO2. Some of the potential energy released is used to: (1) Reduce NAD+ to NADH. (2) Reduce flavin adenine dinucleotide (FAD) to FADH2 (another electron carrier). (3) Phosphorylate ADP to make ATP. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Electron Transport During the fourth step in glucose oxidation, the high potential energy of the electrons carried by NADH and FADH2 is gradually decreased by molecules that participate in a series of redox reactions. The proteins involved in these reactions make up what is called an electron transport chain (ETC). The energy released from these electrons through the ETC is used to pump protons across the plasma membrane, forming a strong electrochemical gradient. The final electron acceptor at the end of the electron transport chain is O2. The transfer of electrons along with protons to oxygen forms water. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Methods of Producing ATP The electrochemical gradient produced by the ETC causes protons to move back across the membrane; the membrane protein ATP synthase uses this energy to phosphorylate ADP to form ATP. This process is called oxidative phosphorylation. In another form of ATP production, substrate-level phosphorylation occurs when ATP is produced by the enzyme- catalyzed transfer of a phosphate group from an intermediate substrate to ADP. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Alternative Pathways for Producing Energy Cellular respiration If electron acceptor PYRUVATE ELECTRON TRANSPORT AND PROCESSING KREBS CYCLE (such as oxygen) OXIDATIVE PHOSPHORYLATION GLYCOLYSIS is present Glucose Pyruvate If electron acceptor (such as oxygen) FERMENTATION is NOT present Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley A Closer Look at the Glycolytic Reactions Figure 9.13 shows all ten reactions of glycolysis, which occurs in the cytosol. In the energy investment phase of glycolysis, two ATP molecules are used, but in the energy payoff phase four ATP molecules are produced by substrate-level phosphorylation. The net yield of glycolysis is two NADH, two ATP, and two pyruvate molecules per glucose molecule. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Glycolysis Pathway All 10 reactions of glycolysis occur in cytosol GLYCOLYSIS What goes in: Glucose Glucose- Fructose- Fructose- 6-phosphate 6-phosphate 1,6-bisphosphate What comes out: Glycolysis begins with an energy- investment phase of 2 ATP Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Glycolysis Pathway The “2” indicates that glucose has been split into two 3-carbon sugars Pyruvate During the energy payoff phase, 4 ATP are produced for a net gain of 2 ATP Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Glycolysis Energy Investment Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Glycolysis Energy Payoff Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley How Is Glycolysis Regulated? When an enzyme in a pathway is inhibited by the product of that pathway, feedback inhibition occurs. For example, during glycolysis, high levels of ATP inhibit the enzyme phosphofructokinase. The product of this enzyme can be used only in glycolysis; thus, this is a good regulation point. Phosphofructokinase has two binding sites for ATP—one is in the active site and the other is a regulatory site. When ATP concentrations are high, ATP also binds at the regulatory site and the enzyme changes shape, making the reaction rate at the active site drop dramatically. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Pyruvate Processing Pyruvate produced by glycolysis is transported from the cytoplasm to mitochondria. Pyruvate processing occurs in the mitochondrial matrix (or in the cytosol in bacteria and archaea). The enzyme pyruvate dehydrogenase converts pyruvate to acetyl- CoA. Pyruvate processing is under both positive and negative control. Large supplies of products inhibit the enzyme complex; large supplies of reactants and low supplies of products stimulate it. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Pyruvate processing/bridge step Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley The Krebs Cycle A series of carboxylic acids is oxidized in the Krebs cycle, but not one of them is used up. Citrate (the first molecule in the Krebs cycle) is formed from pyruvate (from glycolysis) and oxaloacetate (the last molecule in the cycle). The Krebs cycle completes the oxidation of glucose. The energy released by the oxidation of one molecule of acetyl-CoA is used to produce three NADH, one FADH2, and one GTP, which is then converted to ATP. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley The Krebs Cycle Completes the Oxidation of Glucose The two red THE KREBS CYCLE carbons enter the cycle via acetyl CoA Pyruvate Acetyl CoA Citrate Isocitrate In each turn of the cycle, the two blue carbons are converted to CO2 All 8 reactions of the Krebs cycle occur in the -Ketoglutarate mitochondrial matrix, Oxaloacetate outside the cristae In the next cycle, this red carbon becomes a blue carbon Succinyl CoA Malate Succinate Fumarate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley How Is the Krebs Cycle Regulated? The Krebs cycle is slowed when ATP and NADH are abundant, and is speeded up when ATP and NADH are scarce. The Krebs cycle can be turned off at multiple points via several different mechanisms of feedback inhibition. