Lecture 22

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Lecture 22
2010-11-14 17:25:29

Bacterial Metabolic diversity -2
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  1. Photosynthesis
    • Use of sunlight to generate proton motive force
    • Energy of PMF is used to make ATP and reduce NAD to NADH or NADP to NADPH
    • The ATP and NAD(P)H are used to fix CO2 into organic molecules with which make cellular material
  2. Phototrophs
    • Obtain their energy for ATP synthesis from sunlight
    • Photoautotroph- CO2 as carbon source, H2S or H2O as the source of electrons and protons for reducing NAD
    • Photoheterotroph- Organic molecules as carbon source and as source of electrons and protons for reducing NAD
  3. Anoxygenic Photosynthesis
    H2S (or other reduced compounds, but not H2O) as the reductant; yields sulfur (So) or sulfate (SO42-)
  4. Oxygenic Photosynthesis
    • H2O as the reductant; yields oxygen
    • Carried out by only cyanobacteria (and Eukarya phototrophs- plants, algae, and certain protists)
  5. Chlorophyll
    • Pigments that absorb light and release the energy as electrons
    • Similar in structure to the heme portion of cytochromes, but with Mg in place of Fe
    • Chl a (chlorophyll a)- in oxygenic phototrophs
    • Bchl a (bacterial chlorophyll a)- in anoxygenic phototrophs
  6. Photosynthetic Membranes
    • Bchl in embedded in cytoplasmic membrane
    • Many bacteria have extensive internal photosynthetic membrane systems
    • These are extensions of the cytoplasmic membrane and are seen as vesicles or membrane stacks
  7. Reaction Centers and Antenna Pigments
    • Reaction centers participate directly in conversion of light energy into ATP
    • Antenna pigments collect light and channel it to the reaction center
  8. Accessory Pigments
    • Carotenoids- give cells colors (red, purple, brown)
    • 1. Help transfer light energy to RC
    • 2. Protect photosynthetic apparatus by absorbing excess light
    • Phycobilins- accessory pigments associated with proteins
    • - Phycobiliproteins help collect different wavelengths of light and transfer that light energy to RC
  9. Anoxygenic Photosynthesis
    • Bchl-based light-mediated ATP synthesis via a series of electron carriers in the membrane
    • Main RC component is P870 (a complex of chlorophylls and protein)
  10. Oxygenic Photosynthesis- Cyanobacteria
    • Two separate but linked reactions
    • Each has a distinct form of reaction center chlorophyll a
    • PSI- chl a absorbs longer wavelenghts of light
    • PSII- chl a absorbs shorter wavelengths of light and releases oxygen
  11. Autotrophy
    Assimilation of CO2 as a carbon source (carbon fixation)- Calvin cycle
  12. Kinds of Photosynthetic Bacteria
    • Purple phototrophic bacteria- anoxygenic photosynthesis, bacteriochlorophyll; and various different carotenoid pigments
    • Purple sulfur bacteria- use H2S as an electron donor- produce granules of elemental sulfur in the cell, found in illuminated anoxic zones of lakes where H2S accumulates (Chromatium)
    • Purple nonsulfur bacteria- can use H2S but at lower concentrations that purple sulfur bacteria, some can grow anaerobically in the dark using fermentation and anaerobic respiration (Rhodopseudomonas)
  13. Kinds of Photosynthetic Bacteria contd.
    • Green sulfur bacteria- anoxygenic, obligately anaerobic, utilize H2S as an electron donor, excrete sulfur as globules outside the cell (Chlorobium)
    • Green nonsulfur bacteria- anoxygenic, filamentous, thermophilic, photoautotrophic or photoheterotrophic (Chloroflexus- can grow anaerobically in the dark as chemoorganotroph)
    • Gram-positive bacteria- Heliobacteria, anoxygenic phototrophs; strictly anaerobic soil bacteria; fix nitrogen and can grow by fermentatively
    • Cyanobacteria- oxygenic photosynthesis; very large and diverse group of microbes