Chapter 9: Cellular Respiration

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Chapter 9 Cellular Respiration
Cellular Respiration
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Cellular Respiration

• Living cells require energy from outside sources
• Organisms use glucose (C6H12O6) as their main
  energy source
• Cellular respiration is the process of breaking
  down food molecules to release energy (as ATP)
• Energy is released in the process of respiration
  when the cells of plants and animals convert
  sugar and oxygen into carbon dioxide and water

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Respiration

• The breakdown of organic molecules is exergonic
• Aerobic respiration consumes organic molecules
  and O2 and yields ATP (oxygen required)
• Anaerobic respiration is similar to aerobic
  respiration but consumes compounds other than O2
  (no oxygen required)
• Fermentation is a partial degradation of sugars
  that occurs without O2

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Cellular Respiration

• Cellular respiration includes both aerobic and
  anaerobic respiration but is often used to refer
  to aerobic respiration
• Although carbohydrates, fats, and proteins are
  all consumed as fuel, it is helpful to trace
  cellular respiration with the sugar glucose
• C6H12O6 6 O2 ? 6 CO2 6 H2O Energy
  (ATPheat)

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Redox Reactions

• The transfer of electrons during chemical
  reactions releases energy stored in organic
  molecules
• This released energy is used to make ATP
• Chemical reactions that transfer electrons
  between reactants are called oxidation-reduction
  reactions, or redox reactions
• In oxidation, a substance loses electrons, or is
  oxidized
• In reduction, a substance gains electrons, or is
  reduced (the amount of positive charge is
  reduced)
• In cellular respiration, the glucose is oxidized
  and O2 is reduced

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NAD

• In cellular respiration, glucose and other
  organic molecules are broken down in a series of
  steps
• Electrons from organic compounds are usually
  first transferred to NAD (nicotinamide adenine
  dinucleotide), a coenzyme
• As an electron acceptor, NAD functions as an
  oxidizing agent
• Each NADH (the reduced form of NAD) represents
  stored energy that is tapped to synthesize ATP
• NADH passes the electrons to the electron
  transport chain

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Electron Transport Chain

• Unlike an uncontrolled reaction, the electron
  transport chain passes electrons in a series of
  steps instead of one explosive reaction
• O2 pulls electrons down the chain in an
  energy-yielding tumble
• The energy yielded is used to regenerate ATP

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Stages of Cellular Respiration

1. Glycolysis - Anaerobic (breaks down glucose into
   two molecules of pyruvate)
2. Citric Acid Cycle - Aerobic (Krebs Cycle -
   completes the breakdown of glucose)
3. Oxidative phosphorylation - Aerobic (ETC -
   accounts for most of the ATP synthesis)

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Mitochondria

• 1) Glycolysis
• Cytoplasm
• 2) Citric Acid Cycle
• Matrix of mitochondria
• 3) Oxidative Phosphorylation (ETC)
• Cristae of mitochondria

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Step 1 Glycolysis

• Splitting of sugar
• Breaks down glucose (C6H12O6) into two molecules
  of pyruvic acid - AKA pyruvate (C3H4O3)
• Anaerobic
• Occurs in the cytoplasm
• NAD picks up H and electrons to form NADH2

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Glycolysis Summary
Location Cytoplasm

• Products
• 2 Pyruvates (3-C)
• 2 NADH
• 4 ATP total
• 2 ATP NET since 2 are used initially

• Reactants
• Glucose (6-C)
• 2 NAD
• 2 ATP

Simple Summary Summary total
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Bridge Reaction

• In the presence of O2, pyruvate enters the
  mitochondrion
• Before the citric acid cycle can begin, pyruvate
  must be converted to acetyl CoA, which links the
  cycle to glycolysis
• In the mitochondria matrix
• 1) Pyruvic Acid loses a C to form acetic acid
  (2-C)
• 2) The lost carbon binds with O2 making CO2
• 3)Acetic acid binds with Coenzyme-A forming
  Acetyl Co-A

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Step 2 The Krebs Cycle(Citric Acid Cycle)

• Takes place within the mitochondrial matrix
• There are 8 steps, each catalyzed by a specific
  enzyme
• The acetyl group of acetyl CoA joins the cycle by
  combining with oxaloacetate (4-C molecule),
  forming a 6-C molecule known as citric acid
  (citrate)
• The next seven steps decompose the citrate back
  to oxaloacetate, making the process a cycle

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Step 2 The Krebs Cycle(Citric Acid Cycle)

• 2 molecules of CO2 are released
• NAD and FAD (flavin adenine dinucleotide -
  another ion carrier) pick up electrons and H
  becoming NADH and FADH2
• The NADH and FADH2 produced by the cycle relay
  electrons extracted from food to the electron
  transport chain
• The cycle generates 1 ATP, 3 NADH, and 1 FADH2
  per turn
• Recall that two molecules of pyruvate are formed
  during glycolysis resulting in two turns of the
  Krebs cycle for each glucose molecule!

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Krebs Cycle Summary
Location Mitochondrial Matrix

• Products
• 8 NADH (2 from transition)
• 2 FADH2
• 2 ATP
• 6 CO2 (2 from transition)

• Reactants
• 2 Acetyl Co-A

Krebs Summary Kreb's Summary 2
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Step 3 Electron Transport Chain (ETC)

• Aerobic process
• Requires oxygen as the final electron acceptor
• Takes place in the cristae of the mitochondria
• A series of molecules that excited electrons pass
  along, to release energy as ATP
• Most of the chains components are proteins,
  which exist in multiprotein complexes

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Step 3 Electron Transport Chain (ETC)

• Following glycolysis and the citric acid cycle,
  NADH and FADH2 account for most of the energy
  extracted from food
• These two electron carriers donate electrons to
  the electron transport chain, which powers ATP
  synthesis via oxidative phosphorylation
• The carriers alternate reduced and oxidized
  states as they accept and donate electrons
• Electrons drop in free energy as they go down the
  chain
• They are finally passed to O2 (final electron
  acceptor), forming H2O

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NADH and FADH2

• Dump the electrons and protons theyve gathered
  throughout glycolysis and the citric acid cycle
• Again, oxygen is the final electron acceptor
• O2 2e- 2H ? H2O
• Electrons are passed through a number of proteins
  including cytochromes (each with an iron atom) to
  O2
• The chains function is to break the large
  free-energy drop from food to O2 into smaller
  steps that release energy in manageable amounts
• ETC uses chemiosmosis to generate large amounts
  of ATP

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Chemiosmosis

• Electron transfer in the ETC causes proteins to
  pump H from the mitochondrial matrix to the
  intermembrane space
• H then moves back across the membrane, passing
  through channels in ATP synthase (enzyme that
  acts like an ion pump)
• ATP synthase uses the exergonic flow of H to
  drive phosphorylation of ADP
• This is an example of chemiosmosis, the use of
  energy in a H gradient to drive cellular work
• The H gradient is called the proton-motive force

ETC Summary
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ETC
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ETC Summary
Location Cristae of Mitochondria

• Reactants
• 10 NADH
• 2 FADH2

• Product
• 34 ATP
• Each NADH makes 3
• Each FADH2 makes 2

The bulk of ATP is made in the ETC!!
Simpler ETC Summary Best ETC Summary
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Whole Respiration Process
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Total Energy

• 38 ATPs per 1 glucose broken down

Total ATP from 1 molecule of glucose in AEROBIC
CONDITIONS    Stage
ATP

4 Total Glycolysis
2 NET (b/c 2 are used
in the first step) CA Cycle
2 ETC
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TOTAL   38 During cellular
respiration, most energy flows in this sequence
Glucose -gt NADH -gt electron transport chain -gt
proton-motive force -gt ATP About 40 of the
energy in a glucose molecule is transferred to
ATP during cellular respiration, making about 38
ATP
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Fermentation

• Most cellular respiration requires O2 to produce
  ATP
• Glycolysis can produce ATP with or without O2 (in
  aerobic or anaerobic conditions)
• In the absence of O2, glycolysis couples with
  fermentation or anaerobic respiration to produce
  ATP
• Fermentation uses phosphorylation instead of an
  electron transport chain to generate ATP
• 2 Types
• Lactic Acid Fermentation
• Alcohol Fermentation

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Lactic Acid Fermentation

• In lactic acid fermentation, pyruvate is reduced
  to NADH, forming lactate as an end product, with
  no release of CO2
• Lactic acid fermentation by some fungi and
  bacteria is used to make cheese and yogurt
• Human muscle cells use lactic acid fermentation
  to generate ATP when O2 is scarce

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Lactic Acid Fermentation

• Example Burning feeling in muscles during a
  workout
• From oxygen debt
• Aerobic respiration cannot occur
• Lactate builds up in muscles leaks into blood

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Alcohol Fermentation

• In alcohol fermentation, pyruvate is converted to
  ethanol (type of alcohol) in two steps, with the
  first releasing CO2
• Bacteria and fungi (yeast)
• Alcohol fermentation by yeast is used in brewing,
  winemaking, and baking

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Fermentation

• Obligate anaerobes carry out fermentation or
  anaerobic respiration and cannot survive in the
  presence of O2
• Yeast and many bacteria are facultative
  anaerobes, meaning that they can survive using
  either fermentation or cellular respiration

Review
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Role of Macromolecules

• Catabolic pathways funnel electrons from many
  kinds of organic molecules into cellular
  respiration
• Glycolysis accepts a wide range of carbohydrates
• Proteins must be digested to amino acids
• Amino groups can feed glycolysis or the citric
  acid cycle
• Fats are digested to glycerol (used in
  glycolysis) and fatty acids (used in generating
  acetyl CoA)
• Fatty acids are broken down by beta oxidation and
  yield acetyl CoA
• An oxidized gram of fat produces more than twice
  as much ATP as an oxidized gram of carbohydrate

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Regulation of Cell Respiration

• Feedback inhibition is the most common mechanism
  for control
• If ATP concentration begins to drop, respiration
  speeds up
• When there is plenty of ATP, respiration slows
  down
• Control of catabolism is based mainly on
  regulating the activity of enzymes at strategic
  points in the catabolic pathway

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Review Questions

1. Define cellular respiration and state its
   importance as a life process.
2. Differentiate between aerobic respiration,
   anaerobic respiration, and fermentation.
3. State and explain the chemical equation for
   cellular respiration.
4. Define oxidation and reduction and explain the
   idea of redox reactions.
5. Explain the use of NAD as a coenzyme.
6. Explain the electron transport chain (ETC).
7. Name the 3 major stages of cell respiration,
   along with their locations.
8. Explain glycolysis, stating the reactants,
   products, and major activities.
9. Explain the bridge reaction, stating the
   reactants, products, and major activities.
10. Explain the Krebs cycle, stating the reactants,
    products, and major activities.
11. Explain glycolysis, stating the reactants,
    products, and major activities.
12. Explain the ETC, stating the reactants, products,
    and major activities.
13. Explain the role of oxygen in the ETC.
14. Define chemiosmosis and explain its role in
    cellular respiration.
15. Differentiate between lactic acid fermentation
    and alcohol fermentation.
16. Differentiate between oblicate anaerobes and
    facultative anaerobes.
17. Explain the role of macromolecules in cellular
    respiration.
18. Explain how cell respiration is regulated.