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Evolutionconnection krebs

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  • You might be thinking to yourself “Geez. That’s a lot of reactions. Do I really have to memorize all those? And, even worse, do I have to memorize different sets of reactions for different organisms? After all, all living things have to recharge their ATP in some way . . .” It’s true that we want you to understand the important role that the Krebs cycle plays in metabolism and to understand the reactions that compose it. However, lucky for you, there’s only one set of those reactions for you to memorize.
    (click)
    Why is that?
    (click)
    Because most of the organisms you’re familiar with – from simple bacteria to mammals, plants to fungi) – all rely on the same set of reactions (the set we just studied) to finish the process of breaking down glucose and generating ATP.
  • Why do all those very different organisms (from single-celled bacteria, to photosynthesizing plants, to predatory animals) rely on the same set of reactions? Wouldn’t you expect them each to have a unique way of generating ATP that particularly suits their lifestyle?
    Any ideas? (respond as appropriate)
    (click)
    Because of evolution! All life on Earth is related . . .
    (click)
    and aerobic metabolism evolved quite early in the history of life.
    All these different organisms inherited the same metabolic pathways (with only slight variations) from their common ancestor.
    (click)
  • (click)
    The main message is that all life on Earth is related, and we all inherited the same set of chemical reactions from our common ancestors. The reactions of the Krebs cycle are very similar in you, birds, bees, many bacteria, and tons of other critters!
    So if you’re tired of memorizing chemical reactions, you can thank evolution for not producing more variations in these reactions!!
  • Over millions of years the Krebs cycle has evolved some, but not too much. We can chart that evolution on a phylogeny. On the left is a diagram that represents the different reactions in the Krebs cycle.
    (compare to the diagram style for the Krebs cycle used in class previously)
    On the right is a phylogeny showing major branches of the tree of life and which parts of the Krebs cycle each lineage has. You can see that lineages of Archaea . . .
    (click)
    and bacteria . . .
    (click)
    have many components of the Krebs cycle that we studied in class. This complex set of reactions did not pop into being all at once, but evolved over the course of evolutionary time, with different components arising at different points.
    You can also see that the Eukaryotes . . .
    (click)
    have all the components of the Krebs cycle that we studied in class.
  • We humans are eukaryotes.
    (click)
    Where did our version of the Krebs cycle come from? Well, it didn’t evolve in the way you might expect—via small changes from the ancestor of eukaryotes.
    Does anyone remember where in the cell the Krebs cycle takes place? (respond appropriately)
    The Krebs cycle takes place in the mitochondrion. Eukaryotes got mitochondria in an interesting way—via endosymbiosis. A close relative of the Rickettsia bacterium (the bacteria that cause Typhus, among other things) invaded (or was eaten by) our ancestor—and wound up living permanently inside another organism.
    (click)
    When that ancient bacterium (the Rickettsia relative) invaded another bacterium (our ancestor), it brought it’s own DNA and it’s own version of the Krebs cycle with it. So we eukaryotes are actually co-opting the Krebs cycle of another organism for our own metabolism!
    We’ll learn more about endosymbiosis and evolution when we talk about the parts of the cell.
  • Transcript

    • 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.