Lecture 19 PMB

Candice Young
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Chemolithotrophy

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Candice Young
Created by Candice Young about 2 years ago
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Question Answer
chemolithotrophy the use of an inorganic compound as an electron donor
chemolithotrophs vs chemoorganotrophs ETC generating PMF can happen in both chemolithotrophs still need carbon --> usually are autotrophs and take it up from CO2 in the air still need NADH and FADH2 for both!
mixotrophs chemolithotroph that uses organic compounds as a carbon source
energetics of electron donors better electron donors have LOWER E0' ΔG0 = -nFΔE0′ reaction must have a positive ΔE0′ and negative ΔG0 higher ΔE0' --> NAD+/NADH have to perfrom reverse electron flow to generate reducing equivalents
Hydrogen as an electron donor H2 oxidizers can be anaerobes or aerobes --> aerobes get carbon from C fixation BUT can also grow as chemoorganotrophs if organic carbon source present, sugars repress genes for H2 oxidation and carbon fixation (because -organo is preferred)
electron flow in aerobic hydrogen fixation membrane bound hydrogenase passes e- to quinone --> cytochromes --> oxygen, PMF generated powers ATP synthesis cytoplasmic hydrogenase reduces NAD+ to NADH (not the donor here!!) --> no contribution to PMF any H+ can go to membrane or cytoplasm
Winogradsky's observations about chemolithotrophic bacteria 1a) Bacteria grown with abundant H2S contain dark sulfur granules 1b/c) If deprived of H2S, the bacteria lose their sulfur granules, but keep growing. Growth ceases after long time without H2S. If H2S resupplied, sulfur granules reappear. --> H2S oxidized to S0 --> when H2S is absent, S0 in granules can serve as an energy source instead --> when S0 is being used, its oxidized to sulfate and eventually sulfuric acid
S0 as an electron donor H2S --> S0 --> [(1/2)S2O3-]^2 --> [SO4]^-2 hydrogen sulfide --> elemental sulfur --> thiosulfate --> sulfate sulfur oxidizers typically acidophiles because H+ is a byproduct of these rxns
electron flow in oxidation of sulfur compounds electrons from sulfide & sulfite enter at different points, PMF generated by passing H+ onto quinone pool during O2 reduction sulfide donates at lower reduction potential --> yields more PMF electrons start at higher E0' than NAD+/NADH so energy from PMF used to generate reverse e- flow and gain NADH
Iron Oxidizing Bacteria (FeOB) at different pH values -neutral pH in presence of O2 --> Fe2+ spontaneously oxidizes -neutral pH with low O2 --> nitrate reduction may be coupled to biotic Fe oxidation -low pH --> Fe2+ will not oxidize, O2 is only viable acceptor
FeOB bacteria chemolithotrophs that use ferrous iron as energy source coupled to O2/NO3- reduction FeOB at neutral pH must be in anoxic/microoxic environments autotrophs
two types of FeOB bacteria Type 1) Acid mine drainage systems, where oxygen is the electron acceptor (pH < 3) Type 2) Neutral, anoxic or microaerophilic habitats (pH ~7) where Fe2+ oxidation is coupled to nitrate or oxygen reduction.
coloring of iron oxidation at a low pH really high E0' for Fe2+/Fe3+ at a low pH --> only viable electron acceptor is oxygen because reduction potentials are so close, lots of Fe2+ must get used up --> lots of yellow/orange Fe(OH)3 waste builds up
electron flow of iron oxidation at low pH iron remains extracellular, oxidation performed by OM cytochrome e- enter via RcY in periplasm, are at a higher E0' than NAD+/NADH --> RcY must work upstream to reduce NAD+ and generate NADH at expense of PMF
electron flow of iron oxidation at neutral pH at pH 7, Fe3+/Fe2+ E0′ = +0.2 --> oxidation coupled to nitrate or oxygen reduction Fe2+ directly reduces OM cytochrome c --> ETC begings, PMF generated reverse electron flow still needed to make NADH!
cell biology of neutral iron oxidation Fe2+ oxidation not very energy yielding --> organisms must metabolize a lot of iron oxidized iron is insoluble --> forms stalks of exopolysaccarides that are used for excretion and extracellular storage of oxidized iron (prevents crystals from forming)