Cellular Respiration & Fermentation (Lesson 2)

C E L LU L A R R E S P I R AT I O N & F E R M E N TAT I O N Dr. Szuroczki Chapter 7 Key Concepts 1. Overview of Cellular Respiration 2. Glycolysis 3. Breakdown of Pyruvate 4. Citric Acid Cycle 5. Overview of Oxidative Phosphorylation 6. Connections Among Carbohydrate, Protein, and Fat M...

C E L LU L A R R E S P I R AT I O N & F E R M E N TAT I O N Dr. Szuroczki Chapter 7 Key Concepts 1. Overview of Cellular Respiration 2. Glycolysis 3. Breakdown of Pyruvate 4. Citric Acid Cycle 5. Overview of Oxidative Phosphorylation 6. Connections Among Carbohydrate, Protein, and Fat Metabolism 7. Anaerobic Respiration and Fermentation 1. Overview of Cellular Respiration Process by which living cells obtain energy from organic molecules and release waste products Primary aim to make ATP Aerobic respiration uses oxygen: O2 consumed and CO2 released When glucose is broken down via oxidation, a tremendous amount of free energy is released (- 685kcal/mol) Glucose metabolism C6H12O6 + 6O2 → 6CO2 + 6H2O When glucose is broken down some of the energy is lost as heat but much of it is used to make 3 energy intermediates: A T P, N AD H, F AD H2 Four metabolic pathways: 1. Glycolysis 2. Breakdown of pyruvate 3. Citric acid cycle 4. Oxidative phosphorylation Glycolysis Overview Glucose is broken down to two pyruvate molecules = producing 2 A T P and 2 N A D H molecules Occurs in the cytosol in eukaryotes Substrate level phosphorylation occurs when an enzyme transfers a phosphate from a phosphorylated organic molecule to ADP Breakdown of pyruvate: Occurs in the mitochondrial matrix in eukaryotes Each pyruvate is broken down to an acetyl group and CO2 = one N A D H is generated per pyruvate Glycolysis Glycolysis can occur with or without oxygen Steps in glycolysis nearly identical in all living species Ten enzyme steps in three phases: Energy investment Cleavage Energy liberation Three phases of glycolysis 1. Energy investment Steps 1 to 3 2 ATP hydrolyzed (ATP to ADP) to create fructose-1,6 bisphosphate Three phases of glycolysis 2. Cleavage Steps 4 to 5 6 carbon molecules broken into two 3 carbon molecules of glyceraldehyde-3-phosphate Three phases of glycolysis 3. Energy liberation Steps 6 to 10 Two glyceraldehyde-3-phosphate molecules broken down into two pyruvate molecules = produces 2 NADH and 4 ATP (net yield = 2 ATP)  Summary: For the low price of 2 ATP, the cell produces: 2 N A D H and 4 A T P = net yield of 2 A T P!!! Net reaction and regulation of glycolysis Net reaction: C6H12O6  2 NAD  2 ADP2  2 Pi 2- 2 CH3 (C O)COO   2 H  2 NADH  2 ATP 4  2 H2O Rate of glycolysis regulated by availability of substrates and by feedback inhibition Phosphofructokinase catalyzes third step in glycolysis = believed to be the rate-limiting step At high concentrations, A TP binds to an allosteric Recall Feedback inhibition Breakdown of Pyruvate In eukaryotes, pyruvate is transported into the mitochondrial matrix Broken down by pyruvate dehydrogenase Molecule of CO2 removed from each pyruvate Remaining acetyl group attached to CoA (co- enzyme A) to make acetyl CoA Coenzyme A participates in more than 100 different Citric acid cycle Each acetyl group is incorporated into an organic molecule; later oxidized to liberate two CO2 molecules Occurs in the mitochondrial matrix Total yield: 4 CO2, 2 A T P, 6 N A D H, 2 F A D H2 Glycolysis 2 ATP + Citric Acid cycle 2 ATP = Net gain of 4 ATP Citric Acid Cycle Metabolic cycle: Some molecules enter while others leave Series of organic molecules regenerated in each cycle Acetyl is removed from Acetyl CoA and attached to oxaloacetate to form citrate (aka citric acid) Series of steps releases: 2 CO2, 1 ATP, 3 NADH, and 1 FADH2 Oxaloacetate is regenerated to start the cycle again Where does the Co2 go? Do not need to know this, just showing how cellular respiration ties to metabolism and how we deal with the bi-product! Regulation of the Citric Acid cycle Rate of the citric acid cycle largely regulated by the availability of substrates and by feedback inhibition 3 steps are rate-limiting; those catalyzed by: Citrate synthase Isocitrate dehydrogenase a-ketoglutarate dehydrogenase These enzymes are regulated differently in different species V Oxidative phosphorylation N A D H and 2 F A D H2 from previous steps contain high energy electrons; energy is harnessed to produce an H+ electrochemical gradient During chemiosmosis energy stored in the gradient is used to synthesize A T P Occurs in the cristae in eukaryotes 30-34 A T P molecules made in oxidative phosphorylation Glycolysis 2 ATP + Citric Acid cycle 2 ATP + 30-34 ATP oxidative phosphorylation = Net gain of 34 – 38 ATP Oxidative Phosphorylation First three stages of glucose metabolism yield 6 CO 2, 4 ATP, 10 NADH, 2 FADH2 High energy electrons removed from N ADH and FADH2 to make ATP in oxidative phosphorylation