Cellular Respiration & Fermentation - Chapter 7 PDF

Campbell Biology in Focus Third Edition Chapter 7 Cellular Respiration and Fermentation Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Life Is Wor...

Campbell Biology in Focus Third Edition Chapter 7 Cellular Respiration and Fermentation Copyright © 2022, 2020, 2016 Pearson Education, Inc. All Rights Reserved Life Is Work Energy flows into an ecosystem as sunlight and leaves as heat Photosynthesis generates O2 and organic molecules, which are used as fuel for cellular respiration Cells use chemical energy stored in organic molecules to regenerate AT P, which powers work Catabolic Pathways and Production of A T P Cellular respiration includes both aerobic and anaerobic processes but is often used to refer to aerobic respiration Although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose C6H12O6  6 O2 6 CO2  6 H2O  Energy ( ATP  heat) Oxidation-Reduction (Redox) Cellular respiration consists of many steps – but it is considered a Redox reaction Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions In oxidation, a substance loses electrons, or is oxidized In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) Glucose is oxidized – Releasing CO2 Oxygen is reduced – forming H2O Figure: Redox Reaction The Principle of Redox The electron donor is called the reducing agent The electron acceptor is called the oxidizing agent Some redox reactions do not transfer electrons but change the degree of electron sharing in covalent bonds An example is the reaction between methane andO2 Energy Must be Added to Pull Electrons From an Atom The more electronegative the atom, the more energy that is required to take an electron away from it. Electrons lose potential energy when it shifts from a less electronegative atoms to a more electronegative one. A redox reaction that shifts electrons closer to oxygen, releases chemical energy that can be put to work. In oxidation-reduction reactions (redox reactions), ________ are _______ during reduction. A. neutrons; gained B. electrons; lost C. protons; gained D. neutrons; lost E. electrons; gained Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, fuel (such as glucose) is oxidized, and O2 is reduced Organic molecules with an abundance of hydrogen, like carbohydrates and fats, are excellent fuels As hydrogen (with its electron) is transferred to oxygen (yielding H2O), energy is released that can be used in AT P synthesis Stepwise Energy Harvest via NAD+ and the Electron Transport Chain In cellular respiration, glucose and other organic molecules are broken down in a series of steps Electrons from organic compounds are usually first transferred to NAD+, a coenzyme As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration Each NADH (the reduced form of NAD+) represents stored energy that is trapped to synthesize ATP. Electron Carriers Increase Energy Efficiency An Introduction to Electron Transport Chains 3 Stages of Cellular Respiration For each glucose molecule degraded to CO2 and H2O, the cell makes about 32 molecules of ATP Series of reactions resulting in the transfer of electrons to NAD+, creating NADH. NADH brings the electrons to the electrons transport chain (ETC). Movement of electrons along the ETC used to synthesize 90% of ATP – Oxidative Phosphorylation Substrate-Level Phosphorylation A smaller amount of A T P is formed in glycolysis and the citric acid cycle by substrate-level phosphorylation In this process, an enzyme transfers a phosphate group directly from a substrate molecule to A D P Glycolysis Occurs in cytoplasm (cytosol) Occurs whether or not O2 is present 2 major phases: Energy investment phase Energy payoff phase The net energy yield is 2 ATP & 2 NADH molecules per glucose Glycolysis – Energy Investment 1 - Phosphorylation of glucose traps it in the cell (hexokinase) 3- Additional phosphorylation of glucose. Destabilizes the molecule making it possible to split it in two. (phosphofructokinase) 4/5 – Adolase cleaves glucose into 2 3-carbon molecules. DHAP converted to G3P (adolase/G3P) Energy investment phase ends with 2 G3P molecules. Glycolysis – Energy Payoff 6 – 2 sequential reactions: (1) G3P is oxidized by transfer of electrons to NAD+ forming NADH. (2) Using the energy from this exergonic reaction, a phosphate group