Chapter 9 - Cellular Respiration And Fermentation (ML) PDF

Chapter 9: Cellular respiration and fermentation  Outline Cellular respiration. Redox reactions. Glycolysis. Pyruvate oxidation. The citric acid cycle. The electron transport chain and oxidative phosphorylation. Anaerobic metabolism....

Chapter 9: Cellular respiration and fermentation  Outline Cellular respiration. Redox reactions. Glycolysis. Pyruvate oxidation. The citric acid cycle. The electron transport chain and oxidative phosphorylation. Anaerobic metabolism. Readings: Chapter 9, p189 - p209.  Learning outcomes By the end of this lesson, you should be able to 1. explain the biological function of cellular respiration. 2. distinguish between the following pairs of terms: oxidative vs. substrate-level-phosphorylation; aerobic vs. anaerobic respiration; lactic acid vs. alcohol fermentation. 3. name the two electron carriers and explain that they accept electrons from glucose. 4. name the four stages of cellular respiration, and answer the following questions for each stage, where applicable: “what are the reactants and products?”; “what happens to the carbon?”; “what happens to the energy that is released?”; “where does this step occur?”. 5. recognize examples of redox reactions and substrate-level-phosphorylation that occur in this metabolic pathway. 6. explain the role of the electron transport chain in cellular respiration. 7. describe how cellular respiration can be regulated. 8. discuss how fermentation works and under what circumstances a cell will undergo fermentation.  Cellular respiration Cellular respiration is a metabolic pathway that convert the energy stored in carbohydrates, fatty acids, and proteins into ATP. C6H12O6 + 6 O2 6 CO2 + 6 H2O + Glucose Oxygen gas Carbon Water Energy dioxide 4 Cellular respiration The energy stored in these carbohydrates is released gradually in a series of reactions in the form of ATP and electron carrier molecules. 5  The human body uses energy from ATP for all its activities The average adult human needs about 2000 kilocalorie of energy per day. – This equal about 500 g of glucose (1 gram = 4 kcal) which can be translated to 5.35 x 1025 ATP per day. – About 75% of these calories are used to maintain a healthy body. – The remaining 25% is used to power physical activities (e.g., walking, talking, studying). 6  Four guiding questions of cellular respiration 1. Where does cellular respiration take place? 2. What happens to the carbons in glucose? 3. What happens to the electrons of glucose? 4. What is the overall energy yield? C6H12O6 + 6 O2 6 CO2 + 6 H2O + Glucose Oxygen gas Carbon Water Energy dioxide 7  Cellular respiration Amino acids Fatty Glucose acids Stage 1 Cellular respiration occurs in 4 Glycolysis Glycolysis stages. ATP Pyruvate Stage 2 In stage 1 (glycolysis) and stage 2 Pyruvate (pyruvate oxidation), fuel molecules CO2 Acetyl-CoA oxidation are partially broken down, producing ATP and electron carriers. Stage 3 Citric acid Citric acid cycle In stage 3 (citric acid cycle), fuel CO2 cycle molecules are fully broken down, ATP producing ATP and electron carriers. Electron carriers Stage 4 Oxidative In stage 4 (oxidative Electron transport chain O2 phosphorylation phosphorylation), the electron H2O carriers from stage 1-3 donate ATP electrons to the electron transport chain, leading to the synthesis of a lot of ATP.  Redox reactions The transfer of electrons during chemical reactions releases energy stored in organic molecules. Redox reactions, or oxidation-reduction reactions, are chemical reactions that transfer electrons between reactants. – In oxidation, a substance loses electrons, or is oxidized. – In reduction, a substance gains electrons, or is reduced. becomes oxidized (loses electron) becomes reduced (gains electron) – The electron donor, Xe-, is the reducing agent. – The electron acceptor, Y, is the oxidizing agent. – Oxidation and reduction reactions always occur together. 9  Many redox reactions occur without the transfer of electrons, but change the degree of electron sharing in covalent bonds. Reactants Products becomes oxidized Energy becomes reduced Methane Oxygen Carbon dioxide Water Which is the oxidizing agent? A) methane or B) oxygen Why is oxygen a good oxidizing agent? Which has more potential energy? A) methane or B) carbon dioxide An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one. 10  Oxidation reaction in cellular respiration In cellular respiration, glucose is oxidized to CO2 and O2 is reduced to H2O. C6H12O6 + 6O2 à 6CO2 + 6H2O + energy In glucose, the carbon atoms share their electrons equally with other carbon or hydrogen atoms. In CO2, the electrons are not shared equally. As a result, the carbon atoms have partially lost electrons to oxygen atoms and is oxidized. 11  Reduction reaction in cellular respiration In cellular respiration, glucose is oxidized to CO2 and O2 is reduced to H2O. C6H12O6 + 6O2 à 6CO2 + 6H2O + energy In O2, the electrons are shared equally between the two oxygen atoms. In H2O, the electrons are not shared equally. As a result, the oxygen atoms have partially gained electrons and is reduced. 12  Electron carriers The electrons in glucose are not transferred directly to oxygen but are instead first passed to electron carriers. A major electron carrier is NAD+ (nicotinamide adenine dinucleotide). NAD+ is an important coenzyme in oxidizing glucose. NAD+ can be derived from Niacin, also called vitamin B3. NAD+ accepts electrons and becomes reduced to NADH. NADH represents stored energy that is later used to synthesize ATP. What is the other electron carrier involved in cellular respiration? 13  Electron carriers During cellular respiration, the oxidized forms of electron carriers, NAD+ and FAD (flavin adenine dinucleotide), accepts electrons and becomes reduced to NADH and FADH2. The reduced form has high potential energy, used to synthesize ATP in the final stage of cellular respiration. What is the energy yield of a glucose molecule? Which has more potential energy? A) NAD+ or B) NADH? 14  How does NAD+ accept electrons from glucose? Ø First, the enzyme dehydrogenase removes a pair of hydrogen atoms from glucose, thereby oxidizing it, and transfers the hydrogen atoms to NAD+, thereby reducing it to NADH. Becomes oxidized 2H Dehydrogenase Organic Fuel molecule Becomes reduced NAD+ 2H NADH H+ (carries 2 H+ 2 2 electrons) 15  Glycolysis Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate. How many carbons are in glucose? In pyruvate? This process can occur anaerobically. What is an anaerobic process? Where does Glycolysis occur? Glycolysis can be divided into 3 phases. 16  Glycolysis - preparatory phase During phase 1, glucose is prepared by the addition of two phosphate groups, and an input of 2 ATP producing fructose 1,6- bisphosphate. The phosphorylation of glucose destabilizes it making it easier to split. 17  Glycolysis - cleavage phase During phase 2, fructose 1,6-bisphosphate splits into two molecules of glyceraldehyde 3-phosphate at the end of phase 2. 18  Glycolysis - payoff phase During the third phase of glycolysis, two pyruvate molecules are formed and two NADH molecules are produced. Four molecules of ATP are produced, but two are consumed in phase 1, so the net production of ATP from a single molecule of glucose is two. 19  Substrate-level phosphorylation The cell can produce ATP by substrate level phosphorylation, in which an enzyme transfers a phosphate group from a phosphorylated organic molecule to ADP to produce ATP. However, only a small amount of ATP is generated this way. Substrate phosphorylation occurs during stages 1 (glycolysis) and 3 (the citric acid cycle) of cellular respiration. 20  How other sugars contribute to glycolysis Other sugars than glucose are converted into glycolysis intermediates that come later in the pathway. E.g., fructose receives a phosphate group to form fructose 6- phosphate (which can enter glycolysis at step 3). 21  Four guiding questions of glycolysis 1. Where does glycolysis take place in the cell? 2. What happened to the carbons from glucose? 3. What happened to the electrons from glucose? 4. What was the net energy yield from glycolysis per glucose molecule? 22 Most of cellular respiration take place in the mitochondria A mitochondrion has an inner and an outer membrane that define two spaces. ─ The space between the two membranes is called the intermembrane space, and ─ the space inside the inner membrane is the mitochondrial matrix. Why does the inner membrane have so much fold? 23  Pyruvate oxidation In the presence of O2, pyruvate is transported into the mitochondrial matrix, where it is converted to acetyl-CoA. 1. pyruvate is oxidized to form acetyl group and CO2 2. The acetyl group is then transferred to coenzyme A forming Acetyl-CoA, which is carried to the citric acid cycle. Where does pyruvate come from? One molecule of pyruvate produces 1 CO2 molecule, 1 NADH molecule, and 1 acetyl-CoA molecule. 24  Four guiding questions of pyruvate oxidation 1. Where does pyruvate oxidation take place in the cell? 2. What happened to the carbons of pyruvate? 3. What happened to the electrons of pyruvate? 4. What was the net energy yield from pyruvate oxidation per glucose molecule? 25 The citric acid cycle During the citric acid cycle, glucose is completely oxidized. The chemical energy in the bonds of acetyl-CoA is transferred to ATP by substrate level phosphorylation and to the electron carriers NADH and FADH2. 