Ch. 3 How Cells Obtain Energy BIOL 100 Metabolism Metabolism The totality of an organism s chemical reac9ons Sum of anabolism and catabolism emergent property of life that arises from interac9ons between molecules within the cell But in a controlled manner homeostasis Metabolic Pathways Metabolic pathway begins with a specific molecule and ends with a specific product Each step is catalyzed by a specific enzyme 1
Catabolic pathways Release energy by breaking down complex molecules into simpler compounds Cellular respira4on The breakdown of glucose in the presence of oxygen Anabolic pathways Consume energy to build complex molecules from simpler ones Ie - The synthesis of protein from amino acids Bioenerge4cs Catabolism The study of how organisms manage their energy resources All life boils down to energy budget Kine4c energy energy associated with mo9on Heat (thermal energy) kine9c energy associated with random movement of atoms or molecules Poten4al energy energy that majer possesses because of its loca9on or structure Chemical energy Types of Energy poten9al energy available for release in a chemical reac9on Energy can be converted from one form to another The Laws of Energy Transforma9on Thermodynamics The study of energy transforma9ons Closed system Isolated from its surroundings Open system Liquid in a thermos Energy and majer can be transferred between the system and its surroundings Organisms are open systems 2
The First Law of Thermodynamics First law of thermodynamics (Law of Conserva9on of Energy) The energy of the universe is constant: Energy cannot be created or destroyed but transferred and/or transformed The Second Law of Thermodynamics Second law of thermodynamics: Every energy transfer or transforma5on increases the entropy (disorder) of the universe During every energy transfer or transforma9on Some energy is unusable, osen lost as heat Increases entropy Fuel Energy conversion Waste products Heat energy Gasoline Carbon dioxide Combustion Kinetic energy of movement Oxygen Water Energy conversion in a car Energy Living cells unavoidably convert organized forms of energy to heat 2nd law Spontaneous processes occur without energy input can happen quickly or slowly to occur without energy input must increase the entropy of the universe Generally corresponds to breakdown Fuel Energy conversion Waste products Heat Cellular respiration Carbon dioxide Oxygen Water Energy for cellular work Energy conversion in a cell 3
Biological Order and Disorder Cells Create ordered structures from less ordered materials Anabolism Equals less entropy Requires the input of energy Cells Also replace ordered forms of majer and energy with less ordered forms Catabolism Energy flows into an ecosystem in the form of light and exits in the form of heat Order and Disorder Evolu9on yields more complex organisms Does not violate the second law of thermodynamics Entropy (disorder) may decrease in an organism but the universe s total entropy increases cells dismantle (catabolic) macromolecules to make their own (anabolic) Free Energy, Stability, and Equilibrium Free energy measure of a system s instability, its tendency to change to a more stable state During spontaneous change free energy decreases and the stability of a system increases Equilibrium state of maximum stability Lowest energy A process is spontaneous and can perform work only when it is moving toward equilibrium 4
Exergonic and Endergonic Reac9ons in Metabolism Free energy changes in reac9ons Exergonic reac4on proceeds with a net release of free energy and is spontaneous Results in lower energy, more stable products Endergonic reac4on absorbs free energy from its surroundings and is nonspontaneous Results in higher energy, less stable products Free energy Reactants Energy Products Progress of the reaction (a) Exergonic reaction: energy released Products Energy Reactants Free energy Progress of the reaction (b) Endergonic reaction: energy required Amount of energy released ( G < 0) Amount of energy required ( G > 0) Equilibrium and Metabolism Closed systems G < 0 G = 0 eventually reach equilibrium and then do no more work Cells are open systems Therefore, not in equilibrium (a) An isolated hydroelectric system (b) An open hydroelectric system G < 0 Experiencing a constant flow of materials Metabolism is never at equilibrium A defining feature of life G < 0 G < 0 G < 0 A catabolic pathway in a cell releases free energy in a series of reac9ons (c) A multistep open hydroelectric system The Structure and Hydrolysis of (adenosine triphosphate) Energy currency of the cell composed of ribose (a sugar) adenine (a nitrogenous base) three phosphate groups Adenine Phosphate groups Ribose 5
