1. Cellular Respiration and Fermentation7
Cellular Respiration and Fermentation

2. Life Is Work Living cells require energy from outside sources
Some animals, such as the giraffe, obtain energy by eating plants, and some animals feed on other organisms that eat plants
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3. Photosynthesis in chloroplasts Cellular respiration in mitochondria
Energy flows into an ecosystem as sunlight and leaves as heat
Light energy
ECOSYSTEM
Photosynthesis in chloroplasts
Cellular respiration in mitochondria
CO2  H2O
 O2
Organic molecules
ATP powers most cellular work
ATP
Heat energy
Figure 9.2 Energy flow and chemical recycling in ecosystems.

4. Concept 7.1: Catabolic pathways yield energy by oxidizing organic fuels
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 other than O2

5. 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)

6. Redox Reactions: Oxidation and Reduction
Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions
In oxidation, a substance loses electrons, or is oxidized [“OIL”]
In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) [“RIG”]
becomes oxidized (loses electron)
becomes reduced (gains electron)

7. 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

8. 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+ (nicotinamide adenosine dinucleotide, a coenzyme
Each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP

9. 2 e + 2 H+ 2 e + H+ NAD+ H+ NADH Dehydrogenase Reduction of NAD+
Figure 7.4
2 e + 2 H+
2 e + H+
NAD+
NADH H+
Dehydrogenase
Reduction of NAD+
2[H] H+
(from food)
Oxidation of NADH
Nicotinamide
Nicotinamide
(oxidized form)
(reduced form)
Figure 7.4 NAD+ as an electron shuttle
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10. Stepwise Energy Harvest via NAD+ and the Electron Transport Chain
NADH passes the electrons to the Electron Transport Chain of cellular respiration
Unlike an uncontrolled reaction, the electron transport chain passes electrons in a series of steps instead of one explosive reaction
O2 serves as the final electron acceptor at the end of the chain (2 e- and 2 H+ bond with O forming H2O)
The energy yielded is used to regenerate ATP

11. Controlled release of energy release
Figure 7.5
H2 + 1/2 O2
2 H + 1
/2 O2
Controlled
release of
energy
2 H+ + 2 e
ATP
Free energy, G
Free energy, G
ATP
Explosive
release
chain
Electron transport
ATP
2 e 
Figure 7.5 An introduction to electron transport chains
1/2 O2 2 H+
H2O
H2O
(a) Uncontrolled reaction
(b) Cellular respiration
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12. The Stages of Cellular Respiration: A Preview
Harvesting of energy from glucose has three stages
Glycolysis (breaks down glucose into two molecules of pyruvate)
The citric acid cycle (completes the breakdown of glucose)
Oxidative phosphorylation (accounts for most of the ATP synthesis)

13. Electrons carried via NADH Electrons carried via NADH and FADH2
Figure 9.6-3
Electrons carried via NADH
Electrons carried via NADH and FADH2
Oxidative phosphorylation: electron transport and chemiosmosis
Pyruvate oxidation
Glycolysis
Citric acid cycle
Glucose
Pyruvate
Acetyl CoA
CYTOSOL
MITOCHONDRION
Figure 9.6 An overview of cellular respiration.
ATP
ATP
ATP
Substrate-level phosphorylation
Substrate-level phosphorylation
Oxidative phosphorylation

14. Concept 7.2: Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
Glycolysis (“splitting of sugar”) breaks down glucose into two molecules of pyruvate
Glycolysis occurs in the cytoplasm and has two major phases
Energy investment phase
Energy payoff phase
Glycolysis occurs whether or not O2 is present

15. 1, 3-Bisphosphoglycerate
energy investment phase
energy payoff phase
2 NAD+
NADH 2
+ 2 H+
Triose phosphate
dehydrogenase
P i P C
CHOH O
CH2 O–
1, 3-Bisphosphoglycerate
2 ADP
2 ATP
Phosphoglycerokinase
3-Phosphoglycerate
Phosphoglyceromutase
CH2OH H
2-Phosphoglycerate
2 H2O
Enolase
Phosphoenolpyruvate
Pyruvate kinase
CH3 6 8 7 9 10
Pyruvate
Figure 9.8 B
Dihydroxyacetone
phosphate
Glyceraldehyde-
3-phosphate H OH HO
CH2OH O P
CH2O
CH2 C
CHOH
ATP
ADP
Hexokinase
Glucose
Glucose-6-phosphate
Fructose-6-phosphate
Phosphoglucoisomerase
Phosphofructokinase
Fructose-
1, 6-bisphosphate
Aldolase
Isomerase
Glycolysis 1 2 3 4 5
Oxidative
phosphorylation
Citric
acid
cycle
Figure 9.9 A

