Chapter 9: Cellular Respiration Overview: Life Is Work Living cells Require transfusions of energy from outside sources to perform their many tasks Biology, 7 th Edition Neil Campbell and Jane Reece
The giant panda Obtains energy for its cells by eating plants Figure 9.1
Energy Flows into an ecosystem as sunlight and leaves as heat Light energy ECOSYSTEM CO 2 + H 2 O Photosynthesis in chloroplasts Cellular respiration in mitochondria Organic molecules + O 2 ATP powers most cellular work Figure 9.2 Heat energy
Concept 9.1: Catabolic pathways yield energy by oxidizing organic fuels
Catabolic Pathways and Production of ATP The breakdown of organic molecules is exergonic
One catabolic process, fermentation Is a partial degradation of sugars that occurs without oxygen
Cellular respiration Is the most prevalent and efficient catabolic pathway Consumes oxygen and organic molecules such as glucose Yields ATP
To keep working Cells must regenerate ATP
Redox Reactions: Oxidation and Reduction Catabolic pathways yield energy Due to the transfer of electrons
The Principle of Redox Redox reactions Transfer electrons from one reactant to another by oxidation and reduction
In oxidation A substance loses electrons, or is oxidized In reduction A substance gains electrons, or is reduced
Examples of redox reactions becomes oxidized (loses electron) Na + Cl Na + + Cl becomes reduced (gains electron)
Some redox reactions Do not completely exchange electrons Change the degree of electron sharing in covalent bonds Reactants Products becomes oxidized CH 4 + 2O 2 CO 2 + Energy + 2 H 2 O H becomes reduced C O O O C O O H H H H H Figure 9.3 Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water
Oxidation of Organic Fuel Molecules During Cellular Respiration During cellular respiration Glucose is oxidized and oxygen is reduced becomes oxidized C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + Energy becomes reduced
Stepwise Energy Harvest via NAD + and the Electron Transport Chain Cellular respiration Oxidizes glucose in a series of steps
Electrons from organic compounds Are usually first transferred to NAD +, a coenzyme NAD + 2 e + 2 H + 2 e + H + NADH O O P O O O O P O CH 2 H O HO HO CH 2 H O H N + H OH N N O C NH 2 Nicotinamide (oxidized form) NH 2 N N H + 2[H] (from food) Dehydrogenase Reduction of NAD + Oxidation of NADH H N H O C + NH 2 Nicotinamide (reduced form) H HO H OH Figure 9.4
NADH, the reduced form of NAD + Passes the electrons to the electron transport chain
Free energy, G If electron transfer is not stepwise A large release of energy occurs As in the reaction of hydrogen and oxygen to form water H 2 + 1 / 2 O 2 Explosive release of heat and light energy (a) Uncontrolled reaction Figure 9.5 A H 2 O
The electron transport chain Passes electrons in a series of steps instead of in one explosive reaction Uses the energy from the electron transfer to form ATP
Free energy, G 2 H + 1 / 2 O 2 (from food via NADH) 2 H + + 2 e Controlled release of energy for synthesis of ATP ATP ATP ATP 2 e 2 H + 1 / 2 O 2 H 2 O Figure 9.5 B (b) Cellular respiration
The Stages of Cellular Respiration: A Preview Respiration is a cumulative function of three metabolic stages Glycolysis The citric acid cycle Oxidative phosphorylation
Glycolysis Breaks down glucose into two molecules of pyruvate The citric acid cycle Completes the breakdown of glucose
Oxidative phosphorylation Is driven by the electron transport chain Generates ATP
An overview of cellular respiration Electrons carried via NADH Electrons carried via NADH and FADH2 Glycolsis Glucose Pyruvate Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Cytosol Mitochondrion ATP ATP ATP Figure 9.6 Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation
Both glycolysis and the citric acid cycle Can generate ATP by substrate-level phosphorylation Enzyme Enzyme ADP Substrate P + ATP Figure 9.7 Product
Concept 9.2: Glycolysis harvests energy by oxidizing glucose to pyruvate Glycolysis Means splitting of sugar Breaks down glucose into pyruvate Occurs in the cytoplasm of the cell
Glycolysis consists of two major phases Energy investment phase Energy payoff phase Glycolysis Citric acid cycle Oxidative phosphorylation ATP ATP ATP Energy investment phase Glucose 2 ATP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 2 NAD + + 4 e - + 4 H + 2 NADH + 2 H + 2 Pyruvate + 2 H 2 O Glucose 2 Pyruvate + 2 H 2 O 4 ATP formed 2 ATP used 2 ATP + 2 H + Figure 9.8 2 NAD + + 4 e + 4 H + 2 NADH
A closer look at the energy investment phase
CH 2 OH H H H H HO HO OH H OH Glucose Glycolysis Citric acid Oxidative cycle phosphorylation ATP ADP Hexokinase CH 2 OH P H O H H H HO OH H OH Glucose-6-phosphate 2 Phosphoglucoisomerase CH 2 O P O CH 2 OH H HO H HO HO H Fructose-6-phosphate ATP ADP 3 Phosphofructokinase P O CH 2 O CH 2 O P H HO HO H OH Fructose- 1, 6-bisphosphate 4 Aldolase 1 Figure 9.9 A P O CH 2 C O CH 2 OH Dihydroxyacetone phosphate 5 Isomerase H C O CHOH CH 2 O P Glyceraldehyde- 3-phosphate
A closer look at the energy payoff phase
2 NAD + 2 NADH + 2 H + 6 Triose phosphate dehydrogenase 2 P i 2 P O C O CHOH CH 2 O P 1, 3-Bisphosphoglycerate 2 ADP 7 Phosphoglycerokinase 2 ATP 2 O CH 2 O P 3-Phosphoglycerate 8 Phosphoglyceromutase 2 C CHOH O C H C O P CH 2 OH 2-Phosphoglycerate 9 2 H 2 O Enolase 2 O C C O O P CH 2 Phosphoenolpyruvate 2 ADP 10 2 ATP O Pyruvate kinase Figure 9.8 B 2 O C O C O CH 3 Pyruvate
Concept 9.3: The citric acid cycle completes the energy-yielding oxidation of organic molecules The citric acid cycle Takes place in the matrix of the mitochondrion
Before the citric acid cycle can begin Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis CYTOSOL MITOCHONDRION NAD + NADH + H + O C O 2 S C CoA O C O 1 3 CH 3 CH 3 Acetyle CoA Pyruvate CO 2 Coenzyme A Transport protein Figure 9.10
An overview of the citric acid cycle Pyruvate (from glycolysis, 2 molecules per glucose) Glycolysis Citric acid cycle Oxidative phosphorylatio n ATP ATP ATP CO 2 NADH + 3 H + Acetyle CoA CoA CoA CoA FADH 2 FAD Citric acid cycle ADP + P i 2 CO 2 3 NAD + 3 NADH + 3 H + Figure 9.11 ATP
A closer look at the citric acid cycle
Glycolysis Citric acid cycle Oxidative phosphorylation S CoA C CH 3 O Acetyl CoA CoA SH H 2 O NAD + HO FADH 2 NADH + H + CH 2 1 COO H 2 O COO CH CH 2 COO 7 COO CH HC COO Malate Oxaloacetate Fumarate FAD 8 6 Succinate O C COO COO CH 2 CH 2 COO ADP COO Figure 9.12 CoA Citric acid cycle 5 SH GTP GDP P i Citrate S CH 2 HO C COO CH 2 COO HO CH COO COO CH 2 CH 2 C O CoA Succinyl CoA CoA 2 SH 4 NAD + NADH + H + COO CH 2 HC COO Isocitrate 3 COO CH 2 CH 2 C O COO NAD + NADH a-ketoglutarate CO 2 CO 2 + H + Figure 9.12 ATP
Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis NADH and FADH 2 Donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
The Pathway of Electron Transport In the electron transport chain Electrons from NADH and FADH 2 lose energy in several steps
Free energy (G) relative to O 2 (kcl/mol) At the end of the chain Electrons are passed to oxygen, forming water 50 NADH 40 30 20 FMN I Fe S FADH2 FAD Fe S II O III Cyt b Fe S Multiprotein complexes Cyt c 1 Cyt c IV Cyt a Cyt a 3 10 0 2 H + + 1 2 O 2 Figure 9.13 H 2 O
Chemiosmosis: The Energy-Coupling Mechanism ATP synthase Is the enzyme that actually makes ATP INTERMEMBRANE SPACE H + H + H + H + H + H + H + A rotor within the membrane spins clockwise when H + flows past it down the H + gradient. A stator anchored in the membrane holds the knob stationary. H + A rod (for stalk ) extending into the knob also spins, activating catalytic sites in the knob. Figure 9.14 ADP + P i MITOCHONDRIAL MATRIX ATP Three catalytic sites in the stationary knob join inorganic Phosphate to ADP to make ATP.
At certain steps along the electron transport chain Electron transfer causes protein complexes to pump H + from the mitochondrial matrix to the intermembrane space
The resulting H + gradient Stores energy Drives chemiosmosis in ATP synthase Is referred to as a proton-motive force
Chemiosmosis Is an energy-coupling mechanism that uses energy in the form of a H + gradient across a membrane to drive cellular work
Chemiosmosis and the electron transport chain Glycolysis Oxidative phosphorylation. electron transport and chemiosmosis Inner Mitochondrial membrane Intermembrane space ATP ATP ATP Protein complex of electron carners H + H+ Q Cyt c IV H + H + Inner mitochondrial membrane Mitochondrial matrix NADH + I II FADH 2 (Carrying electrons from, food) Electron transport chain Electron transport and pumping of protons (H + ), which create an H + gradient across the membrane III NAD + FAD + 2 H + + 1 / 2 O 2 H 2 O ADP + P i H + ATP Chemiosmosis ATP synthesis powered by the flow Of H + back across the membrane ATP synthase Figure 9.15 Oxidative phosphorylation
An Accounting of ATP Production by Cellular Respiration During respiration, most energy flows in this sequence Glucose to NADH to electron transport chain to proton-motive force to ATP
There are three main processes in this metabolic enterprise CYTOSOL Electron shuttles span membrane 2 NADH or MITOCHONDRION 2 FADH 2 2 NADH 2 NADH 6 NADH 2 FADH 2 Glycolysis 2 Glucose Pyruvate 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis + 2 ATP + 2 ATP + about 32 or 34 ATP by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol Maximum per glucose: About 36 or 38 ATP Figure 9.16
About 40% of the energy in a glucose molecule Is transferred to ATP during cellular respiration, making approximately 38 ATP
Concept 9.5: Fermentation enables some cells to produce ATP without the use of oxygen Cellular respiration Relies on oxygen to produce ATP In the absence of oxygen Cells can still produce ATP through fermentation
Glycolysis Can produce ATP with or without oxygen, in aerobic or anaerobic conditions Couples with fermentation to produce ATP
Types of Fermentation Fermentation consists of Glycolysis plus reactions that regenerate NAD +, which can be reused by glyocolysis
In alcohol fermentation Pyruvate is converted to ethanol in two steps, one of which releases CO 2
During lactic acid fermentation Pyruvate is reduced directly to NADH to form lactate as a waste product
2 ADP + 2 P 1 2 ATP Glucose Glycolysis 2 NAD + 2 NADH O O C C CH 3 O 2 Pyruvate 2 CO 2 H H C OH H C O CH 3 2 Ethanol (a) Alcohol fermentation CH 3 2 Acetaldehyde 2 ADP + 2 P 1 2 ATP Glucose Glycolysis O 2 NAD + 2 NADH C O H C OH O C O C O CH 3 Figure 9.17 CH 3 2 Lactate (b) Lactic acid fermentation
Fermentation and Cellular Respiration Compared Both fermentation and cellular respiration Use glycolysis to oxidize glucose and other organic fuels to pyruvate
Fermentation and cellular respiration Differ in their final electron acceptor Cellular respiration Produces more ATP
Pyruvate is a key juncture in catabolism Glucose CYTOSOL Pyruvate No O 2 present Fermentation O 2 present Cellular respiration Ethanol or lactate Acetyl CoA MITOCHONDRION Citric acid cycle Figure 9.18
The Evolutionary Significance of Glycolysis Glycolysis Occurs in nearly all organisms Probably evolved in ancient prokaryotes before there was oxygen in the atmosphere
Concept 9.6: Glycolysis and the citric acid cycle connect to many other metabolic pathways
The Versatility of Catabolism Catabolic pathways Funnel electrons from many kinds of organic molecules into cellular respiration
The catabolism of various molecules from food Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde-3- P NH 3 Pyruvate Acetyl CoA Citric acid cycle Figure 9.19 Oxidative phosphorylation
Biosynthesis (Anabolic Pathways) The body Uses small molecules to build other substances These small molecules May come directly from food or through glycolysis or the citric acid cycle
Regulation of Cellular Respiration via Feedback Mechanisms Cellular respiration Is controlled by allosteric enzymes at key points in glycolysis and the citric acid cycle
The control of cellular respiration Glucose Glycolysis Fructose-6-phosphate Phosphofructokinase Stimulates + AMP Inhibits Fructose-1,6-bisphosphate Inhibits ATP Pyruvate Acetyl CoA Citrate Citric acid cycle Figure 9.20 Oxidative phosphorylation