Cellular Respiration and Fermentation 7 Cellular Respiration and Fermentation

Life Is Work Living cells require energy from outside sources Some animals, such as the giraffe, obtain energy by eating plants, and some animals feed on other organisms that eat plants © 2016 Pearson Education, Inc. 2

Photosynthesis in chloroplasts Cellular respiration in mitochondria Energy flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM Photosynthesis in chloroplasts Cellular respiration in mitochondria CO2  H2O  O2 Organic molecules ATP powers most cellular work ATP Heat energy Figure 9.2 Energy flow and chemical recycling in ecosystems.

Concept 7.1: Catabolic pathways yield energy by oxidizing organic fuels The breakdown of organic molecules is exergonic Fermentation is a partial degradation of sugars that occurs without O2 Aerobic respiration consumes organic molecules and O2 and yields ATP Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2

Cellular respiration includes both aerobic and anaerobic respiration 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)

Redox Reactions: Oxidation and Reduction Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions In oxidation, a substance loses electrons, or is oxidized [“OIL”] In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) [“RIG”] becomes oxidized (loses electron) becomes reduced (gains electron)

Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced becomes oxidized becomes reduced

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+ (nicotinamide adenosine dinucleotide, a coenzyme Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

2 e + 2 H+ 2 e + H+ NAD+ H+ NADH Dehydrogenase Reduction of NAD+ Figure 7.4 2 e + 2 H+ 2 e + H+ NAD+ NADH H+ Dehydrogenase Reduction of NAD+ 2[H] H+ (from food) Oxidation of NADH Nicotinamide Nicotinamide (oxidized form) (reduced form) Figure 7.4 NAD+ as an electron shuttle © 2016 Pearson Education, Inc.

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain NADH passes the electrons to the Electron Transport Chain of cellular respiration Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction O2 serves as the final electron acceptor at the end of the chain (2 e- and 2 H+ bond with O forming H2O) The energy yielded is used to regenerate ATP

Controlled release of energy release Figure 7.5 H2 + 1/2 O2 2 H + 1 /2 O2 Controlled release of energy 2 H+ + 2 e ATP Free energy, G Free energy, G ATP Explosive release chain Electron transport ATP 2 e  Figure 7.5 An introduction to electron transport chains 1/2 O2 2 H+ H2O H2O (a) Uncontrolled reaction (b) Cellular respiration © 2016 Pearson Education, Inc.

The Stages of Cellular Respiration: A Preview Harvesting of energy from glucose has three stages Glycolysis (breaks down glucose into two molecules of pyruvate) The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis)

Electrons carried via NADH Electrons carried via NADH and FADH2 Figure 9.6-3 Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Pyruvate oxidation Glycolysis Citric acid cycle Glucose Pyruvate Acetyl CoA CYTOSOL MITOCHONDRION Figure 9.6 An overview of cellular respiration. ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

Concept 7.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate Glycolysis occurs in the cytoplasm and has two major phases Energy investment phase Energy payoff phase Glycolysis occurs whether or not O2 is present

1, 3-Bisphosphoglycerate energy investment phase energy payoff phase 2 NAD+ NADH 2 + 2 H+ Triose phosphate dehydrogenase P i P C CHOH O CH2 O– 1, 3-Bisphosphoglycerate 2 ADP 2 ATP Phosphoglycerokinase 3-Phosphoglycerate Phosphoglyceromutase CH2OH H 2-Phosphoglycerate 2 H2O Enolase Phosphoenolpyruvate Pyruvate kinase CH3 6 8 7 9 10 Pyruvate Figure 9.8 B Dihydroxyacetone phosphate Glyceraldehyde- 3-phosphate H OH HO CH2OH O P CH2O CH2 C CHOH ATP ADP Hexokinase Glucose Glucose-6-phosphate Fructose-6-phosphate Phosphoglucoisomerase Phosphofructokinase Fructose- 1, 6-bisphosphate Aldolase Isomerase Glycolysis 1 2 3 4 5 Oxidative phosphorylation Citric acid cycle Figure 9.9 A

Energy investment phase Glycolysis Overview Glycolysis Citric acid cycle Oxidative phosphorylation ATP 2 ATP 4 ATP used formed Glucose 2 ATP + 2 P 4 ADP + 4 2 NAD+ + 4 e- + 4 H + 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Energy investment phase Energy payoff phase 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H + 2 ATP Invested 4 ATP generated 2 NAD+ reduced to 2 NADH Net Products at end of Glycolysis: 2 pyruvate molecules 2 ATP 2 NADH Figure 9.8

Concept 7.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells) Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA) Pyruvate Transport protein CYTOSOL MITOCHONDRION CO2 Coenzyme A NAD + H NADH Acetyl CoA 1 2 3 Figure 9.10 Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle.

The Citric Acid Cycle Pyruvate (from glycolysis, 2 molecules per glucose) CYTOSOL PYRUVATE OXIDATION NAD+ NADH + H+ Acetyl CoA CoA CITRIC ACID CYCLE FADH2 FAD ADP + P i ATP MITOCHONDRION 3 NAD+ 3 NADH + 3 H+ CO2 2 CO2 The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2 The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per pyruvate

Net Products from Intermediate Step + Citric Acid Cycle: 2 ATP 6 CO2 8 NADH 2 FADH2 NADH 1 Acetyl CoA Citrate Isocitrate -Ketoglutarate Succinyl CoA Succinate Fumarate Malate Citric acid cycle NAD FADH2 ATP + H H2O ADP GTP GDP P i FAD 3 2 4 5 6 7 8 CoA-SH CO2 Oxaloacetate Figure 9.12 A closer look at the citric acid cycle.

Concept 7.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis The two electron carriers, NADH and FADH2, donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation The electron transport chain is in the inner membrane (cristae) of the mitochondrion Most of the chain’s components are proteins, which exist in multiprotein complexes For the Cell Biology Video ATP Synthase 3D Structure — Side View, go to Animation and Video Files. For the Cell Biology Video ATP Synthase 3D Structure — Top View, go to Animation and Video Files.

The Pathway of Electron Transport NADH FADH2 2 H + 1/2 O2 2 e 2 e H2O NAD Multiprotein complexes (originally from NADH or FADH2) I II III IV 50 40 30 20 10 Free energy (G) relative to O2 (kcal/mol) FMN FeS FAD Q Cyt b Cyt c1 Cyt c Cyt a Cyt a3 The carriers alternate reduced and oxidized states as they accept and donate electrons Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O Figure 9.13 Free-energy change during electron transport. It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts

Chemiosmosis: The Energy-Coupling Mechanism Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space H+ then moves back across the membrane, passing through the protein/enzyme, ATP synthase ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work

Electron Transport & Chemiosmosis Protein complex of electron carriers H Cyt c IV Q III I ATP synth- ase II 2 H + 1/2O2 H2O FADH2 FAD Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis. NAD NADH ADP  P i ATP (carrying electrons from food) H 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation

ATP synthase is the enzyme that actually makes ATP Figure 7.13 Intermembrane space Inner mitochondrial membrane Mitochondrial matrix ATP synthase is the enzyme that actually makes ATP INTERMEMBRANE SPACE H+ Stator Rotor Internal rod Figure 7.13 ATP synthase, a molecular mill Catalytic knob ADP + P ATP i MITOCHONDRIAL MATRIX (a) The ATP synthase protein complex (b) Computer model of ATP synthase © 2016 Pearson Education, Inc.

An Accounting of ATP Production by Cellular Respiration Energy flows in this sequence: glucose  NADH  electron transport chain  proton-motive force  ATP About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP Electron shuttles span membrane MITOCHONDRION 2 NADH 6 NADH 2 FADH2 or  2 ATP  about 26 or 28 ATP Glycolysis Glucose 2 Pyruvate Pyruvate oxidation 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis CYTOSOL Maximum per glucose: About 30 or 32 ATP

Most cellular respiration requires O2 to produce ATP Concept 7.5: Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen Most cellular respiration requires O2 to produce ATP Without O2, the electron transport chain will cease to operate In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example sulfate Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP

Types of Fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 Alcohol fermentation by yeast is used in brewing, winemaking, and baking 2 ADP 2 ATP Glucose Glycolysis 2 Pyruvate 2 CO2 2  2 NADH 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation P i NAD 2 H Figure 9.17 Fermentation.

Types of Fermentation In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 (b) Lactic acid fermentation 2 Lactate 2 Pyruvate 2 NADH Glucose Glycolysis 2 ATP 2 ADP  2 P i NAD 2 H Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce Figure 9.17 Fermentation.

Animation: Fermentation Overview © 2016 Pearson Education, Inc.

Comparing Fermentation with Anaerobic and Aerobic Respiration All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration Cellular respiration produces 32 ATP per glucose molecule; Fermentation produces 2 ATP per glucose molecule

The Evolutionary Significance of Glycolysis Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP Glycolysis is a very ancient process Glucose CYTOSOL Glycolysis Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration Ethanol, lactate, or other products Acetyl CoA MITOCHONDRION Citric acid cycle

The Versatility of Catabolism Carbohydrates Proteins Fatty acids Amino acids Sugars Fats Glycerol Glycolysis Glucose Glyceraldehyde 3- P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation Figure 9.19 The catabolism of various molecules from food.

You should now be able to: Explain in general terms how redox reactions are involved in energy exchanges Name the three stages of cellular respiration; for each, state the region of the eukaryotic cell where it occurs and the products that result In general terms, explain the role of the electron transport chain in cellular respiration Explain where and how the respiratory electron transport chain creates a proton gradient Distinguish between fermentation and anaerobic respiration