Cellular Respiration

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Cellular Respiration

• A.P. Biology

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Energy

• Flows into an ecosystem as sunlight and leaves as
  heat

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Catabolic pathways yield energy by oxidizing
organic fuels

• The breakdown of organic molecules is exergonic
• Fermentation
• Is a partial degradation of sugars that occurs
  without oxygen

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Catabolic pathways yield energy by oxidizing
organic fuels

• Cellular respiration
• Is the most prevalent and efficient catabolic
  pathway
• Consumes oxygen and organic molecules such as
  glucose
• Yields ATP

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Redox Reactions Oxidation and Reduction

• Catabolic pathways yield energy
• Due to the transfer of electrons
• Redox reactions
• Transfer electrons from one reactant to another
  by oxidation and reduction
• Oxidation
• A substance loses electrons, or is oxidized
• Reduction
• A substance gains electrons, or is reduced

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becomes oxidized(loses electron)
becomes reduced(gains electron)
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Some redox reactions

• Do not completely exchange electrons
• Change the degree of electron sharing in covalent
  bonds

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Oxidation of Organic Fuel Molecules During
Cellular Respiration

• During cellular respiration
• Glucose is oxidized and oxygen is reduced

G -686 kcal/mol
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Step by step catabolism of glucose

• If electron transfer is not stepwise
• A large release of energy occurs
• As in the reaction of hydrogen and oxygen to form
  water

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ETC

• The electron transport chain
• Passes electrons in a series of steps
• Uses the energy from the electron transfer to
  form ATP

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NAD Nicotinamide Adenine Dinucleotide(Electron
Acceptor)

• Electrons from organic compounds
• Are usually 1st transferred to NAD, a coenzyme

Dehydragenase removes 2 hydrogen atoms
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NADH

• NADH, the reduced form of NAD
• Passes the electrons to the electron transport
  chain
• Electrons are ultimately passed to a molecule of
  oxygen (Final electron acceptor)
• G -53 kcal/mol

Electron path in respiration
Food ? NADH ? ETC ? Oxygen
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Cellular Respiration

• Respiration is a cumulative function of three
  metabolic stages
• Glycolysis
• The citric acid cycle (TCA or Krebbs)
• Oxidative phosphorylation

C6H12O6 6O2   lt----gt   6 CO2 6 H20    e-
---gt    36-38 ATP                                
  DG    -686 Kc/mole                             
   263Kc  38
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Respiration

• Glycolysis
• Breaks down glucose into two molecules of
  pyruvate
• The citric acid cycle
• Completes the breakdown of glucose
• Oxidative phosphorylation
• Is driven by the electron transport chain
• Generates ATP

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Respiration Overview
2 ATP
2 ATP 34 ATP
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Aerobic Respiration Substrates
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Substrate Level Phosphorylation

• Both glycolysis and the citric acid cycle
• Can generate ATP by substrate-level
  phosphorylation

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Glycolysis

• Harvests energy by oxidizing glucose to pyruvate
• Glycolysis
• Means splitting of sugar
• Breaks down glucose into pyruvate
• Occurs in the cytoplasm of the cell
• Two major phases
• Energy investment phase
• Energy payoff phase

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Energy Investment Phase
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Energy Payoff Phase
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Glycolysis Summary
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• The First Stage of Glycolysis
• Glucose (6C) is broken down into 2 PGAL's (3C)
• This requires two ATP's

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• The Second Stage of Glycolysis
• 2 PGAL's (3C) are converted to 2 pyruvates
• This creates 4 ATP's and 2 NADH's
• The net ATP production of Glycolysis is 2 ATP's

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Citric Acid Cyclea.k.a. Krebs Cycle

• Completes the energy-yielding oxidation of
  organic molecules
• The citric acid cycle
• Takes place in the matrix of the mitochondrion

