1. Cellular Respiration and Fermentation
Chapter 9
Cellular Respiration and Fermentation

2. Photosynthesis in chloroplasts Cellular respiration in mitochondria
Overview: Life Is Work
living cells require energy from outside sources
some animals obtain energy by eating plants
some animals feed on other organisms that eat plants
energy flows into an ecosystem as sunlight and leaves as heat
photosynthesis generates O2 and organic molecules - used in cellular respiration
cells use chemical energy stored in organic molecules to regenerate ATP - powers work
Light energy
ECOSYSTEM
Photosynthesis in chloroplasts
Cellular respiration in mitochondria
CO2  H2O
 O2
Organic molecules
ATP powers most cellular work
ATP
Heat energy
For the Discovery Video Space Plants, go to Animation and Video Files.

3. Catabolic Pathways and Production of ATP
catabolism - the breakdown of organic molecules is exergonic
potential energy is stored in chemical bonds
result of the arrangement of electrons in these bonds
breaking of these bonds releases this potential energy
catabolic processes – anaerobic, fermentation & aerobic
Fermentation = partial degradation of sugars that occurs without O2
Aerobic respiration consumes organic molecules (fuel) and O2 to yield ATP
most prevalent and efficient catabolic process
carried out by eukaryotic cells and most prokaryotic
Anaerobic respiration - similar to aerobic respiration but consumes compounds other than O2
prokaryotic cells

4. C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat)
Cellular respiration includes both aerobic and anaerobic respiration
glycolysis, Kreb’s cycle and electron transport chain
is often used to refer to aerobic respiration
uses carbohydrates, fats, and proteins
most cellular respiration starts with the sugar glucose
C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat)
breakdown of glucose is exergonic
ΔG = -686 kcal/mol
creates ATP – powers the work of endergonic processes

5. Redox Reactions: Oxidation and Reduction
How do catabolic pathways decompose glucose and yield energy
through the transfer of electrons = oxidation-reduction reactions or Redox reactions
relocation of electrons releases stored potential energy
this released energy is ultimately used to synthesize ATP

6. becomes oxidized (loses electron) becomes reduced (gains electron)
The Principle of Redox
redox reactions = chemical reactions that transfer electrons between reactants
oxidation = a substance loses electrons, or is oxidized
reduction = a substance gains electrons, or is reduced (the amount of positive charge is reduced)
the electron donor is called the reducing agent
e.g. Xe- = reduces Y
the electron receptor is called the oxidizing agent
e.g. Y = oxidizes Xe by removing its electrons
becomes oxidized
becomes reduced
becomes oxidized (loses electron)
becomes reduced (gains electron)

7. Methane (reducing agent) Oxygen (oxidizing agent)
Reactants
Products
Energy
Water
Carbon dioxide
Methane (reducing agent)
Oxygen (oxidizing agent)
becomes oxidized
becomes reduced
the problem becomes when you have covalent bonds
i.e. not all redox reactions involve the complete transfer of electrons from one substance to another
but change the degree of electron sharing in covalent bonds
an example is the reaction between methane and O2
in methane – covalent electrons are shared equally between the C and the Hs of methane and the two Os of O2 - they are equally electronegative
BUT - O is a very electronegative element – one of the most potent oxidizing agents
when CO2 is formed - electrons are shared less equally between C and O – the O “pulls” on the electrons harder – so the carbon has lost electrons (becomes oxidized)
the electrons are also shared less equally in the water – O has gained electrons & become reduced
Figure 9.3 Methane combustion as an energy-yielding redox reaction.

8. Methane (reducing agent) Oxygen (oxidizing agent)
Reactants
Products
Energy
Water
Carbon dioxide
Methane (reducing agent)
Oxygen (oxidizing agent)
becomes oxidized
becomes reduced
energy must be added to pull an electron away from an atom
the more electronegative an atom is – the more energy will be required for it to become oxidized
like rolling a ball uphill
methane is not very electronegative – little energy is required for it to become oxidized
an electron loses potential energy as it shifts from a less electronegative atom to a more electronegative atom
just as a ball loses potential energy when it rolls downhill
it releases this loss of potential energy as chemical energy - used for work
Figure 9.3 Methane combustion as an energy-yielding redox reaction.

9. an electron loses potential energy as it shifts from a less electronegative atom to a more electronegative atom
a redox reaction that moves electrons closer to oxygen – releases chemical energy
e.g. form CO2 from methane – electrons move from the Carbon (when in methane) closer to the Oxygen of CO2 = ENERGY
Figure 9.3 Methane combustion as an energy-yielding redox reaction.

