Cellular Respiration

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Cellular Respiration
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Outline

• Oxidation-Reduction
• Glycolysis
• Aerobic Respiration

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Oxidation-Reduction Reactions

• Oxidation
• molecule releases electrons and energy, often
  as hydrogen atoms
• Reduction
• molecule accepts electrons and gains chemical
  energy (E)

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Catabolism of Food

• Most carbohydrates are catabolized within a few
  hours of absorption
• C6H12O6 6O2 ? 6CO2 6H2O
• Purpose is to transfer energy from glucose to ATP
• Fats and amino acids can also be used to generate
  ATP.

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Glucose Our Primary Fuel
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Glucose Catabolism

• Glucose catabolism occurs as a series of steps
  where small amounts of energy are transferred to
  ATP and the rest is released as heat.
• Major Pathways
• Glycolysis
• - splits a glucose molecule into 2 equal parts
  and generates 2 ATP
• Aerobic Respiration (requires O2)
• - catabolizes the products of glycolysis and
  generates more than 30 ATP

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Coenzymes

• Enzymes remove electrons as hydrogen atoms during
  the catabolism of glucose.
• transferring 2 protons and 2 electrons (2H and 2
  e-) at a time to coenzymes
• NAD (made from vitamin B) or nicotinamide
  adenine dinucleotide.
• NAD 2H ? NADH H
• FAD (made from vitamin B2) or flavin adenine
  dinucleotide.
• FAD 2H ? FADH2
• Coenzymes are reduced and become temporary
  carriers of the energy.

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Glycolysis

• Phosphate groups are added to glucose.
• Glucose is split in half and modified to form two
  three-carbon molecules (PEP).

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Glycolysis

• Glycolysis refers to the breakdown of the
  six-carbon molecule, glucose, into two
  three-carbon molecules of pyruvic acid.
• 10 step process occurring in cell cytosol
• use two ATP molecules, but produce four, a net
  gain of two (Figure 25.3).

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Glycolysis in Ten Steps
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Substrate-levelPhosphorylation

• Glycolysis yields 2 PEP
• Each PEP is used to produce ATP and pyruvate.

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Glycolysis of Glucose Fate of Pyruvic Acid

• Breakdown of six-carbon glucose molecule into 2
  three-carbon molecules of pyruvic acid
• Pyruvic acid is converted to acetylCoA, which
  enters the Krebs Cycle.
• The Krebs Cycle will require NAD
• NAD will be reduced to the high-energy
  intermediate NADH.

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Aerobic Metabolism

• Most ATP is generated in the mitochondria
• Two principal steps
• Matrix Reactions
• occurs in fluids of mitochondria
• macromolecules are oxidized and electrons are
  transferred to NAD and FAD to form NADH and
  FADH2
• Membrane Reactions (Electron Transport Chain)
• catalyzed by enzymes in the mitochondrial
  membrane
• NADH FADH2 are oxidized, transferring energy to
  ATP and regenerating NAD and FAD

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The Structure of Mitochondria
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Pyruvic Acid

• The fate of pyruvic acid depends on the
  availability of O2.

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Formation of Acetyl Coenzyme A

• Pyruvic acid enters the mitochondria with help
  of transporter protein
• Decarboxylation
• pyruvate dehydrogenase converts 3 carbon pyruvic
  acid to 2 carbon fragment acetyle group plus CO2.

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Formation of Acetyl Coenzyme A

• 2 carbon fragment (acetyl group) is attached to
  Coenzyme A to form Acetyl coenzyme A, which enter
  Krebs cycle
• coenzyme A is derived from pantothenic acid (B
  vitamin).

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Krebs Cycle

• The Krebs cycle is also called the citric acid
  cycle, or the tricarboxylic acid (TCA) cycle. It
  is a series of biochemical reactions that occur
  in the matrix of mitochondria (Figure 25.6).

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Krebs Cycle
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Krebs Cycle

• The large amount of chemical potential energy
  stored in intermediate substances derived from
  pyruvic acid is released step by step.
• The Krebs cycle involves decarboxylations and
  oxidations and reductions of various organic
  acids.
• For every two molecules of acetyl CoA that enter
  the Krebs cycle, 6 NADH, 6 H, and 2 FADH2 are
  produced by oxidation-reduction reactions, and
  two molecules of ATP are generated by
  substrate-level phosphorylation (Figure 25.6).
• The energy originally in glucose and then pyruvic
  acid is primarily in the reduced coenzymes NADH
  H and FADH2.