26 Acetyl-CoA is completely oxidized The two carbon part of acetyl-CoA is added to oxaloacetate (4C) forming citrate (6C). The next seven steps (2-8) convert citrate back to oxaloacetate. The oxidation of carbon atoms in (to produce CO2) steps 3 and 4 is coupled with the reduction of NAD+ to NADH. and FAD to FADH2. 27  Four guiding questions of the citric acid cycle 1. Where does the citric acid cycle take place? 2. What happened to the carbons of Acetyl CoA? 3. What happened to the electrons of Acetyl CoA? 4. What was the net energy yield from the Citric Acid Cycle per glucose molecule? 28  Summary to this point How many ATP have been produced? Where is most of the energy? What happened to the carbons from glucose? Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food. – These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation. 29  The electron transport chain receives electrons from electron carriers The electron transport chain (ETC) allows the energy from the electron carriers generated during cellular respiration to drive ATP synthesis. The electron transport chain involves the process of chemiosmosis and oxidative phosphorylation. The electron transport chain is located in the inner mitochondrial membrane and consists of protein complexes named respiratory complex I - V. The electron transport chain also includes mobile electron carriers such as coenzyme Q (CoQ/Q) and cytochrome C (cyt c). Mobile electron carriers are used to pass electrons between respiratory complexes. 30  The electron transport chain transports electrons from electron carriers to an electron acceptor Electrons enter the Electron transport chain via either complex I or complex II depending on whether they enter as NADH or FADH2. Electrons are passed from electron donors to acceptors until they reach the final electron acceptor, oxygen. When oxygen accepts the electron, it is reduced to water. The energy in these electrons is not directly converted to ATP. What will happen if no oxygen is available to accept electron? 31  The electron transport chain generates a proton gradient The flow of electrons across the electron transport chain is coupled to the pumping of protons (H+) from the mitochondrial matrix across the inner mitochondrial membrane to the intermembrane space, creating a concentration and an electrical gradient. This electrochemical gradient provides a source of potential energy that can be used to synthesize ATP. Mitochondrial matrix Intermembrane space 32 The proton gradient drives ATP synthesis via oxidative phosphorylation For the potential energy of the H+ gradient to be released, H+ must cross the inner mitochondrial membrane from the intermembrane space to the mitochondrial matrix through a transport protein. H+ in the intermembrane space are able to diffuse down their electrical and concentration gradients from the intermembrane space to the mitochondrial matrix through a protein channel called ATP synthase (complex V). The movement of H+ down its electrochemical gradient is called chemiosmosis. As H+ diffuses into the mitochondrial matrix, ATP synthase synthesizes ATP via oxidative phosphorylation. Mitochondrial matrix Intermembrane space 33 The fifth respiratory complex is ATP synthase The movement of H+ through ATP synthase is coupled with the synthesis of ATP. Each pair of electrons released by NADH provides enough energy to produce ~2.5 ATP. Each pair of electrons released by FADH2 provides enough energy to produce ~1.5 ATP. 34 Summary of cellular respiration The complete oxidation of glucose by cellular respiration forms around 32 ATP. The energy of glucose is released gradually in a series of reactions. A small amount of energy is transferred to ATP by substrate-level phosphorylation. Most of the energy is transferred to electron carriers NADH and FADH2. How many NADH and FADH2 are produced per glucose molecule? These carriers donate electrons to the electron transport chain. That energy is used to pump H+ across the inner membrane of the mitochondria. The energy of the electron carriers is thus transformed into energy stored in a H+ electrochemical gradient. ATP synthase then converts the energy of the H+ gradient to rotational energy, which drives the regeneration of ATP. How many ATPs are generated by each NADH and each FADH2 molecule? 35  Summary of cellular respiration CYTOPLASM Electron shuttles Mitochondrion across membrane 2 NADH 2 NADH or 2 FADH2 2 NADH 6 NADH 2 FADH2 Glycolysis Pyruvate Electron transport chain 2 oxidation (chemiosmosis and Glucose The Citric Acid Pyruvate Cycle oxidative 2 Acetyl-CoA phosphorylation) Maximum per glucose: +2 +2 + About ATP ATP 28 ATP About 32 ATP by substrate-level by substrate-level by oxidative phosphorylation phosphorylation phosphorylation 36 Three guiding questions of the electron transport chain 1. Where is the electron transport chain located? 