1. Cellular Respiration: Harvesting Chemical Energy
Chapter 9
Cellular Respiration: Harvesting Chemical Energy

2. Living cells require energy from outside sources.
Overview: Life Is Work
Living cells require energy from outside sources.
Some animals, such as the giant panda, obtain energy by eating plants, and some animals feed on other organisms that eat plants.
For the Discovery Video Space Plants, go to Animation and Video Files.
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3. Energy flows into an ecosystem as sunlight and leaves as heat.
Photosynthesis generates O2 and organic molecules C-H-O, which are used in cellular respiration.
Cells use chemical energy stored in organic molecules C-H-O to regenerate ATP, which powers work.
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4. Energy Flow and Chemical Recycling in Ecosystems ECOSYSTEM
IN: Light Energy
ECOSYSTEM
Photosynthesis in chloroplasts
Organic
Molecules
+ O2
CO2 + H2O
Cellular respiration
in mitochondria
Figure 9.2
ATP
ATP powers most cellular work
OUT: Heat energy

5. Catabolic pathways yield energy by oxidizing organic fuels: C-H-O molecules to Produce ATP
The breakdown of organic molecules is exergonic.
Fermentation is a partial degradation of sugars that occurs without O2
Aerobic respiration consumes organic molecules and O2 and yields ATP.
Anaerobic respiration is similar to aerobic respiration but consumes compounds without O2
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6. 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
(ATP + heat)
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7. Redox Reactions: Oxidation and Reduction
Oxidation is a LOSS (of H or electrons).
Reduction is a GAIN (of H or electrons).
The transfer of electrons during chemical reactions releases energy stored in organic molecules.
This released energy is ultimately used to synthesize ATP.
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8. In oxidation, a substance loses electrons, or is oxidized.
The Principle of Redox
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).
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9. Oxidation / Reduction Reactions
becomes oxidized
(loses electron)
becomes reduced
(gains electron)

10. Oxidation / Reduction Reactions
becomes oxidized
becomes reduced

11. The electron donor is oxidized and is called the reducing agent
The electron receptor is reduced and is called the oxidizing agent
Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds.
An example is the reaction between methane and O2
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12. Methane combustion as an energy-yielding redox reaction
Reactants
Products
becomes oxidized
becomes reduced
Figure 9.3
Methane
(reducing
agent)
Oxygen
(oxidizing
agent)
Carbon dioxide
Water

13. Oxidation of Organic Fuel Molecules During Cellular Respiration
The fuel C-H-O (such as glucose) is oxidized, looses H’s
and O2 is reduced, gains H’s
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14. During Cellular Respiration During cellular respiration, the fuel (such se) is oxidized, and O2 i
becomes oxidized
becomes reduced

15. NAD+ is Reduced to NADH + H+
Dehydrogenase

16. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
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+ = a coenzyme.
As an electron acceptor, NAD+ functions as an oxidizing agent during cellular respiration.
NADH = the reduced form of NAD+ . Each NADH represents stored energy that is tapped to synthesize ATP.
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17. NAD+ as an electron shuttle
2 e– + 2 H+
2 e– + H+
NADH H+
Dehydrogenase
Reduction of NAD+
NAD+ +
2[H] + H+
Oxidation of NADH
Nicotinamide
(reduced form)
Nicotinamide
(oxidized form)
Figure 9.4

18. NADH passes the electrons to the electron transport chain ETC.
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 ETC chain in an energy-yielding tumble.
The energy yielded is used to regenerate ATP.
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19. An introduction to electron transport chains
H2 + 1/2 O2
2 H +
1/2 O2
(from food C-H-O via NADH)
Controlled
release of
energy for
synthesis of
ATP
2 H e–
ATP
Explosive
release of
heat and light
energy
ATP
Electron transport
chain
Free energy, G
Free energy, G
ATP
2 e–
Figure 9.5
1/2 O2
2 H+
H2O
H2O
(a) Uncontrolled reaction
(b) Cellular respiration: step-wise controlled reaction

