Card Set Information
final micro test
The harnessing of photoexcited electrons to power cell growth
-A single-protein, light-driven proton pump
-Found in halophilic archaea
-A homolog, proteorhodopsin, is found in marine proteobacteria.
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.
The energy for photosynthesis derives from
The photoexcitation of a light-absorbing pigment.
All forms of photolysis share a common design:
2.Reaction center complex
3.Electron transport system
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.
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.
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
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
- 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.
Takes up bicarbonate (HCO3–), which is then immediately converted to CO2 by carbonic anhydrase.
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).
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
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.
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.