The Structure and Hydrolysis of Harves9ng power from Break high energy phosphate bonds By hydrolysis Energy released when terminal phosphate bond is broken Release of energy comes from chemical change to state of lower free energy P P P Adenosine triphosphate () H 2O not from the phosphate bonds themselves P + P P i + Energy Inorganic phosphate Adenosine diphosphate (ADP) Energy Energy arrives as sunlight Photosynthesis Plants capture sunlight Make organic molecules and generates O 2 Carbs used in cellular respira9on Cells use energy stored in organic molecules to regenerate Energy eventually leaves as heat ECOSYSTEM CO 2 + H 2O Light energy Photosynthesis in chloroplasts Cellular respiration in mitochondria powers most cellular work Heat energy Organic molecules + O 2 Catabolic Pathways and Produc9on of The breakdown of organic molecules is exergonic Aerobic respira4on Consumes organic molecules and O 2 and yields Typically glucose Fermenta4on Par9al degrada9on of sugars that occurs without O 2 Anaerobic respira4on similar to aerobic respira9on but uses compounds other than O 2 as the final electron acceptor 6
Cellular Respira9on Cellular respira4on includes both aerobic and anaerobic respira9on but is osen used to refer to aerobic respira9on 3 of 4 macromolecule classes may be used as fuel carbohydrates, fats, and proteins Summary Equa4on for Aerobic Cellular Respira4on C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O + Energy ( + heat) The Principle of Redox oxida9on-reduc9on reac9ons Chemical reac9ons that transfer electrons between reactants are called Oxida4on redox reac4ons a substance loses electrons becomes oxidized (loses electron) Reduc4on a substance gains electron OIL-RIG becomes oxidized becomes reduced (gains electron) becomes reduced NAD + and the Electron Transport Chain and other organic molecules Broken down in a series of steps NAD + (nico9namide adenine dinucleo9de) Electron carrier Transfers electrons from organic compounds Func9ons as an oxidizing agent during cellular respira9on Reduced form of NAD + represents stored energy that is used to synthesize Dehydrogenase NAD + + 2[H] Nicotinamide (oxidized form) 2 e + 2 H + 2 e + H + Dehydrogena se Reduction of NAD + Oxidation of Nicotinamide (reduced form) + H + H + 7
Free energy, G Free energy, G 9/21/16 NAD+ and the Electron Transport Chain Delivers electrons to the electron transport chain (ETC) ETC passes electrons in a series of steps instead of one explosive reac9on Slow, controlled energy release O 2 receives electrons from the ETC ASer an energy-yielding tumble down the chain Known as final electron acceptor The energy yielded is used to regenerate H 2 + 1 / 2 O 2 H 2O Explosive release of heat and light energy (a) Uncontrolled reaction 2 H + 1 / 2 O 2 (from food via ) 2 H + + 2 e Electron transport chain 2 e (b) Cellular respiration Controlled release of energy for synthesis of 2 H + H 2 O 1 / 2 O 2 The Stages of Cellular Respira9on: A Preview Cellular respira9on has three stages: 1. Literally sugar breaking breaks down glucose into two molecules of pyruvate 2. Citric acid cycle completes the breakdown of glucose Also called Krebs cycle 3. Oxida4ve phosphoryla4on accounts for most of the synthesis Includes Electron Transport Chain Fig. 9-6-1 Electrons carried via Pyruvate Cytosol Substrate-level phosphorylation 8