20. The Stages of Cellular Respiration: A Preview
Cellular respiration has three stages:
Glycolysis (breaks down glucose into two molecules of pyruvate)
The citric acid cycle: Krebs Cycle (completes the breakdown of glucose)
Oxidative phosphorylation (accounts for most of the ATP synthesis) by chemiosmosis.
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21. An overview of cellular respiration - Glycolysis
Electrons
carried
via NADH
Glycolysis
Glucose
Pyruvate
Cytosol
Figure 9.6
ATP
Substrate-level
phosphorylation

22. An overview of cellular respiration: Glycolysis & Krebs Cycle
Electrons
carried
via NADH
Electrons carried
via NADH and
FADH2
Glycolysis
Krebs
Cycle
Glucose
Pyruvate
Mitochondrion
Cytosol
Figure 9.6 An overview of cellular respiration
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation

23. An overview of Cellular Respiration Basics
Electrons
carried
via NADH
Electrons carried
via NADH and
FADH2
Oxidative
phosphorylation:
electron transport
and
chemiosmosis
Glycolysis
Krebs
Cycle
Glucose
Pyruvate
Mitochondrion
Cytosol
Figure 9.6 An overview of cellular respiration
ATP
ATP
ATP
Substrate-level
phosphorylation
Substrate-level
phosphorylation
Oxidative
phosphorylation

24. The process in cell respiration that generates most of the ATP is oxidative phosphorylation (powered by redox reactions).
Oxidative phosphorylation accounts for almost 90% of the ATP generated by cellular respiration.
A smaller amount of ATP is formed in glycolysis and the citric acid / Krebs Cycle by substrate-level phosphorylation.
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25. Substrate-level phosphorylation
Enzyme
Enzyme
ADP P
Substrate +
ATP
Figure 9.7
Product

26. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis (“sugar splitting”) breaks down glucose into two molecules of pyruvate.
Glycolysis occurs in the cytoplasm and has two major phases:
Energy investment phase = EA
Energy payoff phase = ATP and NADH
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27. Energy investment phase
The
energy
input
and
output of
glycolysis
Energy investment phase
Glucose
2 ADP + 2 P
2 ATP
used
Energy payoff phase
4 ADP + 4 P
4 ATP
formed
2 NAD e– + 4 H+
2 NADH
+ 2 H+
Figure 9.8
2 Pyruvate + 2 H2O
Net
Glucose
2 Pyruvate + 2 H2O
4 ATP formed – 2 ATP used
2 ATP
2 NAD e– + 4 H+
2 NADH + 2 H+

28. A closer look at glycolysis Glucose-6-phosphate 2 Phosphogluco-
ATP 1
Hexokinase
ADP
Glucose-6-phosphate
Glucose-6-phosphate 2
Phosphoglucoisomerase 2
Phosphogluco-
isomerase
Fructose-6-phosphate
Figure 9.9
Fructose-6-phosphate

29. Fructose- 1, 6-bisphosphate
Glucose
ATP 1 1
Hexokinase
ADP
Fructose-6-phosphate
Glucose-6-phosphate 2 2
Phosphoglucoisomerase
ATP 3
Phosphofructokinase: allosteric enzyme
Fructose-6-phosphate
ATP 3 3
ADP
Phosphofructokinase
ADP
Figure 9.9 A closer look at glycolysis
Fructose-
1, 6-bisphosphate
Fructose-
1, 6-bisphosphate

30. Aldolase Isomerase Fructose- 1, 6-bisphosphate 4 5 Dihydroxyacetone
Glucose
ATP 1
Hexokinase
ADP
Glucose-6-phosphate 2
Phosphoglucoisomerase
Fructose-
1, 6-bisphosphate 4
Fructose-6-phosphate
Aldolase
ATP 3
Phosphofructokinase
ADP
Figure 9.9 A closer look at glycolysis 5
Isomerase
Fructose-
1, 6-bisphosphate 4
Aldolase 5
Isomerase
Dihydroxyacetone
phosphate
Glyceraldehyde-
3-phosphate
Dihydroxyacetone
phosphate
Glyceraldehyde-
3-phosphate

31. 2 2 ADP 2 ATP 2 3-Phosphoglycerate 1, 3-Bisphosphoglycerate 7
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
1, 3-Bisphosphoglycerate
2 ADP 2
3-Phosphoglycerate 7
Phosphoglycero-
kinase
2 ATP
Figure 9.9 A closer look at glycolysis 2
3-Phosphoglycerate