10. Oxidation of Organic Fuel Molecules During Cellular Respiration
During cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced
becomes oxidized
becomes reduced
the carbon atoms of glucose are not very electronegative
lose their potential energy when they combine with O2 to form CO2 – move away from C and closer to O (more electronegative)
in general – molecules with carbons are good sources of potential energy
electrons will shift toward more electronegative atoms like oxygen

11. Oxidation of Organic Fuel Molecules During Cellular Respiration
becomes oxidized
becomes reduced
in respiration – the oxidation of glucose (loss of electrons) transfers electrons to a lower energy state (lower G)
as electrons move from a less electronegative compound (sugar) to a more electronegative compound (CO2)
this liberates energy – used to make ATP
only the activation energy barrier prevents the transfer of electrons to the lower energy state
body temperature is not high enough to reach this activation energy barrier
done by enzymes
main sources of ATP in cells – carbohydrates and fats

12. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
so oxidizing glucose produces more electronegative compounds and releases energy
if you release this energy all at once – not very efficient
in cellular respiration - glucose and other organic molecules are broken down to these electronegative compounds in a series of steps
at key steps – electrons are stripped from glucose and eventually transferred to O2
in typical oxidation reactions - electrons that are transferred travel with a proton (H+)

13. NAD+ BUT electrons are not transferred directly
first transferred to NAD+ - functions as a coenzyme
NAD = nicotinamine adenine dinucleotide
as an electron acceptor- NAD+ functions as an oxidizing agent during cellular respiration
each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

14. How does NAD+ trap electrons?
Nicotinamide (oxidized form)
NAD
(from food)
Dehydrogenase
Reduction of NAD
Oxidation of NADH
Nicotinamide (reduced form)
NADH
How does NAD+ trap electrons?
transfer is dependent upon a kind of enzyme called a Dehydrogenase
dehydrogenases remove a pair of hydrogen atoms and a pair of electrons
ACTUALLY – dehydrogenase removes a hydride ion/H- (two electrons and a proton) and a proton (H+)
Figure 9.4 NAD as an electron shuttle.

15. Dehydrogenase one electron gets transferred onto the N+ of NAD  N
Nicotinamide (oxidized form)
NAD
(from food)
Dehydrogenase
Reduction of NAD
Oxidation of NADH
Nicotinamide (reduced form)
NADH
Figure 9.UN04
one electron gets transferred onto the N+ of NAD  N
the second electron gets attached to the C4 carbon directly across from the N
this is REDUCTION and produces a reduced form of NAD (NAD-)
one proton attaches to the nicotinamide ring  NADH
the other is released into the surroundings
Figure 9.UN04 In-text figure, p. 166
Dehydrogenase

16. the electrons lose very little of their potential energy when they are transferred to NAD+
each NADH represents stored energy that can be tapped to make ATP when the electrons “fall” down an energy gradient to oxygen
(a) Uncontrolled reaction
(b) Cellular respiration
Explosive release of heat and light energy
Controlled release of energy for synthesis of ATP
Free energy, G
H2  1/2 O2
2 H 
1/2 O2
H2O
2 H+  2 e
2 e
2 H+
ATP
Electron transport chain
(from food via NADH)
cellular respiration uses an Electron Transport Chain to break this fall into several energy releasing steps
NADH shuttles the electrons to the top of the hill
O2 captures these electrons along with H+ to form water

17. electron transfer from NADH to O2 = -53 kcal/mol
use of several redox reactions to allow electrons to fall from one carrier to another
(a) Uncontrolled reaction
(b) Cellular respiration
Explosive release of heat and light energy
Controlled release of energy for synthesis of ATP
Free energy, G
H2  1/2 O2
2 H 
1/2 O2
H2O
2 H+  2 e
2 e
2 H+
ATP
Electron transport chain
(from food via NADH)
each “downhill” carrier is more electronegative than its “uphill” carrier & capable of oxidizing it
electrons eventually fall down the ETC to a far more stable location in an oxygen atom

18. BioFlix: Cellular Respiration
© 2011 Pearson Education, Inc. 18

19. The Stages of Cellular Respiration
Harvesting of energy from glucose has three stages:
Glycolysis –anaerobic process that breaks down glucose into two molecules of pyruvate
cytosol
the citric acid cycle (Kreb’s Cycle) - completes the breakdown of glucose
starting material = acetyl CoA (derived from pyruvate)
matrix of the mitochondria (cytosol in prokaryotes)
Oxidative phosphorylation = ETC and chemiosmosis
generates most of the ATP
powered by redox reactions
inner mitochondrial membrane
(plasma membrane in prokaryotes)
Electrons carried via NADH
Electrons carried via NADH and FADH2
Citric acid cycle
Pyruvate oxidation
Acetyl CoA
Glycolysis
Glucose
Pyruvate
Oxidative phosphorylation: electron transport and chemiosmosis
CYTOSOL
MITOCHONDRION
ATP
Substrate-level phosphorylation
Oxidative phosphorylation

