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					   Cellular Respiration:
Harvesting Chemical Energy




                             1
                 Cellular Respiration
•   Energy flows into an ecosystem                  Light
    as sunlight and leaves as heat                  energy


•   Photosynthesis generates            ECOSYSTEM


    oxygen and organic molecules,
    which are used in cellular                           Photosynthesis
                                                         in chloroplasts
    respiration                            CO2 + H2O                            Organic + O
                                                                               molecules 2
                                                        Cellular respiration
•   All cells can harvest energy from                    in mitochondria

    organic molecules to power
    work
•   To do this, they break down the                             ATP
    organic molecules and use the
                                                       powers most cellular work
    energy that is released to make
    ATP from ADP and phosphate                         Heat
                                                       energy




                                                                                              2
    Catabolic Pathways and Production of ATP
•   Heterotrophs live off the energy produced by
    autotrophs - extracting energy from food via digestion
    and catabolism
•   There are different catabolic pathways used in ATP
    production:
     • Fermentation - the partial degradation of sugars in
        the absence of oxygen.
     • Cellular respiration - A more efficient and
        widespread catabolic process that consumes
        oxygen as a reactant to complete the breakdown
        of a variety of organic molecules.


                                                             3
    Catabolic Pathways and Production of ATP
•   Although carbohydrates, fats, and proteins are all
    consumed as fuel, it is helpful to trace cellular
    respiration with the sugar glucose:
     • The catabolism of glucose is exergonic with a
       G of −686 kcal per mole of glucose.
     • Some of this energy is used to produce ATP,
       which can perform cellular work

    C6H12O6 + 6O2    6CO2 + 6H2O + Energy (ATP + heat)



                                                         4
                   Redox Reactions
• Catabolic pathways yield energy through the transfer
  electrons from one reactant to another by oxidation and
  reduction
• Redox reactions
   • In oxidation - A substance loses electrons, or is oxidized
   • In reduction - A substance gains electrons, or is
     reduced
                       becomes oxidized
                        (loses electron)

              Na   +   Cl                  Na+   +    Cl–
                                  becomes reduced
                                   (gains electron)




                                                                  5
    Oxidation of Organic Fuel Molecules During
                Cellular Respiration
•   Cellular respiration provides the energy for the cell
    using the exergonic reaction:

                  becomes oxidized

          C6H12O6 + 6O2              6CO2 + 6H2O + Energy ~686kcal/mole

                          becomes reduced




• During cellular respiration glucose is oxidized and
  oxygen is reduced
• Glucose oxidation is accomplished in a series of
  steps
                                                                          6
                  Glucose Oxidation
•   If electron transfer is not stepwise
     • A large release of energy occurs
     • As in the reaction of hydrogen and oxygen to form water


                                               H2 + 1/2 O2



                              Free energy, G




                                                          Explosive
                                                         release of      (a) Uncontrolled reaction
                                                        heat and light
                                                           energy




               Figure 9.5 A                     H2O

                                                                                                     7
                              Glucose Catabolism
•   Glucose catabolism is a series of redox reactions that release energy
    by repositioning electrons closer to oxygen atoms.
•   The high energy electrons are stripped from glucose and picked up by
    NAD+ and FAD.


                                                                   2 e– + 2 H+
                                                                                   2 e– + H+
                    NAD+                                                                       NADH
                          H
                               O                                Dehydrogenase
                                                                                      H        H O
                                                 + 2[H]        Reduction of NAD+                                 + H
                                C       NH2                                                      C    NH2
                                                 (from food)   Oxidation of NADH
                          N+                                                              N     Nicotinamide
                                Nicotinamide
       O   CH2                 (oxidized form)                                                  (reduced form)
                    O
     O P O–
       O    H             H
     O P O– HO          OH     NH2
                 HO
       O   CH2
                          N         N
                    H
                          N    N        H
                    O

           H              H
               HO       OH                       Figure 9.4
                                                                                                                       8
                          The Electron Transport Chain
•   Passes electrons in a series of steps instead of in one explosive reaction
•   Uses the energy from the electron transfer to form ATP
•   Eventually, the electrons, along with H+, are passed to a final acceptor.

