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Chapter 9~ Cellular Respiration: Harvesting Chemical Energy
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Harvesting stored energy
Glucose is the model catabolism of glucose to produce ATP glucose + oxygen energy + water + carbon dioxide respiration + heat C6H12O6 6O2 ATP 6H2O 6CO2 + Movement of hydrogen atoms from glucose to water fuel (carbohydrates) COMBUSTION = making a lot of heat energy by burning fuels in one step RESPIRATION = making ATP (& some heat) by burning fuels in many small steps ATP ATP glucose enzymes O2 O2 CO2 + H2O + heat CO2 + H2O + ATP (+ heat)
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How do we harvest energy from fuels?
Digest large molecules into smaller ones break bonds & move electrons from one molecule to another as electrons move they “carry energy” with them that energy is stored in another bond, released as heat or harvested to make ATP • They are called oxidation reactions because it reflects the fact that in biological systems oxygen, which attracts electrons strongly, is the most common electron acceptor. • Oxidation & reduction reactions always occur together therefore they are referred to as “redox reactions”. • As electrons move from one atom to another they move farther away from the nucleus of the atom and therefore are at a higher potential energy state. The reduced form of a molecule has a higher level of energy than the oxidized form of a molecule. • The ability to store energy in molecules by transferring electrons to them is called reducing power, and is a basic property of living systems. loses e- gains e- oxidized reduced + – + e- e- e- oxidation reduction redox
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Coupling oxidation & reduction
REDOX reactions in respiration release energy as breakdown organic molecules strip off electrons from C-H bonds by removing H atoms C6H12O6 CO2 = the fuel has been oxidized electrons attracted to more electronegative atoms in biology, the most electronegative atom? O2 H2O = oxygen has been reduced couple REDOX reactions & use the released energy to synthesize ATP O2 O2 is 2 oxygen atoms both looking for electrons LIGHT FIRE ==> oxidation RELEASING ENERGY But too fast for a biological system C6H12O6 6O2 6CO2 6H2O ATP + oxidation reduction
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Oxidation & reduction Oxidation Reduction adding O removing H
loss of electrons releases energy exergonic Reduction removing O adding H gain of electrons stores energy endergonic C6H12O6 6O2 6CO2 6H2O ATP + oxidation reduction
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Oxidizing agent in respiration
NAD+ (nicotinamide adenine dinucleotide) Removes electrons from food (series of reactions) NAD + is reduced to NADH Oxygen is the eventual e- acceptor
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Cellular respiration: overview
(anaerobic) 1.Glycolysis: cytosol; degrades glucose into pyruvate (aerobic) 2.Kreb’s Cycle: mitochondrial matrix; pyruvate into carbon dioxide 3.Electron Transport Chain: inner membrane of mitochondrion; electrons passed to oxygen
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Glycolysis 6C 3C Breaking down glucose glucose pyruvate 2x
“glyco – lysis” (splitting sugar) ancient pathway which harvests energy where energy transfer first evolved transfer energy from organic molecules to ATP still is starting point for ALL cellular respiration but it’s inefficient generate only 2 ATP for every 1 glucose occurs in cytoplasm In the cytosol? Why does that make evolutionary sense? glucose pyruvate 2x 6C 3C Why does it make sense that this happens in the cytosol? Who evolved first? That’s not enough ATP for me!
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But can’t stop there! Glycolysis Going to run out of NAD+
Pi NAD+ G3P 1,3-BPG NADH DHAP 7 8 H2O 9 10 ADP ATP 3-Phosphoglycerate (3PG) 2-Phosphoglycerate (2PG) Phosphoenolpyruvate (PEP) Pyruvate NAD+ NADH Pi 6 raw materials products Glycolysis glucose + 2ADP + 2Pi + 2 NAD+ 2 pyruvate + 2ATP + 2NADH Going to run out of NAD+ without regenerating NAD+, energy production would stop! another molecule must accept H from NADH so NAD+ is freed up for another round
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How is NADH recycled to NAD+?
with oxygen aerobic respiration without oxygen anaerobic respiration “fermentation” Another molecule must accept H from NADH pyruvate H2O NAD+ CO2 recycle NADH NADH O2 NADH acetaldehyde acetyl-CoA NADH NAD+ NAD+ lactate lactic acid fermentation which path you use depends on who you are… Krebs cycle ethanol alcohol fermentation
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Fermentation (anaerobic)
Bacteria, yeast 1C 3C 2C pyruvate ethanol + CO2 NADH NAD+ back to glycolysis beer, wine, bread Animals, some fungi Count the carbons!! Lactic acid is not a dead end like ethanol. Once you have O2 again, lactate is converted back to pyruvate by the liver and fed to the Kreb’s cycle. pyruvate lactic acid 3C NADH NAD+ back to glycolysis cheese, anaerobic exercise (no O2)
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Alcohol Fermentation pyruvate ethanol + CO2 Dead end process
bacteria yeast recycle NADH 1C 3C 2C pyruvate ethanol + CO2 NADH NAD+ back to glycolysis Dead end process at ~12% ethanol, kills yeast can’t reverse the reaction Count the carbons!