20. 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 cycle by substrate-level phosphorylation
ALSO = some of the steps of Glycolysis and Citric Acid Cycle are redox reactions involving dehydrogenases removing protons and electrons and creating NADH
for each molecule of glucose degraded to CO2 and water by respiration- the cell makes up to 32 molecules of ATP
Substrate
Product
ADP P
ATP
Enzyme

21. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Energy Investment Phase
Glucose
2 ADP  2 P
4 ADP  4 P
Energy Payoff Phase
2 NAD+  4 e  4 H+
2 Pyruvate  2 H2O
2 ATP used
4 ATP formed
2 NADH  2 H+
Net
2 ATP
2 NADH  2 H+
4 ATP formed  2 ATP used
Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate
Glycolysis has two major phases
Energy investment phase
Energy payoff phase
Glycolysis occurs whether or not O2 is present

22. Hexokinase – transfers a phosphate group from ATP
Figure 9.9-1
Glycolysis: Energy Investment Phase
ATP
Glucose
Glucose 6-phosphate
ADP
Hexokinase 1
Hexokinase – transfers a phosphate group from ATP
to glucose  glucose-6-phosphate
- makes the glucose it more chemically reactive
Figure 9.9 A closer look at glycolysis.

23. Phosphogluco- isomerase
Figure 9.9-2
Glycolysis: Energy Investment Phase
ATP
Glucose
Glucose 6-phosphate
Fructose 6-phosphate
ADP
Hexokinase
Phosphogluco- isomerase 1 2
2. Phosphoglucoisomerase – enzyme that catalyzes the rearrangement of G6P into fructose-6-phosphate
Figure 9.9 A closer look at glycolysis.

24. Phosphogluco- isomerase Phospho- fructokinase
Figure 9.9-3
Glycolysis: Energy Investment Phase
ATP
Glucose
Glucose 6-phosphate
Fructose 6-phosphate
Fructose 1,6-bisphosphate
ADP
Hexokinase
Phosphogluco- isomerase
Phospho- fructokinase 1 2 3
3. Phosphofructokinase – transfers a phosphate group from ATP to the opposite end of F6P  fructose-1,6-bisphosphate
-phosphofructokinase is the rate-limiting enzyme for the glycolytic portion of cellular respiration
Figure 9.9 A closer look at glycolysis.

25. Phosphogluco- isomerase Phospho- fructokinase
Figure 9.9-3
Glycolysis: Energy Investment Phase
ATP
Glucose
Glucose 6-phosphate
Fructose 6-phosphate
Fructose 1,6-bisphosphate
ADP
Hexokinase
Phosphogluco- isomerase
Phospho- fructokinase 1 2 3
-phosphofructokinase catalyzes an irreversible step in glycolysis
-same is true for hexokinase and pyruvate kinase
-its activity is regulated by the reversible binding of an allosteric
effector
-inhibited by high levels of ATP – acts to lower its affinity for
fructose-6-phosphate
-ATP binds to the enzyme at a regulatory site – changes the shape of
the enzyme’s catalytic site
-reduced binding to F6P results
-enzyme can also can be inhibited through a drop in pH
Figure 9.9 A closer look at glycolysis.

26. 4. Aldolase – cleaves the 6 carbon F-1,6-BP into two 3-Carbon
Figure 9.9-4
Glycolysis: Energy Investment Phase
ATP
Glucose
Glucose 6-phosphate
Fructose 6-phosphate
Fructose 1,6-bisphosphate
Dihydroxyacetone phosphate
Glyceraldehyde 3-phosphate
To step 6
ADP
Hexokinase
Phosphogluco- isomerase
Phospho- fructokinase
Aldolase
Isomerase 1 2 3 4 5
Figure 9.9 A closer look at glycolysis.
4. Aldolase – cleaves the 6 carbon F-1,6-BP into two 3-Carbon
sugars  dihydroxyacetone phosphate and glyceraldehyde-3-phosphate - are isomers of each other
-DHAP is a aldose, G3P is a ketose
-if this could reach equilibrium – 96% would be DHAP
-BUT G3P is used as soon as it is formed – never reaches Eq.