                              2H                    +               1/
                                                                         2   O2
                                                                                                                                    NADH
                      (from food via NADH)                                                                                    50




                                                                                  Free energy (G) relative to O2 (kcal/mol)
                                                                                                                                                     FADH2
                                              Controlled
                                              release of                                                                                                                 Multiprotein
                         2 H+ + 2 e–                                                                                          40            I      FAD                   complexes
                                              energy for                                                                           FMN
                                             synthesis of                                                                                Fe•S   Fe•S II
                                                 ATP                                                                                            Q
                                                                                                                                                                   III
                                                              ATP                                                                                   Cyt b
                                                                                                                                                            Fe•S
     Free energy, G




                                                                                                                              30
                                                                                                                                                                   Cyt c1                      IV
                                                            ATP
                                                                                                                                                                            Cyt c
                                                                                                                                                                                    Cyt a
                                                             ATP                                                                                                                            Cyt a3
                                                                                                                              20




                                                                                                                              10
                                             2 e–
                                                                    1/       O2
                                                                         2
                                             2   H+                                                                                                                           2 H+ + 1/2 O2
                                                                                                                              0


                                                            H2O
                                                                                                                                                                                               H2O9
              Glucose Catabolism
•   If molecular oxygen (O2) is the final electron
    acceptor, the process is called aerobic
    respiration.
•   If some other inorganic molecule is the final
    electron acceptor, the process is called
    anaerobic respiration.
•   If an organic molecule is the final electron
    acceptor, the process is called fermentation.


                                                     10
        The Stages of Cellular Respiration
•   Respiration is a cumulative function of three
    metabolic stages
    • Glycolysis - breaks down glucose into two
      molecules of pyruvate
    • The Citric Acid Cycle (Kreb’s) - completes the
      breakdown of glucose
    • Oxidative phosphorylation - driven by the
      electron transport chain and Generates ATP



                                                       11
               Cellular Respiration

       Electrons                            Electrons carried
         carried                             via NADH and
       via NADH                                  FADH2



                                                            Oxidative
                                      Citric             phosphorylation:
       Glycolsis
                                      acid                   electron
Glucose        Pyruvate               cycle               transport and
                                                          chemiosmosis

   Cytosol
                          Mitochondrion



         ATP                          ATP                            ATP

                               Substrate-level                     Oxidative
     Substrate-level
                               phosphorylation                  phosphorylation
     phosphorylation

                                                                                  12
                  Substrate Phosphorylation
•   Both glycolysis and the citric acid cycle can
    generate ATP by substrate-level phosphorylation
                  Enzyme                            Enzyme


      Substrate               ADP

                  P
                                          Product    +       ATP




                      PEP

                       P

                       P
         Enzyme                     ADP
                       P
                  Adenosine
                                                                   13
                   Glycolysis

•   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




                                              14
                         Glycolysis
•   Occurs in the cytoplasm of
    the cell
•   Results in the partial
    breakdown of glucose
•   Anaerobic – no oxygen is
    used during glycolysis
•   For each molecule of
    glucose that passes
    through glycolysis, the cell
    nets two ATP molecules.
                                   Glycolysis   Citric
                                                acid        Oxidative
                                                cycle    phosphorylation




                                      ATP       ATP             ATP




                                                                           15
                            ATP/NADH Ledger
  Glycolysis
                            - 2 ATP

   The energy
   investment phase


                                      carbons

       Energy
       coupling

ATP  ADP + P: exergonic

Glu  Glu-6-P :endergonic
                                                16
                   ATP/NADH Ledger
  Glycolysis       -2ATP   +2 NADH
                   +2ATP
    The energy
    payoff phase



Redox reactions



Energy coupling


                                     17
                            ATP/NADH Ledger
                            -2ATP
Glycolysis                  +2ATP
                                    +2 NADH
                            +2ATP




 More energy
 coupling


End-products of
glycolysis are 2 pyruvate
molecules
                                              18
                          Glycolysis Summary
•   Occurs in the cytoplasm
                                      Energy investment phase
•   Glucose converted to two
    3-C chains                                          Glucose

•   Anaerobic - no oxygen
•   2 ATP used, 4 ATP                              2 ADP + 2 P           2 ATP   used
    produced
•   Inefficient - net yield only 2
    ATPs                              Energy payoff phase

•   Not discarded by evolution                     4 ADP + 4 P           4 ATP   formed
    but used as starting point
    for energy production
                                       2 NAD+ + 4 e– + 4 H+             2 NADH   + 2 H+
•   If no O2 - Fermentation
    occurs
•   End products:                                                      2 Pyruvate + 2 H2O

      • 2 ATP
                                       Net
      • Pyruvate (3 C)                                   Glucose    2 Pyruvate + 2 H2O

      • 2 x NADH                     4 ATP formed – 2 ATP used      2 ATP

                                             2 NAD+ + 4 e– + 4 H+   2 NADH + 2 H+
                                                                                          19
     The Citric Acid (Krebs) Cycle
The Krebs cycle is named after Hans Krebs and is a metabolic event
that follows glycolysis. This process occurs in the fluid matrix of the
mitochondrion, uses the pyruvic acid from glycolysis and is aerobic. To
begin the Krebs cycle, pyruvic acid is converted to acetyl CoA.