Fig. 9-6-2 Electrons carried via Electrons carried via and FADH 2 Pyruvate Citric acid cycle Cytosol Mitochondrion Substrate-level phosphorylation Substrate-level phosphorylation Fig. 9-6-3 Electrons carried via Electrons carried via and FADH 2 Pyruvate Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis Cytosol Mitochondrion Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation Oxida9ve Phosphoryla9on Oxida4ve phosphoryla4on accounts for almost 90% of the generated by cellular respira9on 32 of 36-38 total Substrate-level phosphoryla4on formed in glycolysis and the citric acid cycle Enzyme Enzyme P Substrate ADP Product + 9
Breaks down glucose into two molecules of pyruvate Energy investment phase 2 ADP + 2 P 2 used Occurs in the cytoplasm Energy payoff phase Two major phases: 4 ADP + 4 P 4 formed Energy investment phase Energy payoff phase 2 NAD + + 4 e + 4 H + 2 + 2 H + 2 Pyruvate + 2 H 2O Net 2 Pyruvate + 2 H 2O 4 formed 2 used 2 2 NAD + + 4 e + 4 H + 2 + 2 H + Intermediate Step If O 2 is present pyruvate enters the mitochondrion Two per original glucose acetyl CoA formed when Pyruvate added to coenzyme A As it crosses the mitochondrial membranes Yields first CO 2 wastes Reduces a NAD+ to Enters the citric acid cycle CYTOSOL Pyruvate Transport protein MITOCHONDRION NAD + + H + 2 1 3 Acetyl CoA CO Coenzyme A 2 Citric Acid Cycle Citric acid cycle Also called the Krebs cycle Occurs in the mitochondrial matrix oxidizes organic fuel derived from pyruvate generates 1, 3, and 1 FADH 2 per cycle and 2 CO 2 Two cycles per original glucose! Pyruvate FADH 2 FAD NAD + + H + Acetyl CoA CoA Citric acid cycle CO 2 CoA CoA ADP + P i 2 CO 2 3 NAD + 3 + 3 H + 10
Citric Acid Cycle Citric acid cycle Eight steps Each catalyzed by a specific enzyme Acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate Forming citrate Coenzyme A returns to intermediate step The next seven steps decompose the citrate back to oxaloacetate Makes the process a cycle Pyruvate FADH 2 FAD NAD + + H + Acetyl CoA CoA Citric acid cycle CO 2 CoA CoA ADP + P i 2 CO 2 3 NAD + 3 + 3 H + The and FADH 2 Deliver electrons to the electron transport chain Fig. 9-12-8 Acetyl CoA CoA SH +H + 1 H 2O NAD + 8 Oxaloacetate 2 H 2O 7 Malate Citric acid cycle Citrate Isocitrate NAD + 3 CO 2 + H + Fumarate 6 CoA SH CoA SH 4 α-ketoglutarate FADH 2 FAD Succinate 5 GTP GDP P i Succinyl CoA CO NAD + 2 + H + ADP Electron Transport Electron transport chain On the inner membrane (cristae) of the mitochondrion Mostly in mul9protein complexes Carriers alternate reduced and oxidized states as they accept and donate electrons Electron bucket brigade Electrons Drop in free energy as they go down the chain Finally passed to O 2, forming H 2 O Free energy (G) relative to O 2 (kcal/ mol) 50 40 30 20 2 e NAD + FADH 2 FM N Fe S Ι Q 2 e FAD FAD Fe S ΙΙ Cyt b Fe S ΙΙΙ Multiprotein complexes Cyt c 1 Cyt c Cyt a IV Cyt a 3 10 2 e (from or FADH 2) 0 2 H + + 1 / 2 O 2 H 2 O 11
Electron Transport Chain Electron transport chain generates no (directly) But creates H + gradient Concentrated in the intermembrane space breaks the large free-energy drop from glucose to O 2 into smaller steps that release energy in manageable amounts Chemiosmosis: The Energy-Coupling Mechanism H + in the intermembrane space INTERMEMBRANE SPACE then move back across the membrane Down concentra9on gradient Rotor H + Stator passing through channels in synthase synthase uses the exergonic flow of H + to drive phosphoryla9on of This is Chemiosmosis The use of energy in a H + gradient to drive cellular work Internal rod Catalytic knob ADP + P i MITOCHONDRIAL MATRIX Fig. 9-16 PLAY H + H + H + Protein complex of electron carriers Cyt c H + Q ΙV Ι ΙΙ FADH 2 FAD ΙΙΙ 2 H + + 1 / 2O 2 H 2 O synthase (carrying electrons from food) NAD + ADP + P i H + 1 Electron transport chain 2 Chemiosmosis Oxidative phosphorylation 12
Energy flows in this sequence: Aerobic Respira9on Summary glucose electron transport chain proton-mo9ve force About 37% of the energy in a glucose molecule is transferred to during cellular respira9on Makes about 38 36 net! (Actually about 30 in Eukaryotes) Roughly 3 per reduced electron carrier (/FADH 2 each with a pair of electrons) In addi9on to those formed by substrate level phosphoryla9on Fig. 9-17 CYTOSOL Electron shuttles span membrane 2 or 2 FADH 2 MITOCHONDRION 2 2 6 2 FADH 2 2 Pyruvate 2 Acetyl CoA Citric acid cycle Oxidative phosphorylation: electron transport and chemiosmosis + 2 + 2 + about 32 or 34 Maximum per glucose: About 36 or 38 Fermenta9on and anaerobic respira9on Aerobic cellular respira9on requires O 2 to produce can produce with or without O 2 in aerobic or anaerobic condi9ons Without O 2 Electron transport chain can t release electrons System backs up and shuts down couples with fermenta4on or anaerobic respira4on to produce Free energy (G) relative to O 2 (kcal/mol) 50 2 e NAD + FADH 2 2 e FAD Multiprotein 40 FMN Ι FAD complexes Fe S Fe S ΙΙ Q ΙΙΙ Cyt b 30 Fe S Cyt c 1 IV Cyt c Cyt a Cyt a 3 20 10 2 e (from or FADH 2 ) 0 2 H + + 1 / 2 O 2 H 2 O 13
Fermenta9on and anaerobic respira9on Anaerobic respira9on uses ETC with an electron acceptor other than O 2 sulfate Fermenta9on uses substrate level phosphoryla9on aser glycolysis instead of an electron transport chain to generate Fermenta9on Consists of glycolysis plus reac9ons that regenerate NAD + To be reused by glycolysis Two common types alcohol fermenta9on Plants, fungi, bacteria lac9c acid fermenta9on Animals and a few bacteria and fungi Types of Fermenta9on Fermenta9on Alcohol fermenta4on Pyruvate is converted to ethanol in two steps 2 ADP + 2 P i 2 2 Pyruvate first releases CO 2 Used by yeast in brewing, winemaking, and baking 2 NAD + 2 2 CO 2 + 2 H + 2 Ethanol 2 Acetaldehyde (a) Alcohol fermentation 14
Fermenta9on and anaerobic respira9on Lac4c acid fermenta4on Pyruvate is reduced by forms lactate as an end product 2 ADP + 2 P i 2 no release of CO 2 Used by some fungi and bacteria to make cheese and yogurt Human muscle cells use to generate when O 2 is scarce or absent Early in strenuous exercise as sugar catabolism outpaces oxygen delivery 2 NAD + 2 + 2 H + 2 Pyruvate 2 Lactate (b) Lactic acid fermentation Fermenta9on and Aerobic Respira9on Compared Both processes Use glycolysis to oxidize glucose and other organic fuels to pyruvate Have different final electron acceptors: Fermenta9on an organic molecule pyruvate or acetaldehyde Aerobic Cellular respira9on O 2 Cellular respira9on Produces 36 (or 30 (38)) per glucose molecule Fermenta9on Produces 2 (net) (4) per glucose molecule Fermenta9on and Anaerobic respira9on Obligate anaerobes carry out fermenta9on or anaerobic respira9on cannot survive in the presence of O 2 Faculta4ve anaerobes Yeast and many bacteria can survive using either fermenta9on or cellular respira9on pyruvate is a fork in the metabolic road that leads to two alterna9ve catabolic routes 15
Fig. 9-19 CYTOSOL No O 2 present: Fermentation Pyruvate O 2 present: Aerobic cellular respiration Ethanol or lactate MITOCHONDRION Acetyl CoA Citric acid cycle The Versa9lity of Catabolism Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respira9on accepts a wide range of carbohydrates Not just glucose or polymers of glucose Proteins must be digested to amino acids amino acids can feed glycolysis or the citric acid cycle Must be deaminated first Creates nitrogenous wastes such as urea, uric acid, and ammonia The Versa9lity of Catabolism Fats digested to glycerol (used in glycolysis) converted to G3P fajy acids (used in genera9ng acetyl CoA) FaJy acids broken down by beta oxida4on yield 2-carbon fragments becomes acetyl CoA Also yields some and FADH 2 An oxidized gram of fat produces more than twice as much as an oxidized gram of carbohydrate NH 3 Proteins Amino acids Carbohydrates Fats Sugars Glycerol Fatty acids Glyceraldehyde-3- P Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation 16
Now You Should Know 1. Explain in general terms how redox reac9ons are involved in energy exchanges 2. Name the three stages of cellular respira9on; for each, state the region of the eukaryo9c cell where it occurs and the products that result 3. In general terms, explain the role of the electron transport chain in cellular respira9on 4. Explain where and how the respiratory electron transport chain creates a proton gradient 5. Dis9nguish between fermenta9on and anaerobic respira9on 17