27. -the following reactions actually run twice!!! (two molecules of
Figure 9.9-4
Glycolysis: Energy Investment Phase
ATP
Glucose
Glucose 6-phosphate
Fructose 6-phosphate
Fructose 1,6-bisphosphate
Dihydroxyacetone phosphate
Glyceraldehyde 3-phosphate
To step 6
ADP
Hexokinase
Phosphogluco- isomerase
Phospho- fructokinase
Aldolase
Isomerase 1 2 3 4 5
Figure 9.9 A closer look at glycolysis.
5. Triose phosphate Isomerase – converts DAHP  G3P immediately after G3P is used
-G3P forms the main glycolytic pathway so the isomerase funnels DHAP into this pathway by converting it into G3P
-the following reactions actually run twice!!! (two molecules of
G3P are produced)

28. 1,3-Bisphospho- glycerate Triose phosphate dehydrogenase
Figure 9.9-5
Glycolysis: Energy Payoff Phase
2 NADH
2 NAD
+ 2 H
2 P i
1,3-Bisphospho- glycerate 6
Triose phosphate dehydrogenase
6. Triose phosphate Dehydrogenase
-also known as Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)
-first step: removes 2 electrons and 2 protons from G3P and transfers them to NAD+ to make NADH
-so G3P is oxidized and NAD+ is reduced
-transfer of electrons to a more electronegative compound (NAD+) releases energy
-second step: energy released by this redox reaction is used for phosphorylation of G3P  1,3-bisphosphoglycerate
Figure 9.9 A closer look at glycolysis.

29. 7. Phosphoglycerokinase:
Figure 9.9-6
Glycolysis: Energy Payoff Phase
2 ATP
2 NADH
2 NAD
+ 2 H
2 P i
2 ADP
1,3-Bisphospho- glycerate
3-Phospho- glycerate 2
Phospho- glycerokinase 6 7
Triose phosphate dehydrogenase
7. Phosphoglycerokinase:
– 1,3-BPG is unstable and has a high energy potential
-allows for the transfer of the phosphate group from C1 to ADP 
ATP is made via Substrate level Phosphorylation
-1,3-BPG is oxidized  3-phosphoglycerate
(when the phosphate group is bound to 1,3-BPG - one electron is pulled closer to the O of the sugar – O is more electronegative than P
– when the P group is released, it takes it’s electron & the sugar loses it – i.e. oxidized)
Figure 9.9 A closer look at glycolysis.

30. 8. Phosphoglyceromutase:
Figure 9.9-7
Glycolysis: Energy Payoff Phase
2 ATP
2 NADH
2 NAD
+ 2 H
2 P i
2 ADP
1,3-Bisphospho- glycerate
3-Phospho- glycerate
2-Phospho- glycerate 2
Phospho- glycerokinase
Phospho- glyceromutase 6 7 8
Triose phosphate dehydrogenase
8. Phosphoglyceromutase:
transfer of the remaining phosphate group from C3 to C2  2-phosphoglycerate
Figure 9.9 A closer look at glycolysis.

31. phosphoenolpyruvate (PEP)
Figure 9.9-8
Glycolysis: Energy Payoff Phase
2 ATP
2 NADH
2 NAD
+ 2 H
2 P i
2 ADP
1,3-Bisphospho- glycerate
3-Phospho- glycerate
2-Phospho- glycerate
Phosphoenol- pyruvate (PEP) 2
2 H2O
Phospho- glycerokinase
Phospho- glyceromutase
Enolase 6 7 8 9
Triose phosphate dehydrogenase
9. Enolase: extracts a water molecule from the sugar and forms a double bond between C2 and C3 
phosphoenolpyruvate (PEP)
Figure 9.9 A closer look at glycolysis.

32. 10. Pyrvuate kinase: PEP is unstable and has a high energy potential
Figure 9.9-9
Glycolysis: Energy Payoff Phase
2 ATP
2 NADH
2 NAD
+ 2 H
2 P i
2 ADP
1,3-Bisphospho- glycerate
3-Phospho- glycerate
2-Phospho- glycerate
Phosphoenol- pyruvate (PEP)
Pyruvate 2
2 H2O
Phospho- glycerokinase
Phospho- glyceromutase
Enolase
Pyruvate kinase 6 7 8 9 10
Triose phosphate dehydrogenase
10. Pyrvuate kinase: PEP is unstable and has a high
energy potential
-allows the transfer of the remaining P group to ADP
for the second round of Substrate Level Phosphorylation  ATP is made
-this final step of glycolysis produces Pyruvate
Figure 9.9 A closer look at glycolysis.

