Respiration as an energy Respiration as an energy

transfer processtransfer process

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RespirationRespiration• A process in which organic molecules act as A process in which organic molecules act as

a fuela fuel• Molecules broken down to release chemical Molecules broken down to release chemical

potential energy which is used to synthesise potential energy which is used to synthesise ATPATP

• Many cells can only use glucose as their Many cells can only use glucose as their respiratory substrate but others break down respiratory substrate but others break down fatty acid, glycerol and amino acids in fatty acid, glycerol and amino acids in respirationrespiration

• Glucose breakdown occur in 4 stages:Glucose breakdown occur in 4 stages:– GlycolysisGlycolysis– Link reactionLink reaction– Krebs cycleKrebs cycle– Oxidative phosphorylationOxidative phosphorylation

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Glycolysis Glycolysis

• Splitting/lysis of glucoseSplitting/lysis of glucose• 6C glucose molecule to 2 molecules of 6C glucose molecule to 2 molecules of

3C pyruvate3C pyruvate• ATP needed at first but energy ATP needed at first but energy

released in later steps can be used to released in later steps can be used to make ATPmake ATP

• Net gain of 2 ATPNet gain of 2 ATP molecules per molecules per glucose molecule broken downglucose molecule broken down

• Takes place in cytoplasmTakes place in cytoplasm• Pyruvate enters link reaction in Pyruvate enters link reaction in

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Link reactionLink reaction

• Pyruvate passed from cytoplasm into the Pyruvate passed from cytoplasm into the mitochondrial matrix (active transport)mitochondrial matrix (active transport)

• Decarboxylated (CODecarboxylated (CO22 removed), removed), dehydrogenated and combined with dehydrogenated and combined with coenzyme A (CoA) to give acetyl coenzyme Acoenzyme A (CoA) to give acetyl coenzyme A

• Coenzyme A:Coenzyme A:– adenine + ribose + pantothenic acidadenine + ribose + pantothenic acid– acts as a carrier of acetyl groups to Krebs cycle acts as a carrier of acetyl groups to Krebs cycle

• Pyruvate + CoA + NAD Pyruvate + CoA + NAD ↔ acetyl CoA + CO↔ acetyl CoA + CO22 + reduced + reduced NADNAD

• Fatty acids from fat metabolism also used to Fatty acids from fat metabolism also used to produce acetyl coenzyme Aproduce acetyl coenzyme A

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Krebs cycleKrebs cycle• aka citric acid cycle/tricarboxylic acid cycleaka citric acid cycle/tricarboxylic acid cycle• Closed pathway of enzyme-controlled Closed pathway of enzyme-controlled

reactions:reactions:– Acetyl CoA + oxaloacetate (4C) Acetyl CoA + oxaloacetate (4C) → citrate (6C)→ citrate (6C)– Citrate decarboxylated and dehydrogenated to Citrate decarboxylated and dehydrogenated to

yield COyield CO22 (waste) and hydrogens (accepted by (waste) and hydrogens (accepted by NAD and FAD)NAD and FAD)

– Oxaloacetate is regenerated to combine with Oxaloacetate is regenerated to combine with another acetyl CoAanother acetyl CoA

• For each turn of the cycle, 2 COFor each turn of the cycle, 2 CO22 produced, 1 produced, 1 FAD and 3 NAD reduced and 1 ATP generatedFAD and 3 NAD reduced and 1 ATP generated

• OO22 not used not used• Most important contribution: release of Most important contribution: release of

hydrogen (used in oxidative phosphorylation hydrogen (used in oxidative phosphorylation to provide energy to make ATP) to provide energy to make ATP)

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Oxidative phosphorylationOxidative phosphorylation• Energy for the phosphorylation of ADP to Energy for the phosphorylation of ADP to

ATP comes from the activity of the electron ATP comes from the activity of the electron transport chain (mitochondrial membranes)transport chain (mitochondrial membranes)

• NADH and FADHNADH and FADH22 are passed to the electron are passed to the electron transport chain (ETC)transport chain (ETC)

• Hydrogen HHydrogen H++ and e and e--

• ee-- transferred to the first of a series of transferred to the first of a series of electron carriers; Helectron carriers; H++ remains in solution in remains in solution in mitochondrial matrixmitochondrial matrix

• ee-- transferred to O transferred to O22 (in solution), H (in solution), H++ drawn drawn from solution to reduce Ofrom solution to reduce O22 to H to H22OO

• Transfer of eTransfer of e-- along series of electron along series of electron carriers makes energy available which is carriers makes energy available which is used to convert ADP + Pused to convert ADP + Pii to ATP to ATP

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• PotentiallyPotentially, 3 ATP from each NADH , 3 ATP from each NADH molecule/2 ATP from each FADHmolecule/2 ATP from each FADH22 moleculemolecule

• This yield cannot be realised unless This yield cannot be realised unless ADP and Pi are available inside the ADP and Pi are available inside the mitochondrionmitochondrion

• 25% of total energy yield is used to 25% of total energy yield is used to transport ADP into the mitochondrion transport ADP into the mitochondrion and ATP into cytoplasmand ATP into cytoplasm

• Each NADH molecule entering the Each NADH molecule entering the chain produces on average 2½ chain produces on average 2½ molecules of ATP and each FADHmolecules of ATP and each FADH22 produces 1½ molecules of ATPproduces 1½ molecules of ATP

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Sites of events of respiration in a Sites of events of respiration in a cellcell

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Anaerobic respirationAnaerobic respiration

• When free oxygen is not present, hydrogen When free oxygen is not present, hydrogen cannot be disposed of by combination with cannot be disposed of by combination with oxygenoxygen

• ‘‘dumping’ hydrogen stops ETC and affects dumping’ hydrogen stops ETC and affects glycolysisglycolysis

• 2 anaerobic pathways in cytoplasm which 2 anaerobic pathways in cytoplasm which solve the problem:solve the problem:– Ethanol pathwayEthanol pathway (yeast and some plant (yeast and some plant

tissues)tissues)– Lactate pathway Lactate pathway (microorganisms and (microorganisms and

mammalian muscles)mammalian muscles)

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• Ethanol pathwayEthanol pathway– Hydrogen from NADH is passed to ethanal Hydrogen from NADH is passed to ethanal

(CH(CH33CHO), releasing NAD and allows glycolysis to CHO), releasing NAD and allows glycolysis to continuecontinue

– Alcoholic fermentationAlcoholic fermentation: pyruvate : pyruvate decarboxylated to ethanal; ethanal reduced to decarboxylated to ethanal; ethanal reduced to ethanol (Cethanol (C22HH55OH) by alcohol dehydrogenaseOH) by alcohol dehydrogenase

• Lactate pathwayLactate pathway– Pyruvate acts as the hydrogen acceptor and is Pyruvate acts as the hydrogen acceptor and is

converted to lactate by lactate dehydrogenaseconverted to lactate by lactate dehydrogenase– NAD is released and allows glycolysis to continueNAD is released and allows glycolysis to continue

• These reactions allow continued production of These reactions allow continued production of ATP even though oxygen is not available as ATP even though oxygen is not available as the hydrogen acceptorthe hydrogen acceptor

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• However, ethanol and lactate are toxic, and However, ethanol and lactate are toxic, and so reactions cannot continue indefinitelyso reactions cannot continue indefinitely

• Pathway leading to ethanol cannot be Pathway leading to ethanol cannot be reversed and the remaining chemical reversed and the remaining chemical potential energy of ethanol is wastedpotential energy of ethanol is wasted

• Lactate pathway can be reversed in Lactate pathway can be reversed in mammals (carried by blood plasma to the mammals (carried by blood plasma to the liver and converted back into pyruvate; liver and converted back into pyruvate; 20% oxidised to CO20% oxidised to CO22 and H and H22O via aerobic O via aerobic respiration when Orespiration when O22 is available again; is available again; remainder converted to glycogen)remainder converted to glycogen)

• OO22 needed to allow this removal of lactate is needed to allow this removal of lactate is called called oxygen debtoxygen debt

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