1. The Evolution of Aerobic Cellular Respiration
Nick Watkins and Kyle Hayes
Biology 346
Christian Brothers University

2. What is Cellular Respiration?
The process in which a cell breaks down sugar or other organic compounds to release energy used for cellular work.
Energy is in the form of ATP.
May be anaerobic or aerobic, depending on the availability of oxygen.

3. Anaerobic- Glycolysis
Glycolysis is the energy-producing conversion of glucose to pyruvate under anaerobic conditions.
Occurs in the cytosol.
The most elemental metabolic pathway.
Almost universal; all living creatures share various sections of this pathway

4. Glycolitic Pathway

5. Aerobic Respiration Requires oxygen to generate ATP.
Is composed of the Krebs Cycle and Electron Transport Chain, which are linked to anaerobic glycolysis
Yields more energy than anaerobic respiration
Where did it come from?

6. History of Earth Hadean Eon Archean Eon Proterozoic Eon
4.5 B.Y.A. – 3.8 B.Y.A
Archean Eon
3.8 B.Y.A. – 2.5 B.Y.A.
Proterozoic Eon
2.5 B.Y.A. – 542 M.Y.A
Phanerozoic Eon
542 M.Y.A. – Present

7. Hadean Eon Earth’s surface was molten due to extreme temperatures
Unable to support life

8. Archean Eon Earth began to cool, and oceans began to form.
Less hostile environments allowed for the emergence of prokaryotic life, which used the process of photosynthesis to supply food.

9. Photosynthetic Bacteria
Some bacteria used hydrogen sulfide (H2S) as their electron donor for photosynthesis
Used by green sulfur bacteria
Does not produce oxygen
H2S + CO2 + Light  Sugars + Sulfur

10. Photosynthetic Bacteria
Cyanobacteria developed a more efficient method of photosynthesis.
Oxygenic Photosynthesis
Occurred ~ 2.8 B.Y.A.
Water (H2O) was used as the electron donor
Oxygen gas (O2) was introduced into the earth’s atmosphere as a by-product
H2O +CO2 +Light  Sugars + O2

11. Great Oxidation Event (GOE)
Occurred ~2.4 Billion Years Ago
Oxygen levels rose due to photosynthetic cyanobacteria
From 1% of today’s atmospheric oxygen concentration to 10% of today’s concentration.

12. Great Oxidation Event

13. Effects of the GOE High oxygen levels were toxic to anaerobes.
Generated a need for oxygen to be utilized.
Led to creation of the Krebs Cycle.
Enable a stratosphereic atmosphere to form that screens out UV radiation, thus enabling expansion of life to ocean surfaces and land.

14. Origination of Krebs Cycle
The Krebs Cycle itself does not immediately depend on oxygen.
The cycle theoretically originated from two metablically different pathways in anaerobes.
One pathway was reductive and one pathway was oxidative.
Both pathways still function in some organisms, such as cyanobacteria and anaerobically grown E. coli bacteria.

15. Reductive Pathway
Pyruvate  Oxaloacetate  Malate  Fumarate Succinate Succinyl-CoA
Coupled to the reactions above was the oxidation of NADH to NAD.
NAD is used in glycolysis, which was necessary for ATP formation
Succinyl-CoA could also be used in synthesizing important amino acids.

16. Oxidative Pathway
Pyruvate  Acetyl-CoA  Citrate cis-Aconitate  Isocitrate  α-ketoglutarate
Reactions in this oxidative arm had the function of providing reduced nucleotides (NADH and NADPH) for carbohydrate synthesis.
α-ketoglutarate could also be used to synthesize amino acids.

17. The Missing Link
In order to connect the two pathways together, an enzyme was needed to convert α-ketoglutarate to Succinyl-CoA.
α-ketoglutarate dehydrogenase was the missing link.
Genetic variant of pyruvate dehydrogenase

18. Complete Krebs Cycle

19. How does the Krebs Cycle Provide Energy?
The Krebs Cycle supplies electrons and associated protons to be transferred to a terminal electron acceptor.
Such electron transfers occur along an electrical potential gradient that provides sufficient energy to let ADP molecules be phosphorylated to ATP

20. Electron Transport
Earlier in evolutionary history, membrane-bound anaerboic respiratory chains used fumarate as a terminal electron acceptor
Some scientist believe sulfur could have been used as an electron acceptor, too.
However, oxygen was more energy efficient and supplied more ATP for the cells

21. Complete Cellular Respiration (Anaerboic + Aerobic)

22. Endosymbiosis
In order for anaerobes to survive, they had to find a way to utilize the energy of aerobes
The answer = Endosymbiosis
According to this theory, the ancestral anaerobic cell engulfed aerobic organisms in order to survive the oxygen-abundant atmosphere
So what’s the advantage?

23. Aerobic vs. Anaerobic
Complete aerobic oxidation of a molecule of glucose produces maximally 38 ATP
Anaerobic glycolysis only produces ATP
Thus, aerobic respiration gave a selective advantage because of it’s high energy yields, paving the way for multicellular life forms with high energy demands.

24. Sources
1. Strickberger, M. W Evolution. 3rd ed. Jones and Bartlett Publishers, Sudbury, MA, pp
2. Hemp, J., Pace, L.A. Evolution of Aerobic Respiration. Astrobiology Science Conference LPI Contribution No. 1538, p
3. Lane, N Life’s a Gas. New Scientist, 6 Feb:
4. Van Hellemond, J. J., Van der Klei, A., Van Weelden, S. W. H., Tielens, A. G. M Biochemical and Evolutionary Aspects of Anaerobically Functioning Mitochondria. Phil. Trans. R. Soc. Lond. B :
5. Falkowski, P., Godfrey, L.V Electrons, Life and the Evolution of Earth’s Oxygen Cycle. Phil. Trans. R. Soc. B :
6. Andersson, S.G.E., Karlberg, O., Canback, B., Kurland, C. G On the Origin of Mitochondria: a Genomics Perspective. Phil. Trans. R. Lond. B :