33. Glycolysis Energy Tally
2 NADH from glycolysis = 4 ATP from ETC
4 ATP made through substrate phosphorylation
2 ATP consumed

34. After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules
glycolysis releases less than a quarter of the chemical energy in glucose
most energy is still contained in pyruvate
oxidation of pyruvate releases the remaining energy
in eukaryotes and in the presence of O2- pyruvate enters the mitochondrion - the oxidation of glucose is completed
takes place in the cytosol in prokaryotes

35. Transition Phase: Oxidation of Pyruvate to Acetyl CoA
before the citric acid cycle can begin- pyruvate must be converted to acetyl Coenzyme A (acetyl CoA)
links glycolysis to the citric acid cycle
this transition step is carried out by a multienzyme complex in the matrix that catalyzes three reactions = pyruvate dehydrogenase complex
requires a transport pump to bring pyruvate into the matrix of the mitochondria
powered by the H+ gradient of the electron transport chain
Pyruvate
Transport protein
CYTOSOL
MITOCHONDRION
CO2
Coenzyme A
NAD
+ H
NADH
Acetyl CoA 1 2 3
removal of the carboxyl group as a molecule of CO2  2 carbon compound
oxidation into acetate & transfer of electrons to NAD+
coenzyme A attached via its sulfur atom  acetyl coA

36. Transition Phase Energy Tally
2 NADH from pyruvate oxidation = 6 ATP from ETC

37. 1. ethanol 2. lactate 3. acetyl coA 4. PEP  glucose 5. amino acids
the production of lactate or ethanol is a reduction reaction and regenerates NAD+
this sustains glycolysis by replenshing NAD+

38. The Citric Acid Cycle citric acid cycle- also called the Krebs cycle
Pyruvate
NAD
NADH
+ H
Acetyl CoA
CO2
CoA
2 CO2
ADP + P i
FADH2
FAD
ATP
3 NADH
3 NAD
Citric acid cycle
+ 3 H
citric acid cycle- also called the Krebs cycle
completes the break down of pyruvate to CO2
two more CO2 produced per pyruvate
6 total (including 2 from transition)
generates 1 GTP/ATP, 3 NADH, and 1 FADH2 per turn
turns once for every pyruvate made
2 pyruvates are made for each glucose

39. The Citric Acid Cycle
The citric acid cycle has eight steps, each catalyzed by a specific enzyme
two carbons (red) enter in the reduced form as an acetyl group
two different carbons (blue) leave as the oxidized form of CO2
The acetyl group of acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate
The next seven steps decompose the citrate back to oxaloacetate, making the process a cycle
The NADH and FADH2 produced by the cycle relay electrons extracted from food to the electron transport chain

40. Addition of Acetyl coA to oxaloacetate to produce citrate 1
Acetyl CoA
Citrate
Citric acid cycle
CoA-SH
Oxaloacetate
Addition of Acetyl coA to oxaloacetate to produce citrate
-catalyzed by Citrate synthase or CoA Synthase
Figure 9.12 A closer look at the citric acid cycle.

41. -intermediate step creates cis-Aconitate (unstable)1
Acetyl CoA
Citrate
Isocitrate
Citric acid cycle
H2O 2
CoA-SH
Oxaloacetate
2. Isomerization of citrate to iso-citrate: dehydration reaction, followed by a hydration step creates this isomer of citrate
-intermediate step creates cis-Aconitate (unstable)
Figure 9.12 A closer look at the citric acid cycle.

42. -creates an unstable intermediate that loses a CO2 (oxidation)
Figure
Acetyl CoA
CoA-SH 1
H2O
Oxaloacetate 2
Citrate
Isocitrate
NAD
Citric acid cycle
NADH 3
+ H
CO2
-Ketoglutarate
Figure 9.12 A closer look at the citric acid cycle.
3. Oxidation of isocitrate alpha-ketoglutarate: : 1st of the 4 redox reactions in the citric acid cycle
-creates an unstable intermediate that loses a CO2 (oxidation)
-coupled to the reduction of NAD+ to NADH
- catalyzed by isocitrate dehydrogenase

43. -loss of another CO2 in the oxidation reaction 1
Acetyl CoA
Citrate
Isocitrate
-Ketoglutarate
Succinyl CoA
Citric acid cycle
NADH
+ H
NAD
H2O 3 2 4
CoA-SH
CO2
Oxaloacetate
4. Oxidation of a-ketoglutarate  succinyl coA: 2nd of the 4 redox reactions in the citric acid cycle
-loss of another CO2 in the oxidation reaction
-reduction of NAD+ to NADH
-CoA is added through an unstable bond – ΔG is -8kcal/mol
catalyzed by a-ketoglutarate dehydrogenase
3 enzyme complex
reactions similar to the creation of acetyl coA
Figure 9.12 A closer look at the citric acid cycle.

44. -in other cells – ATP is made at this step 1
Acetyl CoA
Citrate
Isocitrate
-Ketoglutarate
Succinyl CoA
Succinate
Citric acid cycle
NADH
ATP
+ H
NAD
H2O
ADP
GTP
GDP
P i 3 2 4 5
CoA-SH
CO2
Oxaloacetate
5. Conversion to succinate: the energy released by the cleavage of the CoA powers the phosphorylation of GDP  GTP
-the GTP can be used to generate another ATP through phosphate transfer
-catalyzed by Succinyl CoA synthase (similar enzyme as that added CoA to oxaloacetate)
-in other cells – ATP is made at this step
-bacteria, plants & some animal cells
Figure 9.12 A closer look at the citric acid cycle.