Typically requires oxygen Oxidative process involves electron transport chain Phosphorylation occurs by ATP synthase Oxidation by the Electron Transport Chain Protein complexes and small organic molecules embedded in the inner mitochondrial membrane Accept and donate electrons in a linear manner in a series of redox reactions Electrons originally found in N ADH or FADH2 are transferred to components of the E TC Each component has an increasingly higher electronegativity At the end of the chain is oxygen; E TC is also called the respiratory chain because oxygen we breathe is used in this process Where does Oxygen come from? Do not need to know this, just showing you where the ETC gets the oxygen needed via respiration (breathing)! Oxidation by the Electron Transport Chain (ETC) Some of the energy released during movement of electrons is used to pump protons (H+) across the mitochondrial membrane into the intermembrane space Generates a large proton electrochemical gradient This provides energy for the next step: synthesizing A TP! Oxidation by the Electron Transport Chain (ETC) As the electrons travel through the chain, they go from a higher to a lower energy level, moving from less electron-hungry to more electron-hungry molecules Energy is released in these “downhill” electron transfers, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient Free-Energy Changes Drive Oxidative Phosphorylation Releasing energy in small increments allows cells to couple glucose breakdown with useful chemical processes Free energy is released as electrons move along the electron transport chain Some of the energy is used to pump H+ across the inner mitochondrial membrane and establish an H+ electrochemical gradient that powers ATP synthesis ATP synthase – Worlds smallest motor It allows protons to pass through the membrane and uses the free energy difference to convert phosphorylate adenosine diphosphate (ADP) into ATP Lipid bilayer of inner mitochondrial membrane is relatively impermeable to H+ Protons can only pass through ATP synthase ATP synthase harnesses free energy released as H+, Chemical synthesis of A TP = pushing H+ across a membrane ATP Synthase consists of subunits ATP Synthase consists of subunits ATP synthase is a rotary machine; synthesis of A TP involves a mechanical rotation of part of A TP synthase Conformational changes produce A TP3 Regulation of Oxidative Phosphorylation Process regulated by availability of E TC substrates such as NADH and O2 and by the ATP/ADP ratio When ATP is high it binds to a subunit of cytochrome oxidase and inhibits the E TC and oxidative phosphorylation When ADP is high, oxidative phosphorylation is stimulated Chemical Reactions of Oxidative Phosphorylation NADH  H   1 2 O2 NAD  H2O  ADP  Pi ATP 2 2- 4  H2O NADH oxidation makes most of the cell’s ATP N A D H oxidation creates the H+ electrochemical gradient used to synthesize ATP Yield = up to 30 to 34 ATP molecules / glucose But rarely achieve maximal amount because: NADH also used in anabolic pathways H+ gradient is used for other purposes Cellular respiration Review Video Connections Among Carbohydrate, Protein, and Fat Metabolism Besides glucose, other molecules also used for energy: carbohydrates, proteins, fats Enter into glycolysis or citric acid cycle at different points Utilizing the same pathways for breakdown increases efficiency What happens when There's no oxygen? Anaerobic Respiration and Fermentation Anerobic describes an environment that lacks oxygen Two strategies to metabolize organic molecules in the absence of oxygen: Use substance other than O2 as final electron acceptor in electron transport chain: called anaerobic respiration Produce A T P only via substrate-level phosphorylation: called fermentation Anaerobic Respiration E. coli uses nitrate as the final electron acceptor under anaerobic respiration Preferably makes ATP via chemiosmosis under aerobic conditions Without oxygen, nitrate produces less energy per oxidized molecule Anaerobic respiration is less efficient but better than death! Fermentation Fermentation is the breakdown of organic molecules without net oxidation Many organisms can only use O2 as final electron acceptor, so under anaerobic conditions, they need a different way to produce A TP, like using glycolysis But glycolysis uses up NAD+ and makes too much NADH under anaerobic conditions (dangerous situation) Muscle cells solve problem by reducing pyruvate into lactate Yeast solve problem by making ethanol Fermentation produces far less A TP than oxidative phosphorylation Providing Energy for Muscle Contraction Anaerobic glycolysis and lactic acid formation: better than nothing! Reaction that breaks down glucose without oxygen Glucose is broken down to pyruvic acid to produce about 2 AT P vs 34 – 38 via aerobic respiration Pyruvic acid is converted to lactic acid Fermentation Review Video