2. What happened in the electron transport chain in terms of redox reactions? 3. What was the net energy yield from the oxidative phosphorylation (per glucose molecule)? 37 Regulation of cellular respiration The energy level in the cell can regulate the reactions of cellular respiration. The level of ATP in the cell is an indicator of how much energy a cell has available. When ATP levels are low or ADP levels are high, the cell activates or upregulates the pathways that lead to ATP synthesis. When ATP levels are high, the cell slows down or downregulates the pathways that lead to ATP synthesis. The level of NAD+/NADH in the cell are also used to regulate cellular respiration. What happens if the cell has high levels of NAD+? What happens if the cell has high levels of NADH? 38 Phosphofructokinase-1 (PFK-1) regulates glycolysis In step 3 of glycolysis, the conversion of fructose 6-phosphate to fructose 1,6- bisphosphate is considered a committed step in glycolysis and is under tight control. This reaction is catalyzed by the enzyme phosphofructokinase-1 (PFK-1). PFK-1 is an allosteric enzyme with many activators and inhibitors. When levels of ADP or AMP are high in the cell, these molecules will bind to PFK-1 and activate the enzyme so that glycolysis continues. When levels of ATP are high, ATP will bind to PFK-1 to inhibit its activity and cause glycolysis to slow down. 39 Cellular respiration can take other forms of fuel Many organic molecules can be used in cellular respiration. Fats are hydrolyzed into glycerol and fatty acid. Fats have many energy rich C-H bonds in their hydrocarbon tails. Fats are converted to glyceraldehyde 3-phosphate and acetyl-CoA. Proteins are burned for fuel last. Amino acids are converted to molecules such as acetyl-CoA, pyruvate and various intermediates of the citric acid cycle. Interrupting cellular respiration can have harmful effects Three categories of cellular poisons obstruct the process of oxidative phosphorylation. These poisons 1. block the electron transport chain (e.g., Rotenone, cyanide, CO). 2. block the flow of H+ through ATP synthase (e.g., the antibiotic Oligomycin). 3. make the membrane leaky to H+ (e.g., dinitrophenol [DNP]). Rotenone Cyanide, carbon Oligomycin monoxide H+ H+ H+ ATP H+ H+ Synthase H+ H+ H+ H+ DNP FADH2 FAD 1 O2 + 2 H+ NADH NAD+ 2 H+ H+ + H2O ADP + P ATP H Chemiosmosis 41 Electron Transport Chain  Interrupting cellular respiration can have beneficial effects Brown fat is a special type of fat tissue associated with the generation of heat. – Brown fat is more abundant in hibernating mammals and newborn infants. In brown fat, – the cells are packed full of mitochondria. – the inner membrane contains a protein that allows H+ to flow back down its electrochemical gradient without generating ATP and generate heat. 42  Anaerobic respiration and fermentation In most organisms, cellular respiration requires oxygen. Oxygen is required for electron transport chain, as the final electron acceptor. In the presence of oxygen, the cell uses cellular respiration to generate ATP. Cellular respiration begins with glycolysis, followed by pyruvate oxidation, the citric acid cycle, and the electron transport chain. In the absence of oxygen, the cell uses anaerobic respiration or fermentation to generate ATP. Anaerobic respiration begins with glycolysis, followed by pyruvate oxidation, the citric acid cycle, and an electron transport chain with an electron acceptor other than O2, such as sulfate or nitrate. – Fermentation begins with glycolysis but is followed by reactions to regenerate NAD+. 43  Fermentation In anaerobic conditions, cells use fermentation to generate ATP. Fermentation consists reactions that regenerate NAD+. There are two common types of fermentation reactions. Lactic acid fermentation. Alcohol (ethanol) fermentation. 44  Lactic acid fermentation In the absence of oxygen, pyruvate is broken down by fermentation. 2 Glucose Lactic acid Lactic acid Glycolysis Lactic acid fermentation fermentation 2 ADP + 2 Pi 2 NAD+ occurs in animals NAD+ produced in and bacteria. 2 ATP fermentation is used in glycolysis. 2 NADH + 2H+ Here, electrons from NADH are transferred to 2 pyruvate to Pyruvate produce lactic acid and NAD+. Glucose + 2 ADP + 2 Pi à 2 lactic acid + 2 ATP + 2 H20 45  Alcohol (ethanol) fermentation Alcohol (ethanol) fermentation occurs in plants and fungi. Glucose 2 Ethanol Here, pyruvate Glycolysis Ethanol 2 ADP + 2 Pi 2 NAD+ fermentation releases CO2 to 2 ATP form acetaldehyde, NAD+ produced in fermentation is and electrons from 2 NADH + 2H+ used in glycolysis. NADH are Acetaldehyde transferred to acetaldehyde to 2 Pyruvate produce ethanol 2 and NAD+. CO2 Glucose + 2 ADP + 2 Pi à 2 ethanol + 2 ATP + 2 H20 + 2CO2 46