32. 2 3-Phosphoglycerate 8 Phosphoglycero- mutase 2 2-Phosphoglycerate
Fig
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
3-Phosphoglycerate 2
3-Phosphoglycerate 8
Phosphoglyceromutase 8
Phosphoglycero-
mutase 2
2-Phosphoglycerate
Figure 9.9 A closer look at glycolysis 2
2-Phosphoglycerate

33. 2 2-Phosphoglycerate Enolase 2 H2O 2 Phosphoenolpyruvate 9
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
2-Phosphoglycerate 2
3-Phosphoglycerate 8
Phosphoglyceromutase 9
Enolase
2 H2O 2
2-Phosphoglycerate 9
Enolase
Figure 9.9 A closer look at glycolysis
2 H2O 2
Phosphoenolpyruvate 2
Phosphoenolpyruvate

34. Pyruvate 2 Phosphoenolpyruvate 2 ADP 10 Pyruvate kinase 2 ATP 2
2 NAD+ 6
Triose phosphate
dehydrogenase 2
NADH 2 P i
+ 2 H+ 2
1, 3-Bisphosphoglycerate
2 ADP 7
Phosphoglycerokinase
2 ATP 2
Phosphoenolpyruvate
2 ADP 2
3-Phosphoglycerate 8 10
Phosphoglyceromutase
Pyruvate kinase
2 ATP 2
2-Phosphoglycerate 9
Enolase
2 H2O
Figure 9.9 A closer look at glycolysis 2
Phosphoenolpyruvate
2 ADP 10
Pyruvate kinase
2 ATP 2
Pyruvate 2
Pyruvate

35. If O2 is present, pyruvates (3 carbon C-H-O) enter the mitochondria.
The Citric Acid Cycle = Krebs Cycle: completes the energy-yielding oxidation of organic molecules
If O2 is present, pyruvates (3 carbon C-H-O) enter the mitochondria.
Before the Krebs Cycle can begin, the pyruvate must be converted to acetyl CoA
Acetyl CoA = Acetate + CoEnzyme A
Acetate = a 2 carbon C-H-O
CoEnzyme A = a carrier molecule
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36. Conversion of pyruvate to acetyl CoA, the junction between glycolysis and the citric acid cycle
CYTOSOL
MITOCHONDRION
NAD+
NADH
+ H+ 2 1 3
Acetyl CoA
Figure 9.10
Pyruvate
Coenzyme A
CO2
Transport protein

37. The citric acid cycle / Krebs cycle takes place within the mitochondrial matrix.
The Krebs cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn (two turns per glucose molecule from glycolysis).
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38. citric acid / Krebs Cycle Pyruvate CO2 NAD+ CoA NADH + H+ Acetyl CoA
Figure 9.11 An overview of the
FADH2
3 NAD+
FAD 3
NADH
+ 3 H+
ADP + P i
ATP

39. The citric acid / Krebs cycle has eight steps, each catalyzed by a specific enzyme.
The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, OAA, forming citrate (citric acid).
The next seven steps break down the citrate and regenerate oxaloacetate,OAA, making the process a cycle.
The NADH and FADH2 produced by the Krebs Cycle carry electrons extracted from food (C-H-O) to the electron transport chain in the mitochondrial cristae membrane.
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40. A closer look at the citric acid / Krebs cycle Citric Acid Cycle
Acetyl CoA
CoA—SH
NADH
+H+ 1
H2O
NAD+ 8
Oxaloacetate
OAA 2
Malate
Citrate
Isocitrate
NAD+
Citric
Acid
Cycle
NADH 3
+ H+ 7
H2O
CO2
Fumarate
CoA—SH
-Keto-
glutarate
Figure 9.12 4 6
CoA—SH
FADH2 5
CO2
NAD+
FAD
Succinate P
NADH i
GTP
GDP
Succinyl
CoA
+ H+
ADP
ATP

41. During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food.
These two electron carriers: NADH and FADH2 donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation.
For the Cell Biology Video ATP Synthase 3D Structure — Side View, go to Animation and Video Files.
For the Cell Biology Video ATP Synthase 3D Structure — Top View, go to Animation and Video Files.
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42. The Pathway of Electron Transport
The electron transport chain is in the cristae membrane of the mitochondrion.
Most of the chain’s components are proteins, which exist in multiprotein complexes.
The carriers alternate reduced and oxidized states as they accept and donate electrons, redox.
Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O (waste).
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43. ETC cristae Free-energy change during electron transport waste
NADH 50 2 e–
NAD+
FADH2 2 e–
FAD
Multiprotein
complexes  40
FMN
FAD
Fe•S 
Fe•S Q

Cyt b
Fe•S 30
Cyt c1 IV
Free energy (G) relative to O2 (kcal/mol)
Cyt c
Cyt a
Cyt a3 20
Figure 9.13 2 e– 10
(from NADH
or FADH2)
2 H+ + 1/2 O2
waste
H2O

44. The electron transport chain generates no ATP
Electrons are transferred from NADH or FADH2 to the electron transport chain, ETC.
Electrons are passed along the cristae membrane through a number of proteins including cytochromes (each with an iron atom) to O2
The electron transport chain generates no ATP
The chain’s function is to break the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts.
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45. Chemiosmosis The Energy-Coupling Mechanism
Electron transfer, redox, in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space  creating a proton H+ gradient.
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 ATP synthesis.
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46. Chemiosomosis
The energy stored in a H+ (proton) gradient, across a membrane couples the redox reactions of the electron transport chain to ATP synthesis.
The H+ gradient is a proton-motive force, emphasizing its capacity to do work.
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47. Chemiosmosis: Energy Coupling - couples the electron transport chain to ATP synthesis
Protein complex
of electron
carriers
Cyt c V Q
 
ATP synthase 
2 H+ + 1/2O2
H2O
FADH2
FAD
NADH
NAD+
Figure 9.16 Chemiosmosis
ADP + P
ATP i
(carrying electrons
from food) H+ 1
Electron transport chain: redox 2
Chemiosmosis
Oxidative phosphorylation

48. An Accounting of ATP Production by Cellular Respiration
During cellular respiration, most energy flows in this sequence:
glucose  NADH  electron transport chain  proton-motive force  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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49. Aerobic Cellular Respiration
ATP yield per molecule of glucose at each stage of cellular respiration
CYTOSOL
Electron shuttles
span membrane
MITOCHONDRION
2 NADH or
2 FADH2
2 NADH
2 NADH
6 NADH
2 FADH2
Citric
Acid
Cycle
Oxidative
phosphorylation:
electron transport
chemiosmosis
Glycolysis 2
Pyruvate 2
Acetyl
CoA
Glucose
+ 2 ATP
+ 2 ATP
+ about 32 or 34 ATP
Figure 9.17
About
36 or 38 ATP
Maximum per glucose:

50. Most cellular respiration requires O2 to produce ATP.
Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen
Most cellular respiration requires O2 to produce ATP.
Glycolysis produces ATP without O2 (in aerobic or anaerobic conditions).
In the absence of O2, glycolysis couples with fermentation or anaerobic respiration to produce ATP.
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51. Fermentation: Anaerobic / No Oxygen Used
Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis.
Two common types are alcohol fermentation and lactic acid fermentation.
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52. In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2
Alcohol fermentation by yeast is used in brewing, winemaking, and baking / $$ commercial uses.
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53. Fermentation Regenerates NAD+ for use in Glycolysis
2 ADP + 2 Pi
2 ATP
Glucose
Glycolysis
2 Pyruvate
2 NAD+
2 NADH 2
CO2
+ 2 H+
2 Ethanol
2 Acetaldehyde
(a) Alcohol fermentation
2 ADP + 2 Pi
2 ATP
Glucose
Glycolysis
Figure 9.18
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
2 Lactate
(b) Lactic acid fermentation

54. 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; meaning there is an O2 debt. This reaction is reversible when O2 is available.
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55. Lactic acid fermentation: reversible if oxygen is available
2 ADP + 2 P
2 ATP i
Glucose
Glycolysis
2 NAD+
2 NADH
+ 2 H+
2 Pyruvate
Figure 9.18b Fermentation
2 Lactate
Lactic acid fermentation: reversible if oxygen is available

