1. Evolution 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 (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.

2. Evolution 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 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 Cycle

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

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

7. 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.