chapter 13 Brock - metabolic diversity of microorganisms Flashcards
Distinguish between oxygenic and anoxygenic photosynthesis…
In p/syn, ATP is generated from light and then consumed in the reduction of CO2.
Oxygenic - oxygen is produced (eg cyanobacteria)
Anoxygenic - oxygen is not produced (eg purple and green bacteria)
Photosynthetic pigments?
Chlorophylls and bacteriochlorophylls reside in membranes where the light reactions of p/syn are carried out.
Antenna chlorophylls harvest light energy and transfer it to a reaction centre.
Accessory pigments?
eg carotenoids and phycobilins absorb light and transfer the energy to reaction centre chlorophyll - this broadens the wavelengths usable in p/syn.
Carotenoids also have photoprotective role, preventing photooxidative damage to cells.
ATP generation and CO2 fixation in anoxygenic phototrophs…
Electron transport reactions occur in the p/syn reaction centre of anoxygenic phototrophs, forming a PMF and ATP.
Reducing power for CO2 fixation comes from substances like H2S.
NADH production in purple bacteria requires reverse electron transport.
Just one photosystem in anoxygenic phototrophs.
In oxygenic photosyn…
H2O donates electrons to drive CO2 fixation, and O2 is a by-product.
There are two separate but interconnected photosystems in oxygenic phototrophs: PSI and PSII
Tell me about autotrophy…
autotrophy is supported in most phototrophic and chemolithotrophic bacteria by the Calvin cycle - key role of RubisCO.
Carboxysomes contain crystalline RubisCO - they concentrate CO2, the key substrate for this enzyme.
What are the autotrophic pathways for green sulfur and green nonsulphur bacteria?
green sulphur - reverse citric acid cycle
green nonsulphur - hydroxyproprionate cycle
Energy conservation in chemolithotrophs?
chemolithotrophs oxidise inorganic electron donors to conserve energy and obtain reducing power.
Energy conservation occurs from respiratory processes that generate a PMF. Most chemolithotrophs can grow autotrophically.
What about chemolithotrophic hydrogen bacteria?
use H2 as an electron donor, reducing O2 to H2O.
Enzyme hydrogenase is required to oxidise H2, and H2 also supplies reducing power for the fixation of CO2 in these autotrophs.
And sulphur chemolithotrophs?
Reduced sulphur compounds such as H2S, S2O3 2- (thiosulphate ion) and S zero are electron donors for energy conservation.
The electrons enter ETC, yielding a PMF.
Sulphur chemolithotrophs are also autotrophs and fix CO2 by the Calvin cycle.
And chemolithotrophic iron bacteria?
oxidise Fe2+ as an electron donor.
What if there are no external electron acceptors?
organic compounds can be catabolised anaerobically only by fermentation. Most fermentations require that an energy-rich organic compound be formed that can yield ATP by substrate-level phosphorylation. Redox balanced is achieved by the production of fermentation products.
Explain syntrophy
Two organisms cooperate to degrade a compound that neither can degrade alone. In this process H2 produced by one organism is consumed by the partner. H2 consumption affects the energetics of the reaction carried out by the H2 producer, allowing it to make ATP where it otherwise could not.
What’s the most widely used electon acceptor in energy-yielding metabolism?
Oxygen.
However other organic and inorganic compounds can be used as electron acceptors - lower energy yield in anaerobic respiration but can proceed in environments where oxygen is absent.
Tell me about nitrate’s common role as an electron acceptor
Anaerobic respiration.
Nitrate reduction is catalysed by the enzyme nitrate reductase, reducing NO3- to NO2-.
Many bacteria that use NO3- in anaerobic resp. produce gaseous nitrogen compounds (NO, N2O or N2) as final end products of reduction (denitrification).