56. Fermentation and Aerobic Respiration Compared
Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate
The processes have different final electron acceptors:
an organic molecule (such as pyruvate or acetaldehyde) in fermentation.
O2 in aerobic cellular respiration.
Aerobic cellular respiration nets 38 ATP per glucose molecule; fermentation nets only 2 ATP per glucose molecule.
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57. Micro-organisms
Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2
Facultative anaerobes (yeast and many bacteria) can survive using either fermentation or cellular respiration. In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes.
Obligate aerobes carry out aerobic cellular respiration and require O2
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58. O2 present: No O2 present: Pyruvate as a key juncture in catabolism
Glucose
Glycolysis
CYTOSOL
Pyruvate
O2 present:
Aerobic cellular
respiration
No O2 present:
Fermentation
MITOCHONDRION
Ethanol or
lactate
Acetyl CoA
Figure 9.19
Citric
acid
cycle

59. The Evolutionary Significance of Glycolysis
Glycolysis occurs in nearly all organisms.
Glycolysis probably evolved in ancient prokaryotes before there was oxygen in the atmosphere.
Fermentation evolved to recycle NAD+ back to glycolysis so ATP production continues in the absence of O2
Gycolysis and the Citric Acid Cycle are major intersections to various catabolic and anabolic pathways.
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60. Glycolysis and the citric acid cycle connect to many other metabolic pathways
Gycolysis and the citric acid cycle are major intersections to various catabolic and anabolic pathways.
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61. The Versatility of Catabolism: breaking down
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 which can feed glycolysis or the citric acid cycle.
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62. Fatty acids are broken down by beta oxidation and yield acetyl CoA.
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 (fat has more calories = unit of energy for cell work).
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63. Versatility Proteins Carbohydrates Fats Amino acids Sugars Glycerol
Fatty
acids
Glycolysis
Glucose
Glyceraldehyde-3- P
NH3
Pyruvate
Acetyl CoA
Figure 9.20 The catabolism of various molecules from food
Citric
acid
cycle
Oxidative
phosphorylation

64. Biosynthesis - Anabolic Pathways: Building
The body uses small molecules to build larger more complex molecules.
These small molecules may come directly from food, from glycolysis, or from the citric acid cycle.
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65. Regulation of Cellular Respiration via Feedback Mechanisms
Feedback inhibition is the most common mechanism for control, regulation.
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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66. Regulation: Feedback Inhibition
The control of cellular respiration
Glucose
AMP
Glycolysis
Fructose-6-phosphate
Stimulates +
Phosphofructokinase – –
Fructose-1,6-bisphosphate
Inhibits
Inhibits
Pyruvate
ATP
Citrate
Acetyl CoA
Figure 9.21
Citric
acid
cycle
Oxidative
phosphorylation

67. Review Glycolysis: Inputs Outputs 2 ATP Glycolysis + 2 NADH Glucose 2
Pyruvate

68. Review: Citric Acid / Krebs Cycle
Inputs
Outputs
S—CoA 2
ATP
C O
CH3 2
Acetyl CoA 6
NADH
O C COO
Citric acid
cycle
CH2 2
FADH2
COO 2
Oxaloacetate

69. Chemiosmosis = ATP Synthesis Review: Energy Coupling: INTER- MEMBRANE
SPACE H+
ATP
synthase
ADP + P
ATP i
MITO-
CHONDRIAL
MATRIX H+

70. Proton Motive Force = H+ concentration Gradient
Proton Motive Force = H+ concentration Gradient. As H’s increase, the pH drops in the Intermembrane space; so pH difference Increases across the membrane

71. You should now be able to:
Explain in general terms how redox reactions are involved in energy exchanges.
Name the three stages of cellular respiration; for each, state the region of the eukaryotic cell where it occurs and the products that result.
In general terms, explain the role of the electron transport chain in cellular respiration.
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72. Distinguish between fermentation and anaerobic respiration.
Explain where and how the respiratory electron transport chain creates a proton gradient.
Distinguish between fermentation and anaerobic respiration.
Distinguish between obligate and facultative anaerobes.
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