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley The Krebs Cycle Is Regulated by Feedback Inhibition These steps are also regulated via feedback inhibition, This step by ATP and NADH is regulated Citrate by ATP Acetyl CoA Oxaloacetate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley A Summary of Glucose Oxidation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Electron Transport and Chemiosmosis Most of the molecules that make up the ETC are proteins containing chemical groups that facilitate redox reactions. All but one of these proteins are embedded in the inner mitochondrial membrane. In contrast, the lipid-soluble ubiquinone (or Q) can move throughout the membrane. During electron transport, NADH donates electrons to a flavin- containing protein at the top of the chain, but FADH2 donates electrons to an iron–sulfur protein that passes electrons directly to Q. The total free-energy change from NADH to oxygen is 53 kcal/mol. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley A Series of Redox Reactions Occurs in an ETC The electron transport chain takes place in the inner membrane and cristae of the mitochondrion FMN: Nucleotide with a flavin- containing group Fe  S: Protein with an iron- sulfur group Cyt: Protein with a heme group (a cytochrome) Q: Ubiquinone Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley The Chemiosmotic Hypothesis The ETC pumps protons from the mitochondrial matrix to the intermembrane space. The proton-motive force from this electrochemical gradient can be used to make ATP in a process known as chemiosmosis. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley How Is the ETC Organized? Occurs in the inner membrane of the mitochondrion Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Summary of ATP Yield Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley A Summary of Cellular Respiration SUMMARY OF CELLULAR RESPIRATION Oxidative ETC phosphorylation H+ H+ H+ H+ H+ H+ H+ + Electrons H+ H H+H+ H+ H+ H + 6 NADH 2 NADH 2 NADH 2 FADH2 O2 H2O GLYCOLYSIS 26 ADP Glucose 2 Pyruvate 2 Acetyl CoA KREBS 4 CO2 CYCLE 2 CO2 2 ATP 2 ATP 26 ATP Maximum yield of ATP Cytoplasm Mitochondrion per molecule of glucose: 30 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Oxidative Phosphorylation Oxygen is the most effective electron acceptor because of its high electronegativity. There is a large difference between the potential energy of NADH and O2 electrons. This large potential energy difference allows the generation of a large proton-motive force for ATP production. Cells that do not use oxygen as an electron acceptor cannot generate such a large potential energy difference and cannot make as much ATP as cells that use aerobic respiration. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Fermentation Fermentation occurs when pyruvate or its derivative, rather than oxygen, accepts electrons from NADH. Fermentation converts NADH back to NAD+ and allows glycolysis to continue to produce ATP via substrate-level phosphorylation. In lactic acid fermentation, pyruvate produced by glycolysis accepts electrons from NADH. Lactate and NAD+ are produced. In alcohol fermentation, pyruvate is enzymatically converted to acetaldehyde and CO2. Acetaldehyde accepts electrons from NADH. Ethanol and NAD+ are produced. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Fermentation Regenerates NAD+ Fermentation pathways allow cells to regenerate NAD+ for glycolysis. Fermentation Intermediate accepts by-product electrons from NADH Lactic acid fermentation occurs in humans. 2 Pyruvate No intermediate; pyruvate accepts electrons from NADH 2 Lactate Alcohol fermentation occurs in yeast. 2 Pyruvate 2 Ethanol 2 Acetylaldehyde Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Fermentation Regenerates NAD+ Fermentation pathways allow cells to regenerate NAD+ for glycolysis. Fermentation Intermediate accepts by-product electrons from NADH Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Fermentation Regenerates NAD+ Lactic acid fermentation occurs in humans. 2 Pyruvate No intermediate; pyruvate accepts electrons from NADH 2 Lactate Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Fermentation Regenerates NAD+ Alcohol fermentation occurs in yeast. 2 Pyruvate 2 Ethanol 2 Acetylaldehyde Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Fermentation Fermentation is extremely inefficient compared with cellular respiration. Fermentation produces just two ATP molecules per glucose molecule, compared with about 36-38 ATP molecules per glucose molecule in cellular respiration. Consequently, organisms never use fermentation if an appropriate electron acceptor is available for cellular respiration. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Cellular Respiration Interacts with Metabolic Pathways Metabolism includes thousands of different chemical reactions, and the amounts and identities of molecules inside cells are constantly in flux. Catabolic pathways involve the breakdown of molecules and the production of ATP, whereas anabolic pathways result in the synthesis of larger molecules from smaller components. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Processing Proteins and Fats as Fuel Proteins, carbohydrates, and fats can all furnish substrates for cellular respiration. For ATP production, cells first use carbohydrates, then fats, and finally proteins. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Food Molecules: Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Furnishing Substrates for Cellular Respiration Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Control of Cell Respiraton: Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Anabolic Pathways Synthesize Key Molecules Molecules found in carbohydrate metabolism are used to synthesize macromolecules such as RNA, DNA, glycogen or starch, amino acids, fatty acids, and other cell components. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley Intermediates in Carbohydrate Metabolism Pathway for synthesis Fats Phospholipids of RNA, DNA Fatty acids Glycogen Several intermediates Glucose Pyruvate Acetyl CoA KREBS used as substrates in or starch CYCLE GLYCOLYSIS amino acid synthesis Lactate (from fermentation) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings/Addison-Wesley