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

2. powers most cellular work
Light energy
ECOSYSTEM
CO2 + H2O
Photosynthesis in chloroplasts
Cellular respiration in mitochondria
Organic molecules
+ O2
ATP
powers most cellular work
Heat

3. Ask a Cell Biologist: What is Cellular Respiration?
Cellular respiration is the aerobic harvesting of energy from organic molecules
It is a catabolic pathway
It contains mostly exergonic reactions that release energy

4. Summary Equation for Cellular Respiration
C6H12O6 + 6O CO2 + 6H2O + ATP + Heat
glucose
oxygen
Carbon
dioxide

5. Greater number of bonds = Greater potential energy
O = C = O
carbon dioxide
glucose

6. Oxidation-Reduction Reactions
Oxidation-Reduction (Redox) reactions
Transfer electrons from one reactant to another by oxidation and reduction

7. Oxidation-Reduction Reactions
In oxidation
A substance LOSES electrons, or is oxidized
In reduction
A substance GAINS electrons, or is reduced

8. becomes oxidized (loses electron) becomes reduced (gains electron)
Example of Redox Reaction
becomes oxidized (loses electron)
becomes reduced (gains electron)
Figure 9.UN01

9. Oxidation of Organic Fuel
During cellular respiration
Glucose is oxidized and oxygen is reduced
It is the movement of hydrogen atoms and their electrons from glucose that are important
C6H12O6 + 6O CO2 + 6H2O + Energy
BECOMES OXIDIZED
BECOMES REDUCED

10. Mitochondrion: Site of Cellular Respiration
Intermembrane space
Outer
membrane
Free
ribosomes
in the
mitochondrial
matrix
Mitochondrial
DNA
Inner
Cristae
Matrix
100 µm

11. Stages of Cellular Respiration
Glycolysis
Breaks down glucose into two molecules of pyruvate (pyruvic acid)
Citric acid cycle (Krebs Cycle)
Completes the breakdown of energy originally in glucose
Electron transport chain
Generates lots of ATP

12. 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

13. Glycolysis Glycolysis Means “splitting of sugar”
Breaks down glucose into pyruvate
Occurs in the cytoplasm of the cell
Ancient pathway
No oxygen required!

14. Energy investment phase
Glycolysis: 2 Phases
Glycolysis
Citric acid cycle
Oxidative phosphorylation
ATP
2 ATP
4 ATP
used
formed
Glucose
2 ADP + 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 +

15. Gradual Harvesting of Energy
Electrons from organic compounds
Are usually first transferred to NAD+, an electron shuttle
NAD+ becomes reduced to NADH as it accepts electrons and H’s from glucose
Dehydrogenase is an enzyme which helps move the electrons by removing 2 H atoms (and their electrons) from glucose and giving them to NAD+
NAD+ + 2H NADH + H+
Dehydrogenase

16. Hydrogen Ions = H+ = Proton
HYDROGEN ATOM

17. Substrate Level Phosphorylation
Both glycolysis and the citric acid cycle
Can generate ATP by substrate-level phosphorylation
ENZYME
ATP
ADP
PRODUCT
SUBSTRATE P +

18. Citric Acid Cycle
The citric acid cycle completes the energy-yielding oxidation of organic molecules
The citric acid cycle
Takes place in the matrix of the mitochondrion

19. Pre-citric Acid Cycle Before the citric acid cycle can begin
Pyruvate must first be converted to acetyl CoA, which links the cycle to glycolysis
CYTOSOL
MITOCHONDRION
NADH
+ H+
NAD+ 2 3 1
CO2
Coenzyme A
Pyruvate
Acetyl CoA S
CoA C
CH3 O
Transport protein O–

20. Oxidative phosphorylation
Citric Acid Cycle
ATP
2 CO2
3 NAD+
3 NADH
+ 3 H+
ADP + P i
FAD+
FADH2
Citric acid cycle
CoA
Acetyl CoA
NADH
CO2
Pyruvate (from glycolysis, 2 molecules per glucose)
Glycolysis
Oxidative phosphorylation
Oxaloacetate
(4 C)
Citrate
(6 C)

21. Electron Transport Chain
Mitochondrial inner membrane proteins (carrier molecules) pass electrons in a series of steps instead of in one explosive reaction
Uses the energy from the electron transfer to form ATP