                                                                          20
                     Oxidation of Pyruvate
•     More energy can be extracted if oxygen is present
•     Within mitochondria, pyruvate is decarboxylated,
      yielding acetyl-CoA, NADH, and CO2


          CYTOSOL                                 MITOCHONDRION



                            NAD+   NADH    + H+




                                                         Acetyl Co A
    Pyruvate                CO2    Coenzyme A


      Transport protein
                                                                       21
         The Citric Acid (Krebs) Cycle

• Occurs in the
  mitochondrial matrix
• Aerobic – although O2 is
  not used directly in this
  pathway, it will not occur
  unless enough is present
  in the cell.
• Main catabolic pathway
• Acetyl-CoA is oxidized in
  a series of nine reactions




                                         22
Krebs Cycle                 Glycolysis   Citric
                                         acid
                                         cycle
                                                     Oxidation
                                                  phosphorylation




                              ATP        ATP            ATP


•   AcetylCoA reacts with
    oxaloacetate using an                                                Acetyl CoA


    enzyme called citrate
    synthase producing                                                                          H2O

                                                                    Oxaloacetate
    citric acid.
•   Because of this, the                                                              Citrate
                                                                                                      Isocitrate


    Krebs cycle is                                                           Citric
                                                                             acid
    sometimes called the                                                     cycle

    citric acid cycle.




                                                                                                                   23
Krebs Cycle              Glycolysis    Citric
                                       acid
                                       cycle
                                                   Oxidation
                                                phosphorylation




• The next 7 steps         ATP         ATP            ATP




  decompose the
  citrate back to                                                        Acetyl CoA


  oxaloacetate,
• Citric acid is                                                                                    H2O


  systematically                                                  Oxaloacetate


  decarboxylated and                                                                    Citrate

  dehyrogenated in                                                                                         Isocitrate


  order to use up the                                                        Citric
                                                                             acid
                                                                                                                        CO2


  acetyl groups that                                                         cycle                                  NAD+

                                                                                                                        NADH
  were attached to the                                 Fumarate                                                          + H+


  oxaloacetate.                                                                                                a-Ketoglutarate



• This allows                         FADH2
  oxaloacetate and                                FAD
                                                                                                  NAD+
                                                                                                              CO2



  CoA to be used in                                         Succinate

                                                                      GTP GDP
                                                                                 Pi
                                                                                      Succinyl
                                                                                                   NADH
                                                                                                    + H+

  the next cycle.                                                  ADP
                                                                                        CoA




                                                                       ATP



                                                                                                                                 24
Krebs Cycle               Glycolysis      Citric
                                          acid
                                          cycle
                                                        Oxidation
                                                     phosphorylation




                            ATP           ATP              ATP


•   The NADH and
    FADH2 produced                                                            Acetyl CoA



    by the cycle relay                             NADH
                                                    + H+                                                 H2O
    electrons extracted                  NAD+
                                                                       Oxaloacetate


    from food to the                                    Malate                               Citrate

    electron transport                                                                                          Isocitrate

                                                                                                                             CO2
                                                                                  Citric
    chain                          H2O
                                                                                  acid
                                                                                  cycle                                  NAD+

                                                                                                                             NADH
                                                            Fumarate                                                          + H+

      ATP/NADH Ledger                                                                                               a-Ketoglutarate




      + 2 ATP                            FADH2
                                                                                                                   CO2
                                                                                                       NAD+
      + 6 NADH                                         FAD
                                                                 Succinate                              NADH
      + 2 FADH2                                                                       Pi
                                                                                           Succinyl      + H+
                                                                           GTP GDP
                                                                                             CoA

                                                                        ADP

                                                                            ATP


                                                                                                                                      25
Krebs Cycle




              26
    ETC and Oxidative Phosphorylation
•   Occurs along the inner mitochondrial membrane (IMM) in the
    cristae of the mitochondrion
•   NADH/FADH2 molecules carry electrons from glycolysis and the
    citric acid cycle to the inner mitochondrial membrane, where they
    transfer electrons to a series of membrane-associated proteins.




