How Cells Release Chemical Energy

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Chapter 7
How Cells Release Chemical Energy
2
Overview of Carbohydrate Breakdown Pathways

• All organisms (including photoautotrophs) convert
  chemical energy of organic compounds to chemical
  energy of ATP
• ATP is a common energy currency that drives
  metabolic reactions in cells
• Pathways of Carbohydrate Breakdown
• Start with glycolysis in the cytoplasm
• Convert glucose and other sugars to pyruvate
• Fermentation pathways
• End in cytoplasm, do not use oxygen, yield 2 ATP
  per molecule of glucose
• Aerobic respiration
• Ends in mitochondria, uses oxygen, yields up to
  36 ATP per glucose molecule
• Carbohydrate Breakdown Pathways
• All organisms produce ATP by degradative pathways
  that extract chemical energy from glucose and
  other organic compounds
• Aerobic respiration yields the most ATP from each
  glucose molecule
• In eukaryotes, aerobic respiration is completed
  inside mitochondria

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Pathways of Carbohydrate Breakdown

• All organisms produce ATP by degradative pathways
  that extract chemical energy from glucose and
  other organic compounds
• Aerobic respiration yields the most ATP from each
  glucose molecule
• In eukaryotes, aerobic respiration is completed
  inside mitochondria

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Overview of Aerobic Respiration

• Cellular respiration is to extract energy in
  carbohydrates through oxidative, exergonic
  processes and store the energy in ATP molecules
  (for later use in cell reactions).
• Three main stages of aerobic respiration
• 1. Glycolysis
• 2. Krebs cycle
• 3. Electron transfer phosphorylation
• Summary equation
• C6H12O6 6O2 ? 6CO2 6 H2O

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Overview of Aerobic Respiration
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Overview of Aerobic Respiration
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Glycolysis Glucose Breakdown Starts

• Glycolysis can be divided into four stages
• Glucose mobilization priming the pump
• Glucose cleavage 6C --gt two 3C
• Oxidation begin energy gain NADH produced
• ATP generation during formation of PEP and
  pyruvate
• Enzymes of glycolysis use two ATP to convert one
  molecule of glucose to two molecules of
  three-carbon pyruvate
• Reactions transfer electrons and hydrogen atoms
  to two NAD (reduces to NADH)
• 4 ATPs are formed by substrate-level
  phosphorylation
• Products of Glycolysis
• Net yield of glycolysis 2 pyruvate, 2 ATP, and 2
  NADH per glucose
• Pyruvate may
• Enter fermentation pathways in cytoplasm
• Enter mitochondria and be broken down further in
  aerobic respiration

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Glycolysis
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Glycolysis
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Second Stage of Aerobic Respiration

• The second stage of aerobic respiration takes
  place in the inner compartment of mitochondria
• Pyruvate becomes Acetyl CoA loses CO2 and NADH
• Two pyruvates from glycolysis are converted to
  two acetyl-CoA
• Two CO2 leave the cell
• Acetyl CoA enters mitochondrial matrix for Krebs
  cycle

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Second Stage Reactions
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Krebs Cycle

• Preparation Acetyl-CoA combined with starting
  molecule yields citric acid
• Energy extraction Citric acid oxidized and
  rearranged to yield ATP, NADH, FADH2, and CO2
• NADH, FADH2 are electron carriers that transfer
  high energy electrons to the electron transport
  chain for further energy extraction.

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Net Results for Krebs Cycle

• Each turn of the Krebs cycle, one acetyl-CoA is
  converted to two molecules of CO2
• After two cycles
• Two pyruvates are dismantled
• Glucose molecule that entered glycolysis is fully
  broken down
• Second stage of aerobic respiration results in
• Six CO2, two ATP, eight NADH, and two FADH2 for
  every two pyruvates
• Adding the yield from glycolysis, the total is
• Twelve reduced coenzymes and four ATP for each
  glucose molecule
• Coenzymes deliver electrons and hydrogen to the
  third stage of reactions

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Third Stage Aerobic Respirations Big Energy
Payoff

• Coenzymes deliver electrons and hydrogen ions to
  electron transfer chains in the inner
  mitochondrial membrane
• Energy released by electrons flowing through the
  transfer chains moves H from the inner to the
  outer compartment
• Electron transport system, composed of five huge
  complexes, is embedded within the inner membrane
  of mitochondria
• The energy carried by electrons of NADH and FADH2
  is released by raveling along the electron
  transport chain and is used to pump H across the
  inner mitochondrial membrane.
• H return by facilitated diffusion through ATP
  synthase in membrane for the generation of ATP
  molecules.
• Chemiosmosis ATP synthase couples energy (from
  proton gradient in membrane) to ATP formation
• Oxygen molecule is the final electron acceptor of
  electron transport chain.
• water is produced from H and O2

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The Electron Transport Chain
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Hydrogen Ions and Phosphorylation

• H ions accumulate in the outer compartment,
  forming a gradient across the inner membrane
• H ions flow by concentration gradient back to
  the inner compartment through ATP synthases
  (transport proteins that drive ATP synthesis)
• Oxygen combines with electrons and H at the end
  of the transfer chains, forming water
• Overall, aerobic respiration yields up to 36 ATP
  for each glucose molecule

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Summary Aerobic Respiration
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Anaerobic Energy-Releasing Pathways

• Different fermentation pathways begin with
  glycolysis and end in the cytoplasm
• Do not use oxygen or electron transfer chains
• Final steps do not produce ATP only regenerate
  oxidized NAD required for glycolysis to continue
• Lactate fermentation end product is lactate
• Alcoholic fermentation end product is ethyl
  alcohol (or ethanol)
• Both pathways have a net yield of 2 ATP per
  glucose (from glycolysis)

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Alcoholic Fermentation
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The Twitchers

• Slow-twitch and fast-twitch skeletal muscle
  fibers can support different activity levels
• Aerobic respiration and lactate fermentation
  proceed in different fibers of muscles

Muscles and Lactate Fermentation
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Alternative Energy Sources in the Body

• Large, complex food molecules digested to smaller
  building blocks
• proteins --gt amino acids
• starch --gt glucose
• fats --gt fatty acids and glycerol
• Small molecules enter pathway at several points.

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Disposition of Organic Compounds
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Lifes Unity

• Photosynthesis and aerobic respiration are
  interconnected on a global scale
• In its organization, diversity, and continuity
  through generations, life shows unity at the
  bioenergetic and molecular levels