22. Electron Transport Chain
H2O O2
NADH
FADH2
FMN
FE•S O
FAD
Cyt b
Cyt c1
Cyt c
Cyt a
Cyt a3
2 H + + 1⁄2 I II
III IV
CARRIER MOLECULES 10 20 30 40 50
Free energy (G) relative to O2 (kcl/mol)
At the end of the chain
Electrons are passed to oxygen, forming water
This makes oxygen the final electron acceptor
oxygen

23. Electron transport chain Oxidative phosphorylation
Chemiosmosis
Chemiosmosis and the electron transport chain
Oxidative
phosphorylation.
electron transport
and chemiosmosis
Glycolysis
ATP
Inner
Mitochondrial
membrane H+
P i
Protein complex
of electron
carners
Cyt c I II
III IV
(Carrying electrons
from, food)
NADH
FADH2
NAD+
FAD+
2 H+ + 1/2 O2
H2O
ADP +
Electron transport chain
Electron transport and pumping of protons (H+),
which create an H+ gradient across the membrane
Chemiosmosis
ATP synthesis powered by the flow
Of H+ back across the membrane
synthase Q
Oxidative phosphorylation
Intermembrane
space
mitochondrial
matrix

24. Chemiosmosis At certain steps along the electron transport chain
Electron transfer causes protein complexes to pump H+ from the mitochondrial matrix to the intermembrane space
The resulting H+ gradient
Stores energy
Drives chemiosmosis in ATP synthase

25. But what’s Chemiosmosis really?
Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work
Is referred to as a proton-motive force
But what’s Chemiosmosis really?

26. Chemiosmosis ATP synthase Is the enzyme that actually makes ATP
INTERMEMBRANE SPACE H+
P i +
ADP
ATP
MITOCHONDRIAL MATRIX

27. Following the Electrons (Energy)
During respiration, most energy flows in this sequence
Glucose to NADH to electron transport chain to chemiosmosis to ATP
About 40% of the energy in a glucose molecule
Is transferred to ATP during cellular respiration, making approximately 32 ATP

28. Oxidative phosphorylation: electron transport and chemiosmosis
Review of Stages of Cellular Respiration
Electron shuttles span membrane
MITOCHONDRION
2 NADH or
2 FADH2
2 NADH
2 NADH
6 NADH
2 FADH2
Oxidative phosphorylation: electron transport and chemiosmosis
Glycolysis
Pyruvate oxidation
Citric acid cycle
Glucose
2 Pyruvate
2 Acetyl CoA
Figure 9.16 ATP yield per molecule of glucose at each stage of cellular respiration.
 2 ATP
 2 ATP
 about 26 or 28 ATP
About 30 or 32 ATP
Maximum per glucose:
CYTOSOL

29. So what happens when there is little or no O2?
Without O2, the ETC will cease to operate
BUT glycolysis couples with fermentation to still produce 2 ATP’s
Without the ETC however, fermentation needs an alternate way to generate NAD+

30. Fermentation
Two common types of fermentation are:
Alcohol fermentation
Lactic acid fermentation

31. Alcohol Fermentation Pyruvate is converted to ethanol in two steps
Releases CO2 from pyruvate making acetaldehyde
Acetaldehyde reduced by NADH to ethanol which regenerates NAD+
Many bacteria undergo alcohol fermentation in anaerobic conditions
Alcohol fermentation by yeast is used in brewing, winemaking, and baking

32. Lactic Acid Fermentation
Pyruvate directly reduced by NADH to form lactate (ionized form of lactic acid)
No CO2 released
Fungi and bacteria undergo lactic acid fermentation
Used to make cheese and yogurt
Human muscle cells will undergo lactic acid fermentation when O2 scarce during heavy exercise

33. Summary of Types of Fermentation
2 ADP  2 P i
2 ATP
2 ADP  2 P i
2 ATP
Glucose
Glycolysis
Glucose
Glycolysis
2 Pyruvate 2
NAD 
2 NADH
2 CO2 2
NAD 
2 NADH 
2 H 
2 H
2 Pyruvate
Figure 9.17 Fermentation.
2 Ethanol
2 Acetaldehyde
2 Lactate
(a) Alcohol fermentation
(b) Lactic acid fermentation
Fermentation Overview

34. Pyruvate as a key juncture in catabolism
Glucose
Glycolysis
CYTOSOL
Pyruvate
No O2 present: Fermentation
O2 present: Aerobic cellular respiration
MITOCHONDRION
Ethanol, lactate, or other products
Acetyl CoA
Figure 9.18 Pyruvate as a key juncture in catabolism.
Citric acid cycle
Pyruvate as a key juncture
in catabolism