                                                                        27
     The Pathway of Electron Transport
•   Most of the chain’s
                                                                                    NADH
    components are proteins,                                                  50


    which exist in multiprotein                                                                        FADH2


    complexes                                                                 40   FMN
                                                                                            I          FAD
                                                                                                                           Multiprotein
                                                                                                                           complexes




                                  Free energy (G) relative to O2 (kcal/mol)
                                                                                         Fe•S       Fe•S II
•   The carriers alternate                                                                      Q
                                                                                                                     III
                                                                                                      Cyt b
    reduced and oxidized                                                      30
                                                                                                              Fe•S
                                                                                                                     Cyt c1
    states as they accept and                                                                                                 Cyt c
                                                                                                                                                 IV

                                                                                                                                      Cyt a
    donate electrons                                                          20
                                                                                                                                              Cyt a3



•   Electrons drop in free
    energy as they go down                                                    10

    the chain and are finally
    passed to O2, forming                                                      0                                                 2 H+ + 1/2 O2

    water
                                                                                                                                                   28
                                                                                                                                                 H2O
     The Pathway of Electron Transport
•   The electron transport                                                       NADH

    chain generates no                                                     50



    ATP                                                                                             FADH2

                                                                                                                        Multiprotein
                                                                                                                        complexes

    The chain’s function is
                                                                           40            I          FAD
                                                                                FMN
•




                               Free energy (G) relative to O2 (kcal/mol)
                                                                                      Fe•S       Fe•S II
                                                                                             Q

    to break the large free-                                               30
                                                                                                   Cyt b
                                                                                                           Fe•S
                                                                                                                  III




    energy drop from food                                                                                         Cyt c1
                                                                                                                           Cyt c
                                                                                                                                              IV



    to O2 into smaller steps                                               20
                                                                                                                                   Cyt a
                                                                                                                                           Cyt a3


    that release energy in
    manageable amounts                                                     10




                                                                            0                                                 2 H+ + 1/2 O2



                                                                                                                                                29
                                                                                                                                              H2O
          Electron Transport Phosphorylation
•   Electron transfer in the electron transport chain causes proteins to pump H+
    from the mitochondrial matrix to the intermembrane space
•   The ETC uses energy from electrons to pump H+ across a membrane against
    their concentration gradient - potential energy.
•   H+ then moves back across the membrane, passing through channels in ATP
    synthase
•   ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP
•   This is an example of chemiosmosis, the use of energy in a H+ gradient to drive
    cellular work




                                                                                      30
           LE 9-15



                                                                                                                                            Inner
                                                                                                                                            mitochondrial
                                                                                                                                            membrane

                                   Oxidative
   Glycolysis        Citric
                     acid       phosphorylation:
                     cycle     electron transport
                               and chemiosmosis




     ATP             ATP               ATP
                                                                                                           H+
                                                                                    H+

                                                        H+
                                                                                                                                    H+
                        Protein complex                                                   Cyt c
Intermembrane           of electron
space                   carriers

                                                                        Q                             IV

                                                    I                         III
                                                                                                                                             ATP
                                                                   II                                                                        synthase
Inner                                                                                       2H+ + 1/2 O2         H2O
mitochondrial                                                     FADH2
                                                                            FAD
membrane
                                   NADH + H+                   NAD+
                                                                                                                       ADP + P i           ATP
                              (carrying electrons
                              from food)
                                                                                                                                    H+

Mitochondrial                                                             Electron transport chain                             Chemiosmosis
matrix                                                        Electron transport and pumping of protons (H+),        ATP synthesis powered by the flow
                                                             Which create an H+ gradient across the membrane          of H+ back across the membrane

                                                                                         Oxidative phosphorylation



                                                                                                                                                            31
                                        ATP
•   The energy stored in a H+ gradient          Intermembrane
                                                     space        H+
    across a membrane couples the redox                                           H+
                                                                       H+    H+
    reactions of the electron transport chain
    to ATP synthesis                                              H+
                                                                        H+
                                                                             H+
                                                                                  H+
•   The H+ gradient is referred to as a
    proton-motive force, emphasizing its                                               Rotor
    capacity to do work

•   Most of the ATP produced in cells is
    made by the enzyme ATP synthase
                                                       Rod

•   The enzyme is embedded in the
    membrane and provides a channel                                                    Catalytic
    through which protons can cross the                                                head
    membrane down their concentration
    gradient                                     ADP + Pi


•   The energy released causes the rotor                    ATP
    and the rod structures to rotate. This                                        H+
    mechanical energy is converted to
    chemical energy with the formation of         Mitochondrial matrix
    ATP
                                                                                                   32
LE 9-14


          INTERMEMBRANE SPACE
                                      H+              A rotor within the
                                                      membrane spins
          H+           H+
                                                      as shown when
                                                      H+ flows past
                                            H+
                                 H+                   it down the H+
                                                      gradient.
                            H+
                H+
                                                      A stator anchored
                                                      in the membrane
                                                      holds the knob
                                                      stationary.


                                                      A rod (or “stalk”)
                                                      extending into
                                                      the knob also
                                                      spins, activating
                                                      catalytic sites in
                                                      the knob.
                                                 H+

                                                      Three catalytic
                                                      sites in the
               ADP                                    stationary knob
                +                                     join inorganic
                                           ATP        phosphate to
               P
                   i                                  ADP to make
                                                      ATP.
          MITOCHONDRAL MATRIX

                                                                           33
Summary
of Glucose
Catabolism




             34
Theoretical ATP Yield of Aerobic
          Respiration




                                   35
        Catabolism of Proteins and Fats
•   Proteins are utilized by deaminating their amino acids, and
    then metabolizing the product.
•   Fats are utilized by beta-oxidation.




                                                                  36
          Regulating Aerobic Respiration
•   Control of glucose catabolism occurs at two key points in the catabolic
    pathway.
     • Glycolysis - phosphofructokinase
     • Pyruvate Oxidation – pyruvate decarboxylase




                                                                              37
             Recycling NADH
•   As long as food molecules are available to
    be converted into glucose, a cell can
    produce ATP.
•   Continual production creates NADH
    accumulation and NAD+ depletion.
•   NADH must be recycled into NAD+.
     • Aerobic respiration - oxygen as electron
       acceptor
     • Fermentation - organic molecule

                                                  38
             Anaerobic Respiration
•   Final electron acceptor is an inorganic
    molecule other than oxygen
•   Some use NO3 -: E. coli
•   Some use SO42-
•   Important in nitrogen and sulfur cycles
•   ATP varies, less than 38
•   Only part of Krebs cycle & ETC used


                                              39
                           Fermentation
•   In some cases, the high
    energy electrons picked
    up by NAD+ during
    glycolysis are not donated
    to an ETC.

•   Instead, NADH donates
    its extra electrons and H+
    directly to an organic
    molecule, which serves as
    the final electron acceptor.




                                          40
                  Fermentation
•   Pyruvate converted to
    organic product
•   NAD+ regenerated
•   Doesn’t require oxygen
•   Does not use Krebs cycle
    or ETC
•   Organic molecule is final
    electron acceptor
•   Produces 2 ATP max


                                 41
               Alcohol Fermentation
•   Occurs in single-celled fungi called yeast
•   A terminal CO2 is removed from the pyruvic acid (3C)
    produced during glycolysis, producing acetaldehyde (2C)
•   Acetaldehyde accepts 2 e- and a H+ from NADH, producing
    ethanol and NAD+

                         2 ADP + 2 P i
                          Glucose           2 ATP
                                                          H

                2 ADP           G                     H C OH
                                L                        CH3
                     Glucose    Y Glycolysis           2 Ethanol
                 2 ATP          C          2 NAD+
                                O                      2 Pyruvate
                                L
                              2 NAD+
                                Y              NADH
                                             2 2 NADH
                                                          H 2
                                                                CO2
              O H               S           +2H   +

              C    O             I                        C O
                                              CO2
                                S
              C    O                                     CH3
                                                      2 Acetaldehyde
              CH3           2 Pyruvic Acid
                   2 Ethanol                        2 Acetaldehyde
                                                                       42
                   Alcohol fermentation
             Lactic Acid Fermentation
•   Used by most animal cells when O2 is not available
•   NADH donates 2 e- and a H+ directly to the pyruvate (3C)
    produced during glycolysis, producing lactate (3C) and
    NAD+

                             2 ADP + 2 P i    2 ATP

                                Glucose                       O–
                                     G                         C   O
                    Glucose
                    2 ADP            Glycolysis
                                     L                     H C OH
                                     Y                         CH3
                     2 ATP           C                      2 Lactate
                                     O          2 NAD+
                                2 NAD+          2 NADH           2 CO2
                                     L             +
                -                            +2H
            O                        Y             2 NADH 2 Pyruvate
                                     S
            C        O
                                     I
            C        O               S
            CH3               2 Pyruvate
                2 Lactate
                                                                         43
                Lactic acid fermentation
                 Fermentation
Alcoholic fermentation and lactic acid fermentation each
generate 2 ATP / glucose molecule compared to the
theoretical maximum of 36 ATP per glucose during aerobic
respiration.




                                                           44
45

				
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