Cellular Respiration (Chapter 4) PDF
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This document provides a review of cellular respiration, covering glycolysis, pyruvate oxidation, and the citric acid cycle. It details the processes, pathways, and chemical reactions involved in cellular respiration.
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Cellular Respiration (Chapter 4) Cellular Respiration Cellular Respiration and Photosynthesis T hey are the reverse of each other Cellular respiration is a catabolic process Photosynthesis is an anabolic process Definitions of Terms Frequently Used: Metabolis...
Cellular Respiration (Chapter 4) Cellular Respiration Cellular Respiration and Photosynthesis T hey are the reverse of each other Cellular respiration is a catabolic process Photosynthesis is an anabolic process Definitions of Terms Frequently Used: Metabolism:All the chemical reactions in a cell etabolic Pathway:Series of chemical reactions inliving cells that are catalyzed by an M enzyme Phosphorylation:Process of attaching a phosphategroup to an organic molecule atabolism:Metabolic process that involves breakingdown a molecule into smaller C molecules, usually to release energy Anabolism:Metabolic process that uses energy to synthesizea large molecule from smaller molecules Oxidation:Process involving the loss of electrons Reduction:Process involving the gain of electrons Cellular Respiration Catabolic pathway that breaks down energy rich compounds like glucose to produce ATP ATP is very important for all cellular functions Aerobic cellular respiration are pathways that require oxygen AerobicCellular Respiration can be divided into four(4) main stages: ○ Glycolysis- Occurs in the cytosol ○ Pyruvate Oxidation- Occurs in the mitochondrial matrix ○ Citric Acid Cycle (Krebs Cycle)- this occurs in themitochondrial matrix ○ Electron Transport Chain and Chemiosmosis(oxidativephosphorylation)- this occurs in the inner mitochondrial membrane and intermembrane space Total # of ATP depends on the cell, but aerobic respiration will yield either 36OR38 ATP molecules Glycolysis and Pyruvate Oxidation History of Glycolysis It is the first set of reactions to extract free energy from glucose. DOES NOT REQUIRE OXYGEN It is universally found in all organisms, this is what all organisms, this is what allowed organisms to exist for 1.5 billion years before oxygen dependent organisms were predominant Occurs in the cytosol, with less sophisticated soluble enzymes involved Glycolysis Overview xidation of 6-carbon glucose, into two 3-carbon molecules calledpyruvate O Entire process has 10 enzymes controlled steps Divided into two phases- Energy investment ○ 2 ATP molecules use Energy payoff ○ 4 ATP molecules produced nergy Investment E Step 1: Glucose receives phosphate group from ATP Produces glucose-6-phosphate Phosphorylation reaction Step 2: Glucose-6-phosphate is rearranged into its isomer, fructose-6-phosphate Isomerization reaction Step 3: Another phosphate group from ATP is attached to fructose-6-phosphate Produces fructose-1,6-bisphosphate Phosphorylation reaction Step 4 (and 5) Fructose-1,6-bisphosphate is split intoglyceraldehyde-3-phosphate(G3P)and dihydroxyacetone phosphate (DHAP) Lysis reaction DHAP is conveyed into a G3P molecule, so 2G3P molecules go into step 6 Isomerization reaction nergy Payoff E Step 6: Two electrons and two protons are removed for G3P Pi from cytosol, not ATP, is attached NAD+ accepts the electrons, protons are released into the cytosol 1,3-diphosphoglycerate formed Redox reaction Step 7 One of the two phosphate groups of 1,3-bisphosphoglycerate is transferred to ADP to produce ATP (FIRST PAYOFF) Substrate level phosphorylation reaction 3-phosphoglycerate produced Step 8: 3-phosphoglycerate is rearranged, moving phosphate group to the second carbon Produces 2-phosphoglycerate Mutase reaction Step 