45. 6. Oxidation to Fumarate – reduction of a FAD  FADH2
NADH 1
Acetyl CoA
Citrate
Isocitrate
-Ketoglutarate
Succinyl CoA
Succinate
Fumarate
Malate
Citric acid cycle
NAD
FADH2
ATP
+ H
H2O
ADP
GTP
GDP
P i
FAD 3 2 4 5 6 7 8
CoA-SH
CO2
Oxaloacetate
6  8: Oxidation of succinate to oxaloacetate: regeneration of oxaloacetate is done in three steps
6. Oxidation to Fumarate – reduction of a FAD  FADH2
-FAD is used because the free energy change is not enough to reduce NAD+
7. Hydration to Malate – addition of water rearranges fumarate into malate
8. Oxidation to oxaloacetate – coupled to reduction of another NAD+  NADH
Figure 9.12 A closer look at the citric acid cycle.

46. CITRIC ACID CYCLE TALLY -for each acetyl coA entering the cycle
NADH 1
Acetyl CoA
Citrate
Isocitrate
-Ketoglutarate
Succinyl CoA
Succinate
Fumarate
Malate
Citric acid cycle
NAD
FADH2
ATP
+ H
H2O
ADP
GTP
GDP
P i
FAD 3 2 4 5 6 7 8
CoA-SH
CO2
Oxaloacetate
CITRIC ACID CYCLE TALLY
-for each acetyl coA entering the cycle
3 NAD+ reduced to NADH (6 total = 18 ATP)
1 FAD reduced to FADH2 (2 total = 4 ATP)
production of 1 GTP (2 total = 2 ATP in some cells)
generation of 2 CO2 (4 total)
Figure 9.12 A closer look at the citric acid cycle.

47. Cellular Respiration = Theoretical Energy
Glucose contains 686 kcal/mol
ATP contains 7.3 kcal/mol
36 ATP harvested from each molecule of glucose
2 from glycolysis – substrate level phosphorylation
2 from Kreb’s cycle – converted from GTP (not done by all cells)
32 from the ETC & NADH/FADH2
2 NADH – glycolysis
8 NADH – transition + Kreb’s
2 FADH2 – Kreb’s
note: NADH in the cytosol is only worth 2ATP; NADH in the mitochondria are worth 3 ATP
36 moles of ATP x 7.3 kcal/mol = 263 kcal trapped as ATP
efficiency : 263/720 = 36% efficiency in converting the potential energy of glucose into chemical energy
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.

48. 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 donate their electrons to the electron transport chain - powers ATP synthesis via oxidative phosphorylation
donation of electrons to O2 powers the phosphorylation of ADP  ATP
enzyme of the electron transport chain is in the inner mitochodrial membrane (cristae)
most of the chain’s components are multiprotein complexes associated with carriers
carriers alternate reduced and oxidized states as they accept and donate electrons
electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O
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. 48

49. the electron transport chain generates no ATP directly
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
H2O
NAD
Multiprotein complexes
(originally from NADH or FADH2) I II
III IV 50 40 30 20 10
Free energy (G) relative to O2 (kcal/mol)
FMN
FeS
FAD Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
the electron transport chain generates no ATP directly
ATP is generated as a result in the drop of free energy as the electrons “fall” from one protein complex to the next
the release free energy is broken up into smaller steps through the development of separate protein complexes
the “fall” is spontaneous as the uphill complex has a higher G value vs. the downhill
as the electrons fall they reduce the next downhill complex
eventually O2 is reduced
Figure 9.13 Free-energy change during electron transport.

50. NADH
FADH2
2 H + 1/2 O2
2 e
2 e
H2O
NAD
Multiprotein complexes
(originally from NADH or FADH2) I II
III IV 50 40 30 20 10
Free energy (G) relative to O2 (kcal/mol)
FMN
FeS
FAD Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
the ETC couples electron transfer to the generation of a proton-motive force for ATP synthesis
Figure 9.13 Free-energy change during electron transport.

51. four protein complexes coupled to specific prosthetic groups:
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
H2O
NAD
Multiprotein complexes
(originally from NADH or FADH2) I II
III IV 50 40 30 20 10
Free energy (G) relative to O2 (kcal/mol)
FMN
FeS
FAD Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
four protein complexes coupled to specific prosthetic groups:
Complex I – NADH dehydrogenase
flavoprotein
Complex II – Succinate dehydrogenase
adds additional electrons to the ETC from succinate via FADH2
Complex III – Cytochrome bc1
also know as cytochrome c reductase
Complex IV – Cytochrome c oxidase
complexes I, II and III have iron-sulfur groups (heme groups) that are critical to their function
Figure 9.13 Free-energy change during electron transport.

