Lesson 7: Harvesting of Energy “Cellular Respiration” March 2, 2015.
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Transcript of Lesson 7: Harvesting of Energy “Cellular Respiration” March 2, 2015.
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Lesson 7:Harvesting of Energy“Cellular Respiration”
March 2, 2015
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Photosynthesis vs Cellular Respiration
• Organisms can be classified based on how they obtain energy:
• Autotrophs– Able to produce their own organic molecules through
photosynthesis• Heterotrophs– Live on organic compounds produced by other
organisms• ALL organisms use cellular respiration to extract
energy from organic molecules
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Cellular Respiration
• Cellular respiration—the set of metabolic reactions and processes in the cells of organisms that convert biochemical energy from nutrients into ATP
• Cycles between redox reactions– Oxidations – loss of electrons– Reductions—gain of electrons
• Dehydrogenations – lost electrons are accompanied by protons– A hydrogen atom is lost (1 electron, 1 proton)
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Redox
• During redox reactions, electrons carry energy from one molecule to another
• Nicotinamide adenosine dinucleotide (NAD+)– An electron carrier– NAD+ accepts 2 electrons and 1 proton to become
NADH– Reaction is reversible
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Overview of Cellular Respiration
• During the cellular energy harvest– Dozens or redox reactions take place– Number of electron acceptors including NAD+
• In the end, high-energy electrons from initial chemical bonds have lost much of their energy
• Transferred to a final electron acceptor– Final electron acceptor is dependent on the
organism
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Final Electron Acceptors
• Aerobic respiration– Final electron receptor is oxygen (O2)
• Anaerobic respiration– Final electron acceptor is an inorganic molecule
(not O2) • Sulfur• Iron (Fe)
• Fermentation– Final electron acceptor is an organic molecule• Ethanol fermentation
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Aerobic respiration
C6H12O6 + 6O2 6CO2 + 6H2O
DG = -686kcal/mol of glucose • This large amount of energy must be released
in small steps rather than all at once.
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Electron Carriers
• Many types of protein carriers are used– Soluble, membrane-bound, move within
membrane• All carriers can be easily oxidized and reduced• Some carry only electrons; others carry both
electrons and protons• NAD+ acquires 2 electrons and a proton to
become NADH
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ATP
• Cells use ATP to drive endergonic reactions• 2 mechanisms for synthesis of ATP
1. Substrate-level phosphorylation• Transfer phosphate group directly to ADP• During glycolysis stage of cellular respiration
2. Oxidative phosphorylation• ATP synthase uses energy from a proton gradient to
generate ATP
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Oxidation of Glucose
The complete oxidation of glucose proceeds in stages:
1. Glycolysis2. Pyruvate oxidation3. Krebs cycle4. Electron transport chain & chemiosmosis
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Play Animation
• From Beginning through Glycolysis
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Glycolysis
• Converts 1 glucose (6 carbons) to 2 pyruvate (3 carbons)
• 10-step biochemical pathway• OCCURS IN THE CYTOPLASM• Net production of 2 ATP molecules by
substrate-level phosphorylation• 2 NADH produced by the reduction of NAD+
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NADH Must Be Recycled
• For glycolysis to continue, NADH must be recycled to NAD+ by either:
1.Aerobic respiration– Oxygen is available as the final electron acceptor– Produces significant amount of ATP
2.Fermentation– Occurs when oxygen is not available– Organic molecule is the final electron acceptor• EtOh
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Play Animation
• Citric Acid Cycle (Krebs Cycle)
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Fate of Pyruvate
• Depends on oxygen availability.– When oxygen is present, pyruvate is oxidized to
acetyl-CoA which enters the Krebs cycle• Aerobic respiration
– Without oxygen, pyruvate is reduced in order to oxidize NADH back to NAD+ • Fermentation
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Pyruvate Oxidation
• In the presence of oxygen, pyruvate is oxidized– Occurs in the mitochondria in eukaryotes• Multi-enzyme complex called pyruvate dehydrogenase
catalyzes the reaction
– Occurs at the plasma membrane in prokaryotes
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• For each 3 carbon pyruvate molecule:– 1 CO2 • Decarboxylation by pyruvate dehydrogenase
– 1 NADH– 1 acetyl-CoA which consists of 2 carbons from
pyruvate attached to coenzyme A• Acetyl-CoA proceeds to the Krebs cycle
Products of Pyruvate Oxidation
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Krebs Cycle
• Oxidizes the acetyl group from pyruvate• Occurs in the matrix of the mitochondria• Biochemical pathway of 9 steps in three
segments 1. Acetyl-CoA + oxaloacetate → citrate2. Citrate rearrangement and decarboxylation3. Regeneration of oxaloacetate
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Krebs Cycle
• For each Acetyl-CoA entering:– Release 2 molecules of CO2 – Reduce 3 NAD+ to 3 NADH– Reduce 1 FAD (electron carrier) to FADH2 – Produce 1 ATP– Regenerate oxaloacetate
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At This Point…..
• Glucose has been oxidized to:– 6 CO2
– 4 ATP– 10 NADH– 2 FADH2
These electron carriers proceedto the electron transport chain
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Play Animation
• Electron Transport Chain to the end.
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Electron Transport Chain
• ETC is a series of membrane-bound electron carriers
• Embedded in the inner mitochondrial membrane
• Electrons from NADH and FADH2 are transferred to complexes of the ETC
• Each complex– A proton pump creating proton gradient– Transfers electrons to next carrier
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Chemiosmosis
• Accumulation of protons in the intermembrane space drives protons into the matrix via diffusion
• Membrane “relatively” impermeable to ions• Most protons can only re-enter matrix through
ATP synthase– Uses energy of gradient to make ATP from ADP + Pi
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Energy Yield of Respiration
• Theoretical energy yield– 38 ATP per glucose for bacteria– 36 ATP per glucose for eukaryotes
• Actual energy yield– ≈30 ATP per glucose for eukaryotes– Reduced yield is due to • “Leaky” inner membrane• Use of the proton gradient for purposes other than ATP
synthesis
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