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Phototrophy is
The harnessing of photoexcited electrons to power cell growth
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Bacteriorhodopsin is
- -A single-protein, light-driven proton pump
- -Found in halophilic archaea
- -A homolog, proteorhodopsin, is found in marine proteobacteria.
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Retinal
- Is linked to lysine residue and absorbs photon and shift from trans to cis.
- Surround by seven alpha helices of bacteriorphodopsin in alternating directions.
- The cycle of excitation and relaxation back to the trans form is coupled to pumping of 1H+ from the cytoplasm across the membrane.
- The proton gradient thus generated drives ATP synthesis by a typical F1Fo ATP synthase.
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The energy for photosynthesis derives from
The photoexcitation of a light-absorbing pigment.
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All forms of photolysis share a common design:
- 1.Antenna system
- 2.Reaction center complex
- 3.Electron transport system
- 4.Energy carriers
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Photosystem I
- Found in chlorobia, “green sulfur” bacteria
- Separates electrons associated with hydrogens from H2S or an organic electron donor such as succinate, or even from reduced iron (Fe2+)
- Electrons are ultimately transferred to NAD+ or NADP+.
- - The reduced carrier (NADH or NADPH) provides reductive energy for CO2 fixation and biosynthesis.
- Bacteria using PSI also generate a net proton gradient to drive ATP synthesis.
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Photosystem II
- Found in alphaproteobacteria, “purple nonsulfur” bacteria
- Separates an electron from bacteriochlorophyll itself
- Electrons are then transferred to an ETS.
- Ultimately, an electron is returned to bacteriochlorophyll.
- This process, which generates ATP, is called cyclic photophosphorylation.
- PSII, unlike PSI, provides no direct way to make NADH or NADPH for reductive biosynthesis.
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Oxygenic Z Pathway
- Found in cyanobacteria and chloroplasts
- Includes homologs of PSI and PSII
- Eight photons are absorbed and two electron pairs are removed from 2H2O, ultimately producing O2.
- Oxygenic photosynthesis forms 3 ATP + 2 NADPH per 2 H2O photolyzed and O2 produced.
- - The ATP and NADPH are used to fix CO2 into biomass
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Calvin cycle
- 1. Carboxylation and splitting: 6C → 2[3C]
- - Ribulose 1,5-bisphosphate condenses with CO2 and H2O to form a 6C molecule, which immediately splits into two 3-phosphoglycerate (PGA) molecules.
- - Reactions are catalyzed by Rubisco.
- 2. Reduction of PGA to G3P
- - The carboxyl group of PGA is phosphorylated by ATP, and then hydrolyzed and reduced by NADPH.
- - This generates glyceraldehyde 3-phosphate.
- 3. Regeneration of ribulose
- 1,5-bisphosphate
- - One of every six G3P is converted to glucose.
- - The other five molecules enter a series of reactions that regenerate three molecules of ribulose 1,5- bisphosphate.
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Carboxysomes
Takes up bicarbonate (HCO3–), which is then immediately converted to CO2 by carbonic anhydrase.
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The reductive, or reverse, TCA cycle:
- Uses 4–5 ATPs to fix four molecules of CO2 and generate one oxaloacetate
- Reduction is performed by NADPH or NADH and by reduced ferredoxin (FDH2).
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Reductive acetyl-CoA pathway
- Used by anaerobic soil bacteria, autotrophic sulfate reducers, and methanogens
- Two CO2 molecules are condensed through converging pathways to form the acetyl group of acetyl-CoA.
- Carbon monoxide is an intermediate.
- Reducing agent is H2 instead of NADPH
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Biosynthesis of Fatty Acids
- is managed by the fatty acid synthase complex.
- Molecules of acetyl-CoA are carboxylated to malonyl-CoA.
- The coenzyme A is replaced by acyl carrier protein (ACP) making malonyl-ACP.
- Malonyl-ACP condenses with the growing chain.
- The growing chain now contains a ketone, which is reduced to CH2 by 2 NADPH.
- Successive addition can continue many times to build a saturated fatty acid of indefinite length.
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Regulation of Fatty Acid Synthesis
- Acetyl-CoA carboxylase regulates its own transcription.
- Starvation blocks fatty acid synthesis through the “stringent response.”
- Low temperature favors unsaturated fatty acids by inducing expression of the dehydratase enzyme.
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