1. Biochemical Pathways: Cellular Respiration
Chapter 6 2-

2. Energy and Organisms
Organisms are classified based on the kind of energy they use.
Autotrophs
Use the energy from sunlight to make organic molecules (sugar)
Use the energy in the organic molecules to make ATP
Heterotrophs
Obtain organic molecules by eating the autotrophs
Autotrophs use photosynthesis.
To use the energy from light to make organic molecules
All organisms use cellular respiration.
To harvest the energy from organic molecules and use it to make ATP 6-
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3. Energy Transformation6-
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4. Cellular Respiration
Respiration is a metabolic pathway of Redox Reactions
Respiration oxidizes carbohydrates and transfers the energy to produce ATP
The type of molecule that is reduced determines the type of respiration
The energy produced is in the form of ATP 6-
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5. Cellular Respiration Three Types of Respiration Aerobic Respiration
Oxygen is reduced to produce water
Anaerobic Respiration
A molecule other that oxygen is reduced
may produce acids, methane, etc.
Fermentation
Another carbohydrate is reduced to produce alcohols 5-
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6. 1. Aerobic Respiration Glucose is Oxidized to become Carbon Dioxide
Oxygen is reduced to become water
The protons and electrons from the oxidation of glucose are used to produce ATPs 5-
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7. C6H12O6 + 6O2 + 38 ADP + 38 P 6 H2O + 6CO2 + 38 ATP
1. Aerobic Respiration
C6H12O6 + 6 O H2O + 6 CO2 + Energy
C6H12O6 + 6O ADP + 38 P H2O + 6CO ATP 5-
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8. 1. Aerobic Respiration Aerobic Respiration is a three stage process:
Stage 1: Glycolysis
Stage 2: The Krebs Cycle
Stage 3: Oxidative Phosphorylation (Electron Transport Chain)
Each of these stages produce ATP
At the end of all three stages, there is a net gain of ~38 ATP’s 5-
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10. Stage 1: Glycolysis
Glycolysis is a 10 step metabolic pathway that cleaves glucose
Glyo-lysis = “splitting glucose”
Glycolysis occurs in the cell’s cytoplasm 6-
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11. Stage 1: Glycolysis
Glycolysis splits glucose to make two pyruvate molecules
Produces 4 ATP molecules
4 ATP made -2 ATP invested = net production of 2 ATP
Reduces 2 NAD+ to make 2 NADH 6-
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12. Stage 1: Glycolysis
During Glycolysis, Glucose (a 6 carbon molecule) is chopped up into 2 Pyruvates (pyruvate is a 3 carbon molecule)
This is a 9 step metabolic process 6-
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13. Figure 6_07

14. 9 Steps of Glycolysis Summary
Glucose is phosphorylated - costs 1 ATP to become Glucose-6-Phosphate
Glucose-6-P is converted to Fructose-6-P
Fructose-6-P is phosphorylated - costs 1 ATP to become Fructose-1,6-bisphosphate
Fructose-1,6-bisphosphate is split into two molecules - each with 3 carbons and a phosphate:
Dihydroxyacetone (DHAP)
Glyceraldehyde-3-Phoshate (G-3-P) 6-
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15. 9 Steps of Glycolysis Summary
G-3-P DHAP
5. G-3-P is phosphorylated to DHAP is phosphorylated to become 1,3-bisphosphoglycerate become 1,3 bisphosphoglycerate
6. 1,3-bisphosphglycerate gives up 6. 1,3-bisphosphoglycerate gives up a a phosphate to an ADP, G-3-P left phosphate to an ADP, G-3-P left
7. G-3-P converted to G-3-P converted to
2-Phosphoglycerate Phospoglycerate
8. 2-Phosphoglycerate converted to Phosphoglycerate converted to Phosphoenolpyruvate Phosphoenolpyruvate
9. Phosphoenolpyruvate gives up 9. Phosphoenolpyruvate gives up its phosphate to an ADP its phosphate to an ADP 6-
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16. Glucose
Pyruvate1 Pyruvate 2
2 ATP
4 ATP, 2 NADH

17. NAD+, NADH
NAD+ is an electron carrier for the redox reactions of cellular respiration
NAD+ accepts 2 electrons and 1 proton to become NADH
Functions of NAD+, NADH
NAD+ is reduced so Redox reactions can occur
NAD+ is used to carry electrons from one part of the cell to another
NAD+ keeps protons out of solution 6-
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18. NAD+, NADH

19. NAD+ Reduced to NADH

20. FAD, FADH2 FAD is very similar to NAD+
It has the same functions of collecting and carrying electrons and protons
FAD can carry 2 Hydrogen atoms
FAD is Reduced to FADH2 6-
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21. Stage 2: Kreb’s Cycle
Also known as The Citric Acid Cycle or the Tricarboxylic Acid (TCA) Cycle
The Krebs cycle is a metabolic pathway that further oxidizes pyruvate
The Krebs Cycle occurs in the Cell membrane of Prokaryotic Cells and in the mitochondria of Eukaryotic Cells
In mitochondria, a multienzyme complex called pyruvate dehydrogenase catalyzes the reaction 6-
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23. Stage 2: Kreb’s Cycle
The Krebs Cycle begins with pyruvate (from Glycolysis)
Remember, there are 2 pyruvates made from each Glucose, so there are 2 Krebs Cycles for every glucose molecule
During a Kreb’s Cycle the pyruvate (a 3 carbon molecule) will be completely oxidized to become 3 carbon dioxide molecules (one carbon atom each) 6-
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24. Cellular Respiration C6H12O6 + 6O2 6H2O + 6CO2 + Energy
Glucose Oxygen Water Carbon Dioxide
The Kreb’s Cycle produces the CO2 that we exhale 5-
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25. Stage 2: Kreb’s Cycle Important steps in the Kreb’s Cycle
Pyruvate is converted to acetyl-CoA
Acetyl-CoA combine with Oxaloacetate (in the mitochondria) to make Citrate
The cycle ends with the production of oxaloacetate, ready for anther turn of the cycle 6-
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26. Figure 6_05

27. Stage 2: Kreb’s Cycle
During the Kreb’s Cycle enough energy is released from one pyruvic acid molecule to produce:
1 ATP
4 NADH from 4 NAD+
1 FADH2 from 1 FAD. 6-
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28. Figure 6_08

29. Glucose 2 ATP Pyruvate1 Pyruvate 2 4 ATP, 2 NADH 2 ATP 1 ATP, 2 CO2
Pyruate converted to Acetyl Co-A
2 ATP
1 ATP, 2 CO2
4 NADH, 1 FADH2

30. Stage 2: Kreb’s Cycle
After glycolysis and the Krebs cycle glucose has been completely oxidized to form:
6 CO2
~5 ATP
~10 NADH
~2 FADH2
We are still short of our ATP goal
The third stage of cellular respiration uses the protons and electrons of the hydrogens on the NADH’s and FADH2’s that were produced during the first 2 stages 6-
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31. Electron Transport Chain NADH, FADH2
Glucose
Pyruvate1 Pyruvate 2
2 ATP
4 ATP, 2 NADH
Pyruate converted to Acetyl Co-A
2 ATP
2 ATP, CO2
8 NADH, 2 FADH2
34 ATP
Electron Transport Chain
NADH, FADH2

32. Stage 3: Electron-Transport System
The electron transport chain (ETC) is a series of membrane-bound electron carrier molecules called cytochromes embedded in the mitochondrial inner membrane
Electrons from NADH and FADH2 are transferred to cytochromes of the ETC
Each cytochrome transfers the electrons to the next cytochromes in the chain 6-
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33. Fig. 7.13a

34. Stage 3: Electron-Transport System
As the electrons are transferred, some electron energy is released with each transfer
This energy is used by the cytochromes to pump protons (H+) across the membrane from the matrix to the inner membrane space
A proton concentration gradient is established 6-
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35. Stage 3: Electron Transport Chain

36. Figure 6_06

37. Stage 3: Electron-Transport System
There are other channel proteins in the membrane known as ATP synthases
ATP synthases provide a channel for protons to rush through by diffusion
The rushing protons provides the energy for ATP synthase to phosphorylate ADP to ATP 6-
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39. Figure 6_06

42. Figure 6_09

43. Cellular Respiration Recall Three Types of Respiration
Aerobic Respiration
Oxygen is reduced to produce water
Anaerobic Respiration
A molecule other that oxygen is reduced
may produce acids, methane, etc.
Fermentation
Another carbohydrate is reduced to produce alcohols 5-
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44. 1. Aerobic Respiration
Oxygen is the final molecule to receive the electrons as they are passed down the Electron Transport Chain
Oxygen is reduced with two electrons and picks up two protons
The result is water:
2O2 + 8e-’s and 8H+’s H2O
Oxygen is the Final Electron Acceptor in Aerobic Respiration 6-
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45. The Electron Transport Chain

46. Electron Transport Chain NADH, FADH2
Glucose
Pyruvate1 Pyruvate 2
2 ATP
4 ATP, 2 NADH
Pyruate converted to Acetyl Co-A
2 ATP
2 ATP, CO2
8 NADH, 2 FADH2
30+ ATP
And H2O
Electron Transport Chain
NADH, FADH2

47. Aerobic Cellular Respiration: Overview6-
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48. Total Yields for Aerobic Cellular Respiration per Glucose Molecule
Glycolysis
2 ATP
2 NADH (converted to 2 FADH2)
Kreb’s cycle
8 NADH
2 FADH2
Electron transport chain
Each NADH fuels the formation of 3 ATP.
8 NADH x 3 ATP = 24 ATP
Each FADH2 fuels the formation of 2 ATP.
4 FADH2 x 2 ATP = 8 ATP
Total ATP= =36 ATP made from the metabolism of one glucose molecule. 6-
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49. 2. Anaerobic Cellular Respiration
Some cells do not require O2 as the final electron acceptor for the electron transport chain
These cells perform Anaerobic Respiration
Anaerobic Respiration produces fewer ATPs per glucose molecule compared to Aerobic Respiration – it is not as efficient and the exact amount of ATP production depends on the organism and the electron acceptors that are used 6-
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50. 2. Anaerobic Cellular Respiration
Anaerobic respiration by methanogens
methanogens use CO2
CO2 is reduced to CH4 (methane)
Anaerobic respiration by sulfur bacteria
inorganic sulphate (SO4) is reduced to hydrogen sulfide (H2S) 6-
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51. 3. Fermentation
Fermentation reduces organic molecules as the final electron acceptors
Ethanol fermentation occurs in yeast
fermentation of sugars produces alcohol
Lactic acid fermentation
occurs in animal cells (especially muscles)
electrons are transferred from NADH to pyruvate to produce lactic acid 6-
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52. Figure 6_10

53. Alcoholic Fermentation
Starts with glycolysis
Glucose is metabolized to pyruvic acid.
A net of 2 ATP is made.
During alcoholic fermentation
Pyruvic acid is reduced to form ethanol.
Carbon dioxide is released.
Yeasts do this
Leavened bread
Sparkling wine 6-
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54. Lactic Acid Fermentation
Starts with glycolysis
Glucose is metabolized to pyruvic acid.
During lactic acid fermentation
Pyruvic acid is reduced to form lactic acid.
No carbon dioxide is released.
Muscle cells have the enzymes to do this, but brain cells do not.
Muscle cells can survive brief periods of oxygen deprivation, but brain cells cannot.
Lactic acid “burns” in muscles. 6-
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55. Metabolizing Other Molecules
Cells will use the energy in carbohydrates first.
Complex carbohydrates are metabolized into simple sugars.
Cells can use the energy in fats and proteins as well.
Fats are digested into fatty acids and glycerol.
Proteins are digested into amino acids.
Cells must convert fats and proteins into molecules that can enter and be metabolized by the enzymes of glycolysis or the Kreb’s cycle. 6-
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56. Fat Respiration Fats are broken down into
Glycerol
Fatty acids
Converted to glyceraldehyde-3-phosphate
Enters glycolysis
Converted to acetylCoA
Enter the Kreb’s cycle
Each molecule of fat fuels the formation of many more ATP than glucose.
This makes it a good energy storage molecule. 6-
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57. Lipids
Amino
Acids
Preparatory Steps

58. Protein Respiration Proteins are digested into amino acids.
Then amino acids have the amino group removed.
Generates a keto acid (acetic acid, pyruvic acid, etc.)
Enter the Kreb’s cycle at the appropriate place 6-
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59. The Interconversion of Fats, Carbohydrates and Proteins6-
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60. Energy Resources
Carbohydrates, fats and proteins can all be used for energy.
Glycolysis and the Kreb’s cycle allow these types of molecules to be interchanged.
If more calories are consumed than used
The excess food will be stored.
Once the organism has all of the proteins it needs
And its carbohydrate stores are full
The remainder will be converted to and stored as fat. 6-
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