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• Krebs's Cycle (citric acid cycle, TCA cycle)
• Goal take pyruvate and put it into the Krebs's
  cycle, producing NADH and FADH2
• Where the mitochondria
• There are two steps
• The Conversion of Pyruvate to Acetyl CoA
• The Kreb's Cycle proper
• In the Krebs's cycle, all of Carbons, Hydrogens,
  and Oxygeng in pyruvate end up as CO2 and H2O
• The Krebs's cycle produces 2 ATP's, 8 NADH's, and
  2FADH2's per glucose molecule

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Fate of Pyruvate
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Carboxyl (coo-) is cleaved CO2 is released
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Before the citric acid cycle can begin

• Pyruvate must first be converted to acetyl CoA,
  which links the cycle to glycolysis

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An overview of the citric acid cycle
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• The Kreb's Cycle
• 6 NADH's are generated
• 2 FADH2 is generated
• 2 ATP are generated
• 4 CO2's are released

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• Net Engergy Production from Aerobic Respiration
• Glycolysis 2 ATP
• Kreb's Cycle 2 ATP
• Electron Transport Phosphorylation 32 ATP
• Each NADH produced in Glycolysis is worth 2 ATP
  (2 x 2 4) - the NADH is worth 3 ATP, but it
  costs an ATP to transport the NADH into the
  mitochondria, so there is a net gain of 2 ATP for
  each NADH produced in gylcolysis
• Each NADH produced in the conversion of pyruvate
  to acetyl COA and Kreb's Cycle is worth 3 ATP (8
  x 3 24)
• Each FADH2 is worth 2 ATP (2 x 2 4)
• 4 24 4 32
• Net Energy Production 36 ATP

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• Energy Yields
• Glucose 686 kcal/mol
• ATP 7.5 kcal/mol
• 7.5 x 36 270 kcal/mol for all ATP's produced
• 270 / 686 39 energy recovered from aerobic
  respiration

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After the Krebs Cycle

• During oxidative phosphorylation, chemiosmosis
  couples electron transport to ATP synthesis
• NADH and FADH2
• Donate electrons to the electron transport chain,
  which powers ATP synthesis via oxidative
  phosphorylation

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The Pathway of Electron Transport

• In the electron transport chain
• Electrons from NADH and FADH2 lose energy in
  several steps
• At the end of the chain
• Electrons are passed to oxygen, forming water

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Chemiosmosis The Energy-Coupling Mechanism

• ATP synthase
• Is the enzyme that actually makes ATP

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ETC

• Electron transfer causes protein complexes to
  pump H from the mitochondrial matrix to the
  intermembrane space
• The resulting H gradient
• Stores energy
• Drives chemiosmosis in ATP synthase
• Is referred to as a proton-motive force

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Chemiosmosis

• Is an energy-coupling mechanism that uses energy
  in the form of a H gradient across a membrane to
  drive cellular work
• H gradient Proton motive force

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An Accounting of ATP Production by Cellular
Respiration

• During respiration, most energy flows in this
  sequence
• Glucose to NADH to electron transport chain to
  proton-motive force to ATP

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Anaerobic Respiration

• Fermentation enables some cells to produce ATP
  without the use of oxygen
• Glycolysis
• Can produce ATP with or without oxygen, in
  aerobic or anaerobic conditions
• Couples with fermentation to produce ATP

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Anaerobic Respiration

• Fermentation consists of
• Glycolysis plus reactions that regenerate NAD,
  which can be reused by glyocolysis
• Alcohol fermentation
• Pyruvate is converted to ethanol in two steps,
  one of which releases CO2
• Lactic acid fermentation
• Pyruvate is reduced directly to NADH to form
  lactate as a waste product

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Pyruvate is a key juncture in catabolism
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• Glycolysis
• Occurs in nearly all organisms
• Probably evolved in ancient prokaryotes before
  there was oxygen in the atmosphere

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• Cellular respiration
• Is controlled by allosteric enzymes at key points
  in glycolysis and the citric acid cycle