group is attached to the oxidized substrate, making it a high energy product (Triphosphate dehydrogenase) 7 – The phosphate group in 1,3-Bisphosphoglycerate is transferred to ADP via substrate level phosphorylation. (phosphoglycerokinase) 10- A second phosphate group is transferred from PEP to ADP via substrate level phosphorylation, forming pyruvate (pyruvate kinase) At the End of Glycolysis Glycolysis occurs in both the presence or absence of O2. (Can take place with or without oxygen) 4 ATP – 2 ATP used during phase 1 = 2 ATP (net gain) 2 NADH 2 pyruvate The ATP can be used anywhere in the cell. The NADH and pyruvate still carry a lot of energy… Glycolysis results in the partial oxidation of glucose to pyruvate. This means that: A. the electron carriers donate electrons to proteins in the mitochondria that in turn produce ATP. B. glucose is broken down partially to ATP in the cytoplasm. C. glucose combines with oxygen in the cytoplasm to get partially oxidized. D. glycolysis consists only of exergonic reactions so that ATP can be made from the release of energy. E. in the process of the conversion of glucose to pyruvate, some potential energy is transferred to NADH and ATP. Oxygen vs. No Oxygen If O2 is present, then aerobic respiration will take place: Chemical energy stored in pyruvate and NADH can be extracted via 1. Pyruvate oxidation 2. The Citric Acid Cycle (Krebs cycle) 3. Oxidative phosphorylation (Electrons transport chain) In the absence of oxygen, cells will undergo fermentation to continue to synthesize ATP Pyruvate Oxidation: Acetyl-CoA Synthesis Pyruvate is converted to acetyl-CoA and CO2 is released This process yields 1 NADH per pyruvate – 2 NADH per glucose molecule The Citric Acid Cycle Also known as the Krebs cycle, completes the breakdown of pyruvate into CO2. Each turn oxidizes molecules derived from one pyruvate molecule The cycle turns twice, generating 2 ATP, 6 NADH, and 2 FADH2 per glucose molecule The Citric Acid Cycle Begins with the Entry of Two Carbons from Acetyl C o A – CAC1 In the first reaction, C A C-1, the two-carbon acetate group is transferred from acetyl CoA to oxaloacetate (4C) to form citrate (6C). This reaction is catalyzed by citrate synthase (synthesizing citrate). and driven by the free energy of hydrolysis of the acetyl C o A thioester bond. C A C-2 Citrate is converted to isocitrate in the second step. The enzyme aconitase catalyzes the reaction. Isocitrate dehydrogenase has a hydroxyl group that is easily oxidized (or dehydrogenated remove hydrogen from main substrate and add to e- carrier.) in the next step. Product of the reaction is a reduced electron carrier. C A C-3: The First Oxidative Decarboxylation in the Citric Acid Cycle Isocitrate is oxidized by isocitrate dehydrogenase to oxalosuccinate, with N A D+ as the electron acceptor. Oxalosuccinate immediately undergoes decarboxylation to form α- ketoglutarate (5 C) (C A C- 3). C A C-4: The Second Oxidative Decarboxylation in the Citric Acid Cycle α-ketoglutarate is oxidized to succinyl C o A, by α-ketoglutarate dehydrogenase (C A C-4), with N A D+ as the electron acceptor. The Citric Acid Cycle – Half- Way point So far, two carbon atoms have entered and two have left (but not the same two carbons), and two molecules of N A D H have been generated. Direct Generation of G T P (or A T P) Occurs at One Step in the Citric Acid Cycle Succinyl C o A has been generated; like acetyl C o A, it has a high- energy thioester bond. The C A C-5 reaction (succinyl C o A to succinate) is catalyzed by succinyl C o A synthetase. The energy from hydrolysis of this bond is used to generate one A T P (bacteria, plants) or G T P (animals; eventually converted to ATP). CAC – 6: Oxidative Reaction to Generate F A D H2 Succinate is oxidized to fumarate by succinate dehydrogenase (C A C-6); this transfers electrons to F A D (flavin adenine dinucleotide), a lower-energy coenzyme than N A D+. The oxidation of a carbon- hydrogen bond releases less energy than does the oxidation of a carbon-oxygen bond—not enough energy to transfer electrons to NAD+. CAC – 7: Hydration of Fumarate into Malate Fumarate is hydrated to produce malate (C A C-7) by fumarate hydratase. Since Fumerate is symmetrical, the hydroxyl group of water has an equal chance of adding to ether internal carbon atom CAC – 8:Final Oxidation and Regeneration of Oxaloacetate The newly formed hydroxyl group is the target of the final oxidation catalyzed by Malate dehydrogenase NAD+ serves as the electron acceptor, producing NADH. Oxaloacetate is regenerated Acetyl-CoA is completely Oxidized Since 2 Pyruvate molecules are present at the end of glycolysis, we have 2 Acetyl-CoA entering the TCA cycle. Results in: 2 ATP (one from krebs cycle one from the other pyruvate. Most of the energy 6 NADH extracted from glucose 2 FADH2 is stored in the electron carriers In the citric acid cycle: A. the acetic acid from acetyl-CoA is completely oxidized and the energy is captured by ATP B. Pyruvate is oxidized and the energy is captured by ATP C. Acetic acid from acetyl-CoA is reduced and most of the energy is captured by NADH and FADH 2 D. Acetic acid from acetyl-CoA is oxidized and most of the energy is captured by NADH and FADH 2 E. Pyruvate is reduced and most of the energy is captured by NADH and FADH2 Oxidative Phosphorylation Complex I harvests electrons from NADH Complex II harvests electrons from FADH2 Movement of electrons through the ETC is used to pump H+ from the matrix into the intermembrane space. The Electron Transport Chain The electron transport chain is located in the inner membrane of the mitochondrion in eukaryote cells (plasma membrane in prokaryotes) Made up of protein complexes that contain electron carriers – Numbered I to IV Electrons drop in free energy with each transfer between carriers down the chain to O2. The energy released is used to pump protons across the inner membrane into the intermembrane space. Chemiosmosis – ATP Synthesis The ETC establishes a proton (H+) gradient across Energy is stored as potential energy Proton-motive force Chemiosmosis: The use of energy in the H+ gradient to drive cellular work – ATP synthesis H+ can only cross the membrane back into the matrix via a specialized complex – ATP Synthase ATP synthase uses the exergonic flow of H+ down their concentration gradient to drive the phosphorylation of ADP into ATP The energy from the movement of electrons through the electron transport chain is directly used to synthesize ATP. A. true B. false An Accounting of A T P Production by Cellular Respiration During cellular respiration, most energy flows in the following sequence: glucose NADH electron transport chain proton - motive force ATP About 34% of the energy in a glucose molecule is transferred to A T P, making about 30 to 32 A T P depending on cell type. 1 NADH = 2.5 ATP; 1 FADH2 = 1.5 ATP The remaining energy from glucose is lost as heat When considering the transfer and capture of potential energy derived from glucose during cellular respiration, which molecule carries the smallest amount of that potential energy? A. acetyl-CoA B. NADH C. ATP D. pyruvate E. FADH2 Oxygen vs. No Oxygen Without O2 the ETC will stop and oxidative phosphorylation will cease. The the absence of O2, cells generate ATP via: 1. Anaerobic respiration 2. Fermentation Anaerobic respiration take place in prokaryotes that use SO42- as the final electron acceptor in their ETC. Fermentation Fermentation is a way for a cell to regenerate NAD+ so ATP can be synthesized via glycolysis NAD+ is regenerated via electron transfers from NADH to pyruvate 2 types: 1. Alcohol fermentation= 2. Lactate fermentation= used by eukaryotic cells (muscle cells ie; lactic acid) Alcohol Fermentation In alcohol fermentation, pyruvate is converted to ethanol (ethyl alcohol) in two steps 1. CO2 is released from pyruvate, forming acetaldehyde 2. Acetaldehyde is reduced by NADH to ethanol NADH is oxidized regenerating NAD+ Fermentation goal= generate NAD+ Lactic Acid Fermentation In lactic acid fermentation, pyruvate is converted directly to lactate (an ionized form of lactic acid) without producing CO2 NADH is oxidized to produce NAD+ The purpose of fermentation is to regenerate NAD+ so glycolysis can keep taking place. Glycolysis needs a steady supply of NAD+ to get to the ATP producing steps Wait, We don’t Just Eat Straight Glucose… Lipids are a good source of energy because of their chemical structure: they are rich in carbon-carbon and carbon-hydrogen bonds Beta-oxidation: Breakdown of fatty acids 2 carbons at a time to generate NADH, FADH2, and Acetyl-CoA