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Transcript
BIO 2, Lecture 13 FIGHTING ENTROPY II: RESPIRATION • Respiration is the process whereby cells break down complex organic molecules (like starch) and convert them into ATP + heat (not 100% efficient) • The cell then uses the ATP to do work $10,000 bill (Starch; Unusable) $1 bills (ATP; Usable) • There are two types of respiration: • Anaerobic respiration, also called fermentation, occurs without O2 • Example: Ethanol fermentation of glucose C6H12O6 + ADP + Pi 2 C2H5OH + 2 CO2 + ATP + heat • Aerobic respiration relies on O2 • Is more efficient than anaerobic respiration; generates more ATP per organic molecule, loses less energy as heat C6H12O6 + 6O2 + ADP + Pi 6CO2 + 6H2O + ATP + heat • Cells do three types of work: mechanical, transport, and chemical • All must be coupled to the hydrolysis of ATP • Overall, the coupled reactions are catabolic • An example is the creation of the amino acid glutamine from ammonia and glutamic acid NH2 Glu Glutamic acid NH3 + Glu ∆G = +3.4 kcal/mol Glutamine Ammonia (a) Anabolic (endergonic) reaction 1 ATP phosphorylates glutamic acid, making the amino acid less stable. P + Glu ATP Glu + ADP NH2 2 Ammonia displaces the phosphate group, forming glutamine. P Glu + NH3 Glu + Pi (b) Coupled with ATP hydrolysis, a catabolic (exergonic) reaction (c) Overall free-energy change Adenine Phosphate groups Ribose • The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis • Energy is released from ATP when the terminal phosphate bond is broken P P P Adenosine triphosphate (ATP) H20 Pi + P P + Inorganic phosphate Adenosine diphosphate (ADP) Energy • ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP) • The energy to re-phosphorylate ADP comes from catabolic reactions in the cell H2O ATP ATP + H2O Energy from catabolism (exergonic, energy-releasing processes) ADP+ P i Energy for cellular work (endergonic, energyconsuming processes) • Although carbohydrates, fats, and proteins can all be broken down during respiration to produce ATP, it is helpful to trace cellular respiration with the sugar glucose • The step-wise transfer of electrons (from high energy states in complex organic molecules to lower energy states in simple organic molecules) gently releases the energy stored in glucose to regenerate ATP from ADP + P • 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) becomes oxidized (loses electron) becomes reduced (gains electron) • Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds, thereby changing the potential chemical energy stored in the molecules • Reduced organic molecules carry more potential chemical energy than oxidized forms of the same molecules • Starch is a highly reduced form of water and carbon dioxide • Therefore, breaking starch down into water and carbon dioxide releases energy Reactants Products becomes oxidized becomes reduced Methane Oxygen Carbon dioxide Water • During cellular respiration, the fuel (such as glucose) is oxidized, and another molecule (such as O2) is reduced • The chemical potential energy in the reactants is greater than the chemical potential energy in the products; thus energy is released becomes oxidized becomes reduced • Both anaerobic and aerobic respiration begin with glycolysis – Breaks down glucose into two molecules of pyruvate) to produce 2 ATP per glucose – Takes place in the cytoplasm – In anaerobic respiration, the process stops here and only 2 ATP are generated per glucose • Aerobic respiration has two additional steps that break down the pyruvate to carbon dioxide and water to produce an additional 36 ATP – The citric acid cycle (completes the breakdown of glucose) – Oxidative phosphorylation (accounts for most of the ATP synthesis) • Both steps take place in the mitochondria • Glycolysis (“splitting of sugar”) has two major phases: – Energy investment phase – Energy payoff phase • In the energy investment phase, 2 ATP are consumed to “kick start” the process • In the energy payoff phase, four ATP are produced, yielding a net gain of 2 ATP Energy investment phase Glucose 2 ADP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 2 NAD+ + 4 e– + 4 H+ 4 ATP formed 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Glucose 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 H+ 2 Pyruvate + 2 H2O 2 ATP 2 NADH + 2 H+ • In anaerobic respiration, the process stops with the production of 2 ATP • Pyruvate is converted to waste products (like ethanol) but no more ATP is gained • The energy stored in NADH is likewise wasted • It is important to note, however, that NADH carries high energy electrons and can be harvested to produce ATP if a cell has the machinery to do so ... Anaerobic Respiration Electrons carried via NADH Net = 2 NADH Glycolysis Pyruvate Glucose Cytosol Net = 2 ATP Substrate-level phosphorylation Potential source of additional ATP; WASTED in aerobic respiration Potential source of additional ATP; WASTED in aerobic respiration • In anaerobic respiration, the process stops with the production of 2 ATP • Pyruvate is converted to waste products (like ethanol) but no more ATP is gained • The energy stored in NADH is