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6: Fueling and Building Cells

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    Chapter 6 BSC 3271 Learning Outcomes

    (some are repeated from Chapter 5)

    • Identify the two types of metabolic process by which chemoheterotrophs obtain energy
    • Explain why a terminal electron acceptor is necessary for energy-generating metabolic processes (respiration or fermentation).
    • Distinguish between the two mechanisms (__________ phosphorylation and _____________ phosphorylation) by which ATP is formed in cells
    • Differentiate between fermentation and respiration based on the following questions:
      • Is it always, sometimes, or never an anaerobic process?
      • By what mechanism is ATP formed (substrate-level, oxidative)?
      • Is the terminal electron acceptor always, sometimes, or never organic?
      • Is the terminal electron acceptor always, sometimes, or never inorganic?
      • Is the terminal electron acceptor generated within the cell or obtained from the external environment?
      • Is an electron transport chain used to generate energy?
    • Compare the amount of ATP produced from glucose by aerobic respiration, anaerobic respiration, and fermentation (see table 5.4) (most, in between, or least; don't worry about exact numbers).
    • Given a metabolic pathway, determine if it is a usable fermentation pathway based on 1) does it produce a net gain of ATP and 2) are all NADH re-oxidized to NAD+.
    • If given an energy-generating metabolic process (either described or in a figure), determine whether it is fermentation, aerobic respiration, or anaerobic respiration.
    • Sketch the processes of lactic acid fermentation and respiration (general, including cellular location).
    • Identify common products of bacterial fermentations, including the kind of molecule always produced in bacterial fermentations.
    • Explain how oxidation/reduction of electron carriers can be linked to pumping of protons across a membrane.
    • Explain where the electrons for the electron transport chain come from in an organism using glucose as an energy source and on what molecule those electrons are carried.  Consider both the original chemical source of the electrons and the metabolic pathways that harvest those electrons.
    • Define proton motive force and explain why it is necessary for ATP synthesis through respiration.
    • Describe the generation of ATP by ATP synthase (oxidative phosphorylation).
    • Identify the main difference between aerobic and anaerobic respiration.
    • Describe how the metabolites of glycolysis and the Krebs (TCA) cycle can be used by the cell for purposes other than energy generation
    • Describe the biosynthesis of amino acids by both amination and transamination
    • Explain nitrogen fixation, how it differs from the way most organisms obtain nitrogen for their cells, and what domains of organisms nitrogen fixation is limited to.
    • Give one example of a nitrogen fixer and explain its direct role in food production.

    As discussed in Chapter 4, all organisms require both carbon (the cell's main building material) and energy (to build the cell's structures and perform other metabolic functions).  Although microbes can use either organic carbon (heterotrophs) or carbon dioxide (autotrophs) for carbon and light (phototrophs) or chemical energy (chemotrophs), all pathogens are chemoheterotrophs.  The host which they are infecting is their source of both carbon and energy.  Most microbes used for food production are also chemoheterotrophs.  Therefore, we will focus on how chemoheterotrophs fuel and build their cells.

    Chemoheterotrophs can generate ATP from organic molecules using two different mechanisms: respiration and fermentation.  In both mechanisms, the electrons stored on NADH in the metabolic reactions of Central Metabolism are "dumped" on a final electron acceptor, thus regenerating NAD+ to be used again in Central Metabolism.  In respiration, the transfer of electrons from NADH to the final electron acceptor (obtained from the environment) occurs via an electron transport chain (ETC).  The ETC concurrently creates a proton gradient which is used by ATP synthase to generate ATP through oxidative phosphorylation.  In fermentation, however, the transfer of electrons from NADH to the final electron acceptor (a metabolite formed within the cell) does not generate ATP in addition to that already produced by substrate-level phosphorylation.

    The precursor metabolites generated through Central Metabolism are used to build the important molecules (and eventually structures) of the cell in anabolic pathways.  Sometimes additional macronutrients are required to build the molecules of the cell, which cells acquire in various ways.  For example, amino acids require nitrogen for their amino group.  Although there are hundreds of critical anabolic pathways in cells, the acquisition of nitrogen and synthesis of amino acids will be used as an example of anabolism in bacteria.

    • 6.1: Respiration
      Respiration begins when electrons are transferred from an electron donor through a series of chemical reactions to a final inorganic electron acceptor obtained from the environment (either oxygen in aerobic respiration or non-oxygen inorganic molecules in anaerobic respiration). In chemoheterotrophs the electron donors are NADH and FADH2 which carry electrons from glycolysis and the TCA cycle, but in chemoautotrophs the electron donor is another source of chemical energy such as hydrogen sulfide
    • 6.2: Fermentation
      Fermentation uses an organic molecule as a final electron acceptor to regenerate NAD+ from NADH so that glycolysis can continue. Fermentation does not involve an electron transport system, and no ATP is made by the fermentation process directly. Fermenters make very little ATP—only two ATP molecules per glucose molecule during glycolysis. Microbial fermentation processes have been used for the production of foods and pharmaceuticals, and for the identification of microbes.
    • 6.3: Catabolism of Lipids and Proteins
      Collectively, microbes have the ability to degrade a wide variety of carbon sources besides carbohydrates, including lipids and proteins. The catabolic pathways for all of these molecules eventually connect into glycolysis and the Krebs cycle. Several types of lipids can be microbially degraded. Triglycerides are degraded by extracellular lipases, releasing fatty acids from the glycerol backbone. Phospholipids are degraded by phospholipases, releasing fatty acids and phosphorylated head groups.
    • 6.4: Photosynthesis and the Importance of Light
      Heterotrophic organisms ranging from E. coli to humans rely on the chemical energy found mainly in carbohydrate molecules. Many of these carbohydrates are produced by photosynthesis, the biochemical process by which phototrophic organisms convert solar energy (sunlight) into chemical energy. Although photosynthesis is most commonly associated with plants, microbial photosynthesis is also a significant supplier of chemical energy, fueling many diverse ecosystems.
    • 6.5: Biogeochemical Cycles
      Energy flows directionally through ecosystems, entering as sunlight for phototrophs or as inorganic molecules for chemoautotrophs. The six most common elements associated with organic molecules—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath earth’s surface
    • 6.6: Anabolism

    Thumbnail: "Making Traditionally Fermented Pickles" by Chiot's Run is licensed under CC BY-NC 2.0