1. Cellular Respiration - Conclusion
The Electron Transport Chain
and Oxidative Phosphorylation

2. Oxidative phosphorylation: when electron transport is coupled to ATP synthesis through chemiosmosis
NADH and FADH2 (from glycolysis and the citric acid cycle) donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation.

3. The electron transport chain is in the inner membrane (cristae) of the mitochondrion.

4. Figure 9.13
NADH 50
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.
Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O.
2 e
NAD
FADH2
2 e
FAD
Multiprotein complexes I 40
FMN II
FeS
FeS Q
III
Cyt b 30
FeS
Cyt c1 IV
Free energy (G) relative to O2 (kcal/mol)
Cyt c
Cyt a
Cyt a3 20 10
2 e
(originally from NADH or FADH2)
2 H + 1/2 O2
H2O

5. Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to O2.
The electron transport chain generates no ATP directly.
What is its purpose then?

6. 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, ATP synthase.
ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP.
This is an example of chemiosmosis, the use of potential energy in a H+ gradient to drive cellular work.

7. INTERMEMBRANE SPACE H Stator Rotor Internal rod Catalytic knob ADP +
Figure 9.14
INTERMEMBRANE SPACE H
Stator
Rotor
Internal rod
Catalytic knob
ADP +
P i
ATP
MITOCHONDRIAL MATRIX

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

9. The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work.
The potential energy from diffusion of H+ across the membrane powers the synthesis of ATP.

10. Summary
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 36 ATP.
What happens to the rest of the energy?
It’s given off as heat.

11. Cellular Respiration

12. What if there’s no oxygen?
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: electron transport chain with an electron acceptor other than O2 (often sulfate)
Fermentation: substrate-level phosphorylation (like glycolysis)

13. Fermentation
Fermentation = glycolysis + recycling of NAD+ (to use for more glycolysis)
alcohol or lactic acid

14. Compare + Contrast Both do glycolysis
Both reduce NAD+ (electron acceptor)
Final electron receptor is different
Cellular Respiration: O2
Fermentation: pyruvate or acetaldehyde
Produce different amounts of ATP
Cellular respiration = 32 ATP per glucose
Fermentation = 2 ATP per glucose

15. Ethanol, lactate, or other products
Figure 9.18
Glucose
Glycolysis
CYTOSOL
Pyruvate
No O2 present: Fermentation
O2 present: Aerobic cellular respiration
MITOCHONDRION
Ethanol, lactate, or other products
Acetyl CoA
Citric acid cycle

16. Other Fuel Molecules
Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration – not just glucose.
Carbohydrates  glycolysis
Proteins (amino acids)  glycolysis or the citric acid cycle
Fats
Glycerol  glycolysis
Fatty acids  acetyl CoA (Citric Acid Cycle)
An oxidized gram of fat produces more than twice as much ATP as an oxidized gram of carbohydrate.

17. Oxidative phosphorylation
Figure 9.19
Proteins
Carbohydrates
Fats
Amino acids
Sugars
Glycerol
Fatty acids
Glycolysis
Glucose
Glyceraldehyde 3- P
NH3
Pyruvate
Acetyl CoA
Citric acid cycle
Oxidative phosphorylation

18. With 1 molecule of glucose, cellular respiration produces 36-38 ATP molecules.