52. QH2 carries 2 electrons Complex I – NADH dehydrogenase
FADH2
2 H + 1/2 O2
2 e
2 e
H2O
NAD
Multiprotein complexes
(originally from NADH or FADH2) I II
III IV 50 40 30 20 10
Free energy (G) relative to O2 (kcal/mol)
FMN
FeS
FAD Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
Complex I – NADH dehydrogenase
flavoprotein because one of its prosthetic groups is flavin mononucleotide (FMN)
4H+ pumped into intermembrane space
1. 2 electrons transferred to FMN  FMNH2 (reduction in one step)
2. Oxidation of FMNH2 (in 2 steps)  QH2 (ubiquinol)
the two electrons moved via a Fe-S group  Q (ubiquinone)
step A: 1st electron converts Q  semiquinone
step B: 2nd electron converts semiquinone into ubiquinol (QH2)
QH2 is lipid soluble and diffuses through the membrane to Complex III
Figure 9.13 Free-energy change during electron transport.
QH2 carries 2 electrons

53. This complex can “leak” electrons to O2 and create superoxide radicals
Complex III – Cytochrome bc1
transfers its electrons via Q-Cycle
4H+ pumped as a result
QH2 binds to the Qo site on the complex – two electrons take two pathways:
path 1. one electron move to Fe-S group and then to a c1 site
electron then reduces the cytochrome c1 carrier and then a cytochrome c
path 2. 2nd electron moves to a cytochrome bL site  a bH site –> the Qi site of the complex
a molecule of Q has bound the Qi site – accepts the electron from bH  becomes the unstable semiquinone/Q- (not pictured)
ANOTHER CYCLE IS NEEDED – another QH2 binds the Qi site – 1st electron reduces the 2nd cyto c and the 2nd electron travels via bL/bH to reduce semiquinone to QH2
this QH2 can cycle back to deliver its electrons to Qo – continuous cycle
so 2 cytochrome c’s are reduced and transferred to Complex IV
This complex can “leak”
electrons to O2 and
create superoxide radicals
(water-soluble &
found in the
intermembrane
space)
Figure 9.13 Free-energy change during electron transport.
THIS PROTEIN COMPLEX
NEEDS 2 MOLECULES OF
QH2 TO RUN (4 electrons)
-moves 4H+

54. Complex IV – Cytochrome c Oxidase
4 molecules of cyto c deliver their electrons to this complex
4 electrons are transferred to cyto a group  cyto a3 group  O2 to create two molecules of water
2 H+ are pumped by this complex
water production:
2e- + 2H+ + ½ O2  H20
4e- + 4H+ + O2  2H20
this complex can be inhibited by cyanide
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
H2O
NAD
Multiprotein complexes
(originally from NADH or FADH2) I II
III IV 50 40 30 20 10
Free energy (G) relative to O2 (kcal/mol)
FMN
FeS
FAD Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
Figure 9.13 Free-energy change during electron transport.

55. Complex II –Succinate Dehydrogenase
transfers the electrons produced through the reduction of FAD  FADH2
2 electrons moved to Q to create QH2
NADH
FADH2
2 H + 1/2 O2
2 e
2 e
H2O
NAD
Multiprotein complexes
(originally from NADH or FADH2) I II
III IV 50 40 30 20 10
Free energy (G) relative to O2 (kcal/mol)
FMN
FeS
FAD Q
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
Figure 9.13 Free-energy change during electron transport.

56. Chemiosmosis: The Energy-Coupling Mechanism
Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space
H+ then moves back across the membrane, passing through the proton, 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 cellular work

57. The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis
The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work
Protein complex of electron carriers
(carrying electrons from food)
Electron transport chain
Oxidative phosphorylation
Chemiosmosis
ATP synth- ase I II
III IV Q
Cyt c
FAD
FADH2
NADH
ADP  P i
NAD H
2 H + 1/2O2 2 1
H2O
ATP
Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis.

58. ATP-synthase – comprised of a:
stator – two “½ channels” for H+ binding
rotor – multiple H+ binding sites
internal rod
catalytic knob
1. H+ ion flowing down its gradient
enters the 1st half-channel in the stator
2. H+ ions enter binding sites within the
rotor
-binding of the H+ ions changes the rotors
shape so it rotates
3. each H+ ion makes one complete turn
before leaving the rotor and passing through
the second half-channel within the stator
4. spinning of the rotor, rotates the rod and then the
knob below it
5. rotation of the knob activates catalytic sites that
add a P group to ADP
INTERMEMBRANE SPACE
Rotor
Stator H
Internal rod
Catalytic knob
ADP +
P i
ATP
MITOCHONDRIAL MATRIX
Figure 9.14 ATP synthase, a molecular mill.