9: Electrons are removed from one part of 2-phosphoglycerate and delivered to another part of the molecule Most of the energy lost is retained by the product,phosphoenolpyruvate Redox reaction Step 10: Last phosphate group is removed from phosphoenolpyruvate and transferred to ADP T he reaction forms ATP and the final product of glycolysis: PYRUVATE Redox Reactions and Energy Carriers Enzymes calleddehydrogenasesare commonly used tofacilitate the transfer of electrons from food molecules to electron carriers T he most common electron carrier is coenzymeNAD+ NAD+ stands for nicotinamide adenine dinucleotide. Dehydrogenases removes 2 hydrogen atoms but only transfers 2e- (high energy) and a proton to NAD+ reducing the electron carrier to NADH T he other proton is released into the cytosol Substrate Level Phosphorylation T he enzymatic transfer of a phosphate group from a substrate to ADP to form ATP Enzyme has specific binding sites Conformational change to accommodate the substrates Reaction occurs and enzyme releases products Efficiency Glucose has been broken down into 2 pyruvate molecules Produced a NET of 2 ATP 2 ATP x 30.5 kJ/mol = 61 kJ/mol (61kJ/mol / 2870 kJ/mol) x 100% = 2% Summary 2 ATP consumed (in the energy investment phase) 4 ATP and 2 NADH molecules synthesized (energy pay off phase) ○ Net 2 ATP and 2 NADH are produced No carbon is lost because each pyruvate molecule has 3 carbons T he overall reaction up to this stage is T he ATP was produced bySubstrate level phosphorylation Pyruvate Oxidation The Mitochondrion Pyruvate Oxidation T he 2 pyruvate molecules from glycolysis are small enough to move into the matrix of the mitochondrion now T hey cross the OUTER mitochondrial membrane through simple diffusion T hey enter the INNER mitochondrial membrane through a specific carrier protein Steps of Pyruvate Oxidation 1. T he pyruvate molecule which us a 3 carbon compound loses one carbon through a process known as decarboxylation. A two carbon compound called acetyl is left 2. An oxidation/reduction reaction takes place where the pyruvate is oxidized using NAD+ becomes reduced to NADH + H is produced. This reaction is also known as a dehydrogenation reaction. 3. Acetyl group reacts with a sulfur atom from a compound called coenzyme A, producing acetyl-CoA a. Very high energy compound Citric Acid Cycle A lso known as the Krebs Cycle or Tri-Caroxylic Acid Cycle (TCA) is an 8 step process, each catalyzed by a specific enzyme Cyclic process because oxaloacetate, the product of step 8, is the reactant of step 1 Step 1 A 2-carbon acetyl group carried by coenzyme-A (product of pyruvate oxidation) is transferred to oxaloacetate to formcitrate. Step 2 Citrate is transformed into isocitrate, an isomer Step 3 Isocitrate is oxidized to a-ketoglutarate One carbon is removed and released as CO2, and NAD+ is reduced to NADH+ H+ Step 4 A-ketoglutarate is oxidized tosuccinyl CoA One carbon is removed and released as Co2 and NAD+ is reduced to NADH and H+ Step 5 CoA is released to formsuccinate T he energy released converts GDP to GTP which then converts ADP to ATP through substrate level phosphorylation T his is the only ATP made directly in the Citric Acid Cycle Step 6 Succinate is oxidized tofumarate Two electrons and two protons are removed and transferred to FAD, producing FADH2 Step 7 Fumarate is converted to malate by the addition of water Step 8 Malate is oxidized to oxaloacetate NAD+ is reduced to NADH and H+ Oxaloacetate can react with acetyl-CoA from pyruvate oxidation and re-enter the cycle Summary: Acetyl CoA enters the Krebs cycle and reacts with oxaloacetate a 4C compound, producing citrate and a 6-carbon compound Citrate becomes oxidized four times ○ 3 times by NAD+ and once by FAD ○ Becomes decarboxylated twice and releases two CO2 molecules ○ Creates one ATP molecule through substrate level phosphorylation ○ Ends up producing oxaloacetate to begin cycle again SO FAR WE HAVE GLYCOLYSIS PYRUVATE OXIDATION KREB’S CYCLE NADH 2 CO2 2 CO2 4 2 ATP 2 NADH 2 ATP 6 NADH 2 FADH2 4 ATP 10 NADH 2 FADH2 Electron Transport Chain (ETC) and Chemiosmosis Electron Transport Chain Consists of 4 protein complexes as follows: ○ Complex I - NADH dehydrogenase ○ Complex II - FADH2 dehydrogenase ○ Complex III - Cytochrome complex ○ Complex IV - Cytochrome oxidase Complexes I, III and IV areintegralproteins of theinner mitochondrial membrane Complex II is a peripheral protein T hese complexes facilitate movement of electrons from NADH and FADH2 to oxygen Electrons move from one complex to another through two mobile proteins: ○ Ubiquinone (UQ) ○ Cytochrome c (Cyt c) T hese complexes are arranged from high to low free energy relative to oxygen ADH and FADH2 are electron carriers N for the ETC ADH gives up electrons to complex I N (NADH dehydrogenase) ADH2 gives up electrons to complex II F (FADH2 dehydrogenase) W ith the help of mobile proteins (UQ and Cyt c) the electrons are passed from complex to complex until they reach the final electron acceptor O2 ath of NADH Electrons P NADH →Complex 1→ Complex III → Complex IV → O2 ath of FADH2 Electrons P FADH2 →Complex II→ Complex II → Complex IV → O2 FINAL STEP of ETC Once the electrons reach O2 two protons also join the reaction to produce H2O Chemiosmosis Process where ATP is made using energy of an electrochemical gradient and the ATP synthase enzyme Proton Movement Across the Inner Mitochondrial Membrane As electrons move from complex to complex, free energy is generated T his energy is used to move protons (H+) from the mitochondrial matrix to intermembrane space ComplexI & IV arespecific protein components thatpump H+ across inner mitochondrial as energy is released Ubiquinonealso picks up H+ from the matrix and movesit into the inner mitochondrial membrane as it accepts electrons from complex I & II In the process, and electrochemical gradient is created in the intermembrane space T he energy from the electrochemical gradient is used to make ATP byoxidative phosphorylation ATP Synthase T he enzyme responsible for ATP synthesis Has 2 principal components: ○ “Basal” portion: made up of integral membrane proteins and the “head” portion which extends to the matrix Basal unit forms a channel for H+ ions to pass freely via proton motive force Research has show that it takes3 H+to pass throughthe ATP Synthase basal portion to catalyze the formation ofone ATP Regulating Cellular Respiration Uncoupling ETC S ometimes we do not want to produce ATP all the time, we can use the energy for another purpose When this happens ○ Energy is released as thermal energy ○ H+ ions rush back from the mitochondrial intermembrane space over the membrane through uncoupling proteins into the matrix When is this useful? In brown adipose fat tissues, there are a lot of uncoupling protein channels T hese channels allow H+ ions back into the matrix ○ T his process releases thermal energy that can be used to maintain body temperature Calculating ATP Yield/Glucose 2 NADH from cytosol (glycolysis) do not have a way of getting into the mitochondria in some cells So NADH gives its electrons to FAD, to become FADH2 T his means only 2 ATP are produced instead of 3 T his is why sometimes cells there is only 36 instead of 38 Creatine Phosphate (ATP Storage Molecule) ATP Energy demand fluctuate in cells ○ T he cell may need sudden burst or very little ATP ○ E.g. in skeletal muscle cells Creatine phosphate acts as ATP storage molecule ○ T his molecule can be easily released for sudden ATP energy bursts or stored if there are no activity in the cell as shown in the reaction below: ○ Creatine + ATP → creatine phosphate + ADP ○ Creatine phosphate + ADP → creatine + ATP Regulating Aerobic Respiration Many enzymes and transport systems are used to facilitate respiration in cells as we have seen ATP production is controlled/monitored to meet energy requirements ○ Supply and Demand: Cell wont start pathways when its not needed ○ Feedback inhibition: Intermediates provide feedback midway through pathways to change outcomes Regulatory molecules can be inhibitors or activators (Stop or Start a pathway) Example Phosphofructokinase enzyme ○ Pathway is regulated at irreversible steps ○ Main enzyme in glycolysis that can be inhibited or activated ○ High ATP or citrate content = inhibited pathway ○ Insulin (indirectly) and AMP = activated pathway Alternative to Glucose Fats: E x: The breakdown of a triacylglycerol or triglyceride which is made up of a glycerol molecule and 3 fatty acids Hydrolysis of a Triacylglycerol An enzyme called lipasehydrolyzesa triacylglycerolinto a glycerol molecule and 3 fatty acids Glycerol is ○ EITHER converted to glucose through a process called gluconeogenesis ○ OR is converted to dihydroxyacetone phosphate (DHAP) which is converted to glyceraldehyde-3-phosphate (G3P) which enters glycolysis. Fatty Acids T he fatty acids enter the mitochondrial matrix where they are catabolized by a process known asbeta-oxidation Beta-oxidation is the sequential removal of acetyl groups (2 carbon compounds) from the fatty acid T he acetyl groups react with CoA to make acetyl-CoA Acetyl-CoA enters the Krebs cycle and is broken down to CO2 and H2O Energy Produced and Used During the Process Reduced Coenzymes produce 10 theoretical ATP from FADH2 and 15 ATP from NADH in ETC. A total of 25 ATP Less 5 ATP used in beta-oxidation = 20 ATP NET 6 acetyl-CoA produced from Beta-oxidation go through the Krebs cycle (1 cycle per acetyl-CoA) and produce ○ 6 ATP at substrate level phosphorylation ○ 18 NADH that produce 54 ATP in ETC ○ 6 FADH2 that produce 12 ATP in ETC Total = 92 theoretical ATP produce from each laureate Comparing One Laurate Molecule with 2 Glucose Molecules We are comparing 1 laurate with 2 glucose molecules because laurate is 12 carbons and 2 glucose is also 12 carbon Two glucose molecules produce 72 ATP-theoretical (Fatty acids produce 92 ATP) Proteins as Energy Source Second last resort because they have other important uses in the body Proteins are hydrolyzed to amino acids where some are used to make cells own protein Other amino acids have the amino group remove via deamination (ammonia is a product) In plants ammonia is a nutrient, in animals its a waste product Carbon skeleton left after deamination is catabolized ○ Amino acid depicts what type of substrate is produced ○ E.g. alanine is converted to pyruvate ○ Leucine → acetyl-CoA ○ Proline → a-ketoglutarate ○ Phenylalanine → fumarate Anaerobic Cellular Respiration Pathways Anaerobic Pathways Anaerobic cellular respiration is a type of respiration where oxygen is not used as a final electron acceptor. Other molecules are used instead Organisms in oxygen-poor environments use these ○ Wet environments, human digestive tract, deep underground In step 6 of glycolysis it reduces NAD+ to NADH But because there is no oxygen they have to take a different route of oxidizing called fermentation Ethanol Fermentation: T he 2 pyruvates from glycolysis are decarboxylated and form 2 acetaldehydes T hese acetaldehydes are reduced by 2NAH forming 2 ethanol and 2 NAD+ Acetaldehyde is the oxidizing agent. It oxidizes NADH to NAD+ and reduces itself to ethanol Co2 and ethanol are the waste products Hymans use Alcohol Fermentation to make breads, pasterires, beers, wine, liquor Ethanol → acetaldehyde → acetic acid Lactate Fermentation 2 Pyruvates are reduced by 2 NADH from glycolysis forming 2 lactates and 2 NAD+ Pyruvates are the oxidizing agent. It oxidises NADH to NAD+ and reduces itself to lactate NAD+ generated goes back to glycolysis and Lactate accumulates Lactate is reversible (reversed back to pyruvate and back into mitochondria)
