The Krebs cycle reactions are uniform across all life due to inheritance from a common ancestor. While the basic pathways are the same, different lineages have evolved some variations as they have diverged. The eukaryotic version of the Krebs cycle originated from the endosymbiotic bacterium that evolved into the mitochondria in our cells.

1. Evolution connection: The Krebs CycleEvolution connection: The Krebs Cycle
Learning goals:
Students will understand that 1) the uniformity of the Krebs cycle reactions across all life is due to
inheritance from a common ancestor, 2) different lineages have evolved slight variations in the
Krebs cycle reactions as they have diverged, and 3) the eukaryotic version of the Krebs cycle
actually comes from the endosymbiotic bacterium from which our mitochondria evolved.
For the instructor:
This short slide set explains the uniformity of the Krebs cycle across all life using evolutionary
theory. To integrate it best, use these slides immediately after you’ve discussed aerobic respiration.
Each of the following slides comes with a sample script for the instructor. To review this script,
download the PowerPoint file and view the Notes associated with each slide.
Evolution Connection slideshows are provided by Understanding Evolution

2. Evolution connection: The Krebs CycleEvolution connection: The Krebs Cycle
“That’s a lot of reactions. How many of these metabolic
pathways do we need to memorize?”
Not many … why?
Because most organisms you’re familiar with (aerobic
ones) use the Krebs Cycle, electron transport chain, and
oxidative phosphorylation. There’s just one set of
reactions to worry about for them.
Heliobacter pylori photo by Yutaka Tsutsumi, M.D. Professor Department of Pathology Fujita Health University School of Medicine; Blue-green
algae photo by Micrographica.com; Redwood and mountain lion photos by Gerald and Buff Corsi © California Academy of Sciences; Oyster
mushroom photo by Dr. Robert Thomas and Dorothy B. Orr © California Academy of Sciences.

3. But why?
Because these metabolic pathways evolved early in
the history of life and were passed on to many different
lineages.
Aerobic metabolism evolves!
Evolution connection: The Krebs CycleEvolution connection: The Krebs Cycle
Heliobacter pylori photo by Yutaka Tsutsumi, M.D. Professor Department of Pathology Fujita Health University School of Medicine; Blue-green
algae photo by Micrographica.com; Redwood and mountain lion photos by Gerald and Buff Corsi © California Academy of Sciences; Oyster
mushroom photo by Dr. Robert Thomas and Dorothy B. Orr © California Academy of Sciences.

4. These reactions work pretty much the same way
in you, birds, bees, many bacteria, and tons of
other critters!
Evolution connection: The Krebs CycleEvolution connection: The Krebs Cycle

5. The evolution of the Krebs cycle:
Archaea
Bacteria
Eukaryotes
Evolution connection: The Krebs CycleEvolution connection: The Krebs Cycle

6. Where did our
(eukaryotic) version
of the Krebs cycle
come from?
Us
Rickettsia
bacterium
Evolution connection: The Krebs CycleEvolution connection: The Krebs Cycle

7. References:References:
Andersson, S. G. E, et al. 1998. The genome sequence of Rickettsia
prowazekii and the origin of mitochondria. Nature. 396: 133-140.
Huynen, M. A., Dandekar, T., and Bork, P. 1999. Variation and evolution
of the citric acid cycle: a genomic perspective. Trends in Mircrobiology.
7: 281-291.
Evolution Connection slideshows are provided by Understanding Evolution
(understandingevolution.org) and are copyright 2011 by The University of California
Museum of Paleontology, Berkeley, and the Regents of the University of California.
Feel free to use and modify this presentation for educational purposes.