59. 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 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP
Electron shuttles span membrane
MITOCHONDRION
2 NADH
6 NADH
2 FADH2 or
 2 ATP
 about 26 or 28 ATP
Glycolysis
Glucose
2 Pyruvate
Pyruvate oxidation
2 Acetyl CoA
Citric acid cycle
Oxidative phosphorylation: electron transport and chemiosmosis
CYTOSOL
Maximum per glucose:
About 30 or 32 ATP

60. An Accounting of ATP Production by Cellular Respiration
There are several reasons why the number of ATP is not known exactly
1. the ratio of NADH molecules to ATP is not a whole number – phosphorylation and redox are not directly coupled to each other
2. NADH made in the cytosol cannot enter the mitochondria
must use a “shuttle” to take these electrons into the matrix
number of ATP made depends on whether FAD or NAD+ is used as that shuttle
3. use of the proton-motive force can be used for other mitochondrial work
powers the uptake of pyruvate from the cytosol
if this motive force was used exclusively for oxidative phosphorylation = 30 to 32 ATP made
Electron shuttles span membrane
MITOCHONDRION
2 NADH
6 NADH
2 FADH2 or
 2 ATP
 about 26 or 28 ATP
Glycolysis
Glucose
2 Pyruvate
Pyruvate oxidation
2 Acetyl CoA
Citric acid cycle
Oxidative phosphorylation: electron transport and chemiosmosis
CYTOSOL
Maximum per glucose:
About 30 or 32 ATP

61. Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen
Most cellular respiration requires O2 to produce ATP
Without O2, the electron transport chain will cease to operate
In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP
Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example sulfate
Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP

62. Animation: Fermentation Overview Right-click slide / select “Play”
© 2011 Pearson Education, Inc. 62

63. Two common types are alcohol fermentation and lactic acid fermentation
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
in alcohol fermentation - pyruvate is converted to ethanol in two steps, with the first releasing CO2
acetylaldehyde  ethanol is catalyzed by Alcohol Dehydrogenase 1
Alcohol fermentation by yeast is used in brewing, winemaking, and baking
humans can take the ethanol and convert it back into a sugar using Alcohol Dehydrogenase  Acetylaldehyde
2 ADP
2 ATP
Glucose
Glycolysis
2 Pyruvate
2 CO2 2 
2 NADH
2 Ethanol
2 Acetaldehyde
(a) Alcohol fermentation
(b) Lactic acid fermentation
2 Lactate P i
NAD
2 H

64. 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
2 ADP
2 ATP
Glucose
Glycolysis
2 Pyruvate
2 CO2 2 
2 NADH
2 Ethanol
2 Acetaldehyde
(a) Alcohol fermentation
(b) Lactic acid fermentation
2 Lactate P i
NAD
2 H

65. Comparing Fermentation with Anaerobic and Aerobic Respiration
all three pathways use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food
in all three - NAD+ is the oxidizing agent that accepts electrons during glycolysis (i.e. that gets reduced)
BUT these processes have different final electron acceptors:
aerobic respiration – O2
anaerobic respiration – other atoms
fermentation - pyruvate or acetaldehyde
Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

66. Ethanol, lactate, or other products
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
In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes
Glucose
CYTOSOL
Glycolysis
Pyruvate
No O2 present: Fermentation
O2 present: Aerobic cellular respiration
Ethanol, lactate, or other products
Acetyl CoA
MITOCHONDRION
Citric acid cycle

67. Oxidative phosphorylation
Glycolysis and the citric acid cycle connect to many other metabolic pathways
Carbohydrates
Proteins
Fatty acids
Amino acids
Sugars
Fats
Glycerol
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Acetyl CoA
Citric acid cycle
Oxidative phosphorylation

68. Biosynthesis (Anabolic Pathways)
the body uses small molecules to build other substances
these small molecules may come directly from food, from glycolysis, or from the citric acid cycle
about 10 of the 20 amino acids can be made by converting substrates from the citric acid cycle
rest are called essential amino acids
glucose can be made from pyruvate – gluconeogenesis
fatty acids can be made from acetyl coA

69. Regulation of Cellular Respiration via Feedback Mechanisms
Phosphofructokinase
Glucose
Glycolysis
AMP
Stimulates  
Fructose 6-phosphate
Fructose 1,6-bisphosphate
Pyruvate
Inhibits
ATP
Citrate
Citric acid cycle
Oxidative phosphorylation
Acetyl CoA
feedback inhibition is the most common mechanism for control
if ATP concentration begins to drop, respiration speeds up
control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway
key point of regulation:
phosphofructokinase and ATP/AMP + citrate
hexokinase
pyruvate kinase