likewise wasted • It is important to note, however, that NADH carries high energy electrons and can be harvested to produce ATP if a cell has the machinery to do so ... • Aerobic respiration continues the process of glycolysis to breakdown pyruvate and utilize the high energy electrons stored in NADH • Takes place in cells that have mitochondria – The citric acid cycle (breaks down pyruvate to CO2 and H2O in the mitochondrial matrix to produce additional ATP, NADH, and FADH2) – Oxidative phosphorylation (harvests electrons from NADH and FADH2 in the inner mitochondrial membrane and accounts for most of the ATP synthesis) Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Mitochondrion Cytosol ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Electrons carried via NADH and FADH2 Electrons carried via NADH Citric acid cycle Glycolysis Pyruvate Glucose Oxidative phosphorylation: electron transport and chemiosmosis Mitochondrion Cytosol ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation • The Citric Acid Cycle: • In the presence of O2, pyruvate (generated by glycolysis) enters the mitochondrion from the cytoplasm • Prior to the start of the cycle, pyruvate is converted to acetyl CoA, generating one molecule of NADH • The cycle then oxidizes acetyl CoA, generating 1 ATP, 3 NADH, and 1 FADH2 per turn Pyruvate CO2 NAD+ NADH + H+ CoA Acetyl CoA CoA CoA Citric acid cycle 2 CO2 3 NAD+ FADH2 3 FAD NADH + 3 H+ ADP + ATP P i • The cycle is “fed” by acetyl-coA • In the first step, the acetyl-coA is combined with oxaloacetate to form citrate • The citrate is then broken down in a series of steps to produce energy (in the form of ATP, NADH, and FADH2) + CO2 (gas) • The end product of the cycle is oxaloacetate, which can then combine with another molecule of acetyl-coA to run the cycle again ... Acetyl CoA CoA—SH NADH +H NAD+ H2O 1 + 8 Oxaloacetate 2 Malate H2O Citrate Isocitrate NAD + Citric acid cycle 7 Fumarate 3 NADH + H+ CO2 CoA—SH 6 4 CoA—SH 5 FADH2 NAD FAD + Succinate GTP GDP ADP ATP Pi Succinyl CoA NADH + H+ -Ketoglutarate CO2 • Following glycolysis and the citric acid cycle, NADH and FADH2 carry most of the energy extracted from food • These two molecules transport the high energy electrons generated by the breakdown of glucose to pyruvate (during glycolysis) and pyruvate to oxaloacetate and CO2 (during the citric acid cycle) and donate them to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation • The electron transport chain is located in the inner membrane of the mitochondrion • Most of the chain’s components are proteins, which exist in multi-protein complexes • The carriers alternate reduced and oxidized states as they accept (become reduced) and donate (become oxidized) electrons down the chain • Electrons drop in free energy as they go down the chain and are finally passed to O2 (gas), forming H2O NADH 5 0 2 e– NAD+ FADH2 2 e– 4 0 3 0 FM N Fe• S Q FAD FAD Fe• S Cyt b Multiprotein complexes Fe• S Cyt c1 I V Cyt c Cyt a 2 0 1 0 0 Cyt a3 2 e– (from NADH or FADH2) 2 H+ + 1/2 O2 H2O • As electrons are transferred down the electron transport chain, the energy released at each step is used by the protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space • H+ then moves back across the membrane, with its diffusion gradient, passing through channels in a protein complex called ATP synthase • ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP from ADP and Pi • This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work • The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work H+ H+ H+ Protein complex of electron carriers Cyt c V Q FADH2 NADH (carrying electrons from food) H+ ATP synthase 2 H+ + 1/2O2 FAD NAD+ H 2O Why we breathe O2!! 1 Electron transport chain Oxidative phosphorylation ADP + P i ATP H+ 2 Chemiosmosis • During aerobic respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP • About 40% of the energy in a glucose molecule is transferred to ATP during aerobic respiration, generating about 38 ATP • The rest is lost as heat Anaerobic phase Electron shuttles span membrane CYTOSOL Aerobic phase MITOCHONDRION 2 NADH or 2 FADH2 2 NADH Glycolysis Glucose 2 Pyruvate 2 NADH 2 Acetyl CoA + 2 ATP 6 NADH Citric acid cycle + 2 ATP Maximum per glucose: About 38 ATP 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis + about 34 ATP • Comparing aerobic and anaerobic respiration: • Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate • However, 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 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule Proteins Amino acids Respiration can use many different fuels (not just glucose!) Carbohydrates Sugars Glycolysis Glucose Glyceraldehyde-3-P NH3 Pyruvate Acetyl CoA Citric acid cycle Oxidative phosphorylation Fats Glycerol Fatty acids H2 + 1/2 O2 1/ 2 H (from food) 2 H+ + 2 e– 2 O2 Controlled release of energy for synthesis of ATP Explosive release of heat and light energy 1/ 2 O2 (a) Uncontrolled reaction (b) Cellular respiration Light energy ECOSYSTEM CO2 + H2O Photosynthesis in chloroplasts Cellular respiration in mitochondria ATP ATP powers most cellular work Heat energy Organic molecules + O2