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Krebs Cycle (Citric Acid Cycle)

• The oxidation-reduction decarboxylation
  reactions occur in matrix of mitochondria.
• acetyl CoA (2C) enters at top combines with a
  4C compound
• 2 decarboxylation reactions peel 2 carbons off
  again when CO2 is formed

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Krebs Cycle

• Potential energy (of chemical bonds) is released
  step by step to reduce the coenzymes (NAD?NADH
  FAD?FADH2) that store the energy
• Review
• Glucose? 2 acetyl CoA molecules
• each Acetyl CoAmolecule that enters the
  Krebscycle produces
• 2 molecules of C02
• 3 molecules of NADH H
• one molecule of ATP
• one molecule of FADH2

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Summary of Matrix Reactions

• 2 pyruvate 6 H2O ? 6 CO2
• 2 ADP 2 Pi ? 2 ATP
• 8 NAD 8 H2 ? 8 NADH 8 H
• 2 FAD 2 H2 ? 2 FADH2
• Carbon atoms of the glucose have all been carried
  away as CO2 and exhaled.
• Energy has been lost as heat and stored in 2 ATP,
  8 NADH, and 2 FADH2.

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Electron Transport Chain

• The electron transport chain involves a sequence
  of electron carrier molecules on the inner
  mitochondrial membrane, capable of a series of
  oxidation-reduction reactions.
• As electrons are passed through the chain, there
  is a stepwise release of energy from the
  electrons for the generation of ATP.
• In aerobic cellular respiration, the last
  electron receptor of the chain is molecular
  oxygen (O2). This final oxidation is
  irreversible.
• The process involves a series of
  oxidation-reduction reactions in which the energy
  in NADH H and FADH2 is liberated and
  transferred to ATP for storage.

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Electron Transport Chain

• Pumping of hydrogen is linked to the movement of
  electrons passage along the electron transport
  chain.
• It is called chemiosmosis (Figure 25.8.)
• Note location.

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Chemiosmosis

• H ions are pumped from matrix into space between
  inner outer membrane
• High concentration of H is maintained outside of
  inner membrane
• ATP synthesis occurs as H diffuses through a
  special H channels in the inner membrane

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Electron Transport Chain

• The carrier molecules involved include flavin
  mononucleotide, cytochromes, iron-sulfur centers,
  copper atoms, and ubiquinones (also coenzyme Q).

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Electron Carriers

• Flavin mononucleotide (FMN) is derived from
  riboflavin (vitamin B2)
• Cytochromes are proteins with heme group (iron)
  existing either in reduced form (Fe2) or
  oxidized form (Fe3)
• Iron-sulfur centers contain 2 or 4 iron atoms
  bound to sulfur within a protein
• Copper (Cu) atoms bound to protein
• Coenzyme Q is nonprotein carrier mobile in the
  lipid bilayer of the inner membrane

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Steps in Electron Transport

• Carriers of electron transport chain are
  clustered into 3 complexes that each act as a
  proton pump (expelling H)
• Mobile shuttles (CoQ and Cyt c) pass electrons
  between complexes.
• The last complex passes its electrons (2H) to
  oxygen to form a water molecule (H2O)

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Proton Motive Force Chemiosmosis

• Buildup of H outside the inner membrane creates
  charge
• The potential energy of the electrochemical
  gradient is called the proton motive force.
• ATP synthase enzymes within H channels use the
  proton motive force to synthesize ATP from ADP
  and P

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• Complete oxidation of glucose produces 38 ATP (an
  efficiency of only 40 the rest is body heat)

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Summary of Cellular Respiration
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Metabolism ofFats Proteins

• Amino acids enter the Krebs cycle
• Fats are broken into parts that enter glycolysis
  and the Krebs cycle
• While fats and sugars can be stored, amino acids
  are catabolized if they are not used in proteins.

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Aerobic versus Anaerobic Respiration

• In the absence of oxygen, only glycolysis can
  occur.
• Anaerobic metabolism produces a toxic byproduct
  lactic acid
• Anaerobic metabolism is inefficient and cannot be
  sustained for long periods.