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Lactic Acid Fermentation
animals some fungi recycle NADH O2 pyruvate lactic acid 3C NADH NAD+ back to glycolysis Reversible process once O2 is available, lactate is converted back to pyruvate by the liver Count the carbons!
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Pyruvate is a branching point
fermentation anaerobic respiration mitochondria Krebs cycle aerobic respiration
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Krebs cycle 1937 | 1953 aka Citric Acid Cycle in mitochondrial matrix
8 step pathway each catalyzed by specific enzyme step-wise catabolism of 6C citrate molecule Hans Krebs The enzymes of glycolysis are very similar among all organisms. The genes that code for them are highly conserved. They are a good measure for evolutionary studies. Compare eukaryotes, bacteria & archaea using glycolysis enzymes. Bacteria = 3.5 billion years ago glycolysis in cytosol = doesn’t require a membrane-bound organelle O2 = 2.7 billion years ago photosynthetic bacteria / proto-blue-green algae Eukaryotes = 1.5 billion years ago membrane-bound organelles! Processes that all life/organisms share: Protein synthesis Glycolysis DNA replication
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Electron Carriers = Hydrogen Carriers
Krebs cycle produces large quantities of electron carriers NADH FADH2 go to Electron Transport Chain! ADP + Pi ATP What’s so important about electron carriers?
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Energy accounting of Krebs cycle
2x 4 NAD + 1 FAD 4 NADH + 1 FADH2 pyruvate CO2 1 ADP 1 ATP 3C 3x 1C ATP Net gain = 2 ATP = 8 NADH + 2 FADH2
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Electron Transport Chain
series of proteins built into inner mitochondrial membrane along cristae transport proteins & enzymes transport of electrons down ETC linked to pumping of H+ to create H+ gradient yields ~36 ATP from 1 glucose! only in presence of O2 (aerobic respiration) That sounds more like it! O2
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Electron Transport Chain
Building proton gradient! NADH NAD+ + H p e intermembrane space H+ H+ H+ inner mitochondrial membrane H e- + H+ C Q e– e– e– H FADH2 FAD H 1 2 NADH 2H+ + O2 H2O NAD+ NADH dehydrogenase cytochrome bc complex cytochrome c oxidase complex mitochondrial matrix What powers the proton (H+) pumps?…
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O2 is 2 oxygen atoms both looking for electrons
But what “pulls” the electrons down the ETC? H2O Pumping H+ across membrane … what is energy to fuel that? Can’t be ATP! that would cost you what you want to make! Its like cutting off your leg to buy a new pair of shoes. :-( Flow of electrons powers pumping of H+ O2 is 2 oxygen atoms both looking for electrons O2 electrons flow downhill to O2 oxidative phosphorylation
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Electrons flow downhill
Electrons move in steps from carrier to carrier downhill to oxygen each carrier more electronegative controlled oxidation controlled release of energy make ATP instead of fire! Electrons move from molecule to molecule until they combine with O & H ions to form H2O It’s like pumping water behind a dam -- if released, it can do work
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We did it! Set up a H+ gradient
“proton-motive” force We did it! H+ ADP + Pi Set up a H+ gradient Allow the protons to flow through ATP synthase Synthesizes ATP ADP + Pi ATP ATP Are we there yet?
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ATP Pyruvate from cytoplasm Intermembrane space Inner mitochondrial
Electron transport system C Q NADH e- 2. Electrons provide energy to pump protons across the membrane. H+ 1. Electrons are harvested and carried to the transport system. e- Acetyl-CoA NADH e- H2O Krebs cycle e- 3. Oxygen joins with protons to form water. 1 FADH2 O2 2 O2 + 2H+ CO2 H+ ATP ATP H+ ATP 4. Protons diffuse back in down their concentration gradient, driving the synthesis of ATP. ATP synthase Mitochondrial matrix
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Taking it beyond… NAD+ Q C NADH H2O H+ e– 2H+ + O2 FADH2 1 2 NADH dehydrogenase cytochrome bc complex cytochrome c oxidase complex FAD What is the final electron acceptor in Electron Transport Chain? O2 So what happens if O2 unavailable? ETC backs up nothing to pull electrons down chain NADH & FADH2 can’t unload H ATP production ceases cells run out of energy and you die! What if you have a chemical that punches holes in the inner mitochondrial membrane?
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