16. Energy investment phase
Glycolysis Overview
Glycolysis
Citric acid cycle
Oxidative phosphorylation
ATP
2 ATP
4 ATP
used
formed
Glucose
2 ATP + 2 P
4 ADP + 4
2 NAD+ + 4 e- + 4 H +
2 NADH
+ 2 H+
2 Pyruvate + 2 H2O
Energy investment phase
Energy payoff phase
4 ATP formed – 2 ATP used
2 NAD+ + 4 e– + 4 H +
2 ATP Invested
4 ATP generated
2 NAD+ reduced to 2 NADH
Net Products at end of Glycolysis:
2 pyruvate molecules
2 ATP
2 NADH
Figure 9.8

17. Concept 7.3: After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules
In the presence of O2, pyruvate enters the mitochondrion (in eukaryotic cells)
Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA)
Pyruvate
Transport protein
CYTOSOL
MITOCHONDRION
CO2
Coenzyme A
NAD
+ H
NADH
Acetyl CoA 1 2 3
Figure 9.10 Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle.

18. The Citric Acid Cycle
Pyruvate
(from glycolysis,
2 molecules per glucose)
CYTOSOL
PYRUVATE OXIDATION
NAD+
NADH
+ H+
Acetyl CoA
CoA
CITRIC
ACID
CYCLE
FADH2
FAD
ADP + P i
ATP
MITOCHONDRION
3 NAD+
3 NADH
+ 3 H+
CO2
2 CO2
The citric acid cycle, also called the Krebs cycle, completes the break down of pyruvate to CO2
The cycle oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per pyruvate

19. Net Products from Intermediate Step + Citric Acid Cycle:
2 ATP
6 CO2
8 NADH
2 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
Figure 9.12 A closer look at the citric acid cycle.

20. Concept 7.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
The two electron carriers, NADH and FADH2, donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation
The electron transport chain is in the inner membrane (cristae) of the mitochondrion
Most of the chain’s components are proteins, which exist in multiprotein complexes
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.

21. The Pathway of Electron Transport
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 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
Figure 9.13 Free-energy change during electron transport.
It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts

22. 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 protein/enzyme, 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

23. Electron Transport & Chemiosmosis
Protein complex of electron carriers H
Cyt c IV Q
III I
ATP synth- ase II
2 H + 1/2O2
H2O
FADH2
FAD
Figure 9.15 Chemiosmosis couples the electron transport chain to ATP synthesis.
NAD
NADH
ADP  P i
ATP
(carrying electrons from food) H 1
Electron transport chain 2
Chemiosmosis
Oxidative phosphorylation

24. ATP synthase is the enzyme that actually makes ATP
Figure 7.13
Intermembrane
space
Inner
mitochondrial
membrane
Mitochondrial
matrix
ATP synthase is the enzyme that actually makes ATP
INTERMEMBRANE
SPACE H+
Stator
Rotor
Internal
rod
Figure 7.13 ATP synthase, a molecular mill
Catalytic
knob
ADP + P
ATP i
MITOCHONDRIAL MATRIX
(a) The ATP synthase protein complex
(b) Computer model of ATP synthase
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25. An Accounting of ATP Production by Cellular Respiration
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

26. Most cellular respiration requires O2 to produce ATP
Concept 7.5: 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

27. Types of Fermentation
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
2 ADP
2 ATP
Glucose
Glycolysis
2 Pyruvate
2 CO2 2 
2 NADH
2 Ethanol
2 Acetaldehyde
(a) Alcohol fermentation P i
NAD
2 H
Figure 9.17 Fermentation.

28. Types of Fermentation
In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2
(b) Lactic acid fermentation
2 Lactate
2 Pyruvate
2 NADH
Glucose
Glycolysis
2 ATP
2 ADP  2 P i
NAD
2 H
Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce
Figure 9.17 Fermentation.

29. Animation: Fermentation Overview
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30. Comparing Fermentation with Anaerobic and Aerobic Respiration
All 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
The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration
Cellular respiration produces 32 ATP per glucose molecule; Fermentation produces 2 ATP per glucose molecule

31. The Evolutionary Significance of Glycolysis
Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP
Glycolysis is a very ancient process
Glucose
CYTOSOL
Glycolysis
Pyruvate
No O2 present: Fermentation
O2 present: Aerobic cellular respiration
Ethanol, lactate, or other products
Acetyl CoA
MITOCHONDRION
Citric acid cycle

32. The Versatility of Catabolism
Carbohydrates
Proteins
Fatty acids
Amino acids
Sugars
Fats
Glycerol
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Acetyl CoA
Citric acid cycle
Oxidative phosphorylation
Figure 9.19 The catabolism of various molecules from food.

33. 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
Explain where and how the respiratory electron transport chain creates a proton gradient
Distinguish between fermentation and anaerobic respiration