MICRO LEC. Exam 2
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How is Photochemistry mediated?
By membrane-embedded reaction centers containing chlorophyll and other electron carriers
In procaryotes where are the photosythetic membranes?
In cell membrane, invaginations of cell membrane, or in internal membranes called thylakoids (cyanobacteria).
What do Photosynthetic membranes contain?
Reaction centers with prosthetic groups that contain chemicals that can be reversibly oxidized/reduced.
What are reaction centers associated with?
What do Chlorophyll molecules have at reaction center?
pigment antenna to absorb light (other chlorophylls, phycobilins)
Antenna pigments in cyanobacteria are associated with what which is? and have what?
phycobillisomes; the green sulfur bacteria, have antenna pigments in chlorosome
Photosynthesis is based on what?
The cyclic transformations of chlorophyll through three different states
The reaction center undergoes what? (photsythesis)
A cyclical process of excitation, oxidation, and reduction.
How to electrons pass --> how?
Electrons pass from excited chlorophyll molecules pass to prosthetic groups of the reaction center (oxidation). Chlorophyll then acts as a powerful oxidizing agent.
What does Cyclic photophosphorylation generate?
energy in the light with no material input
Electron donors and noncyclic photophosphorylation are needed for what?
autotrophic growth only, not for energy metabolism
Autotrophic growth requires what?
an exogenous electron donor; in cyclic process, the reaction center acts as both donor and recipient.
Photoheterotrophic growth needes an electron doner? T or F ?
FALSE :does not need an electron donor
Photoheterotrophic growth utilizes waht for energy production? Relies on what?
cyclic process for energy production and relies primarily on fermentation end products as a carbon source
Photoheterotrophic growth, for organic material at the same redox level as cell material? more oxidized than cell material? more reduced than cell material?
- a.assimilation involves no oxidation or reduction
- b.assimilation requires a reducing agent
- c. assimilation requires an electron acceptor
How many different types of reaction center?
two fundamentally different types of reaction center
Type I reaction centers, Type II?
- I: (ferredoxin reductases) contain powerful reducing agents, conserving much energy
- I: contain weaker reducing agents, thus less efficient.
How many types of photosynthesis? Name/Describe Them.
There are three distinct types of photosynthesis
Type I photosynthesis and Type II photosynthesis are anoxygenic.
Type I/II photosynthesis uses water as the proton/electron donor and is oxygenic.
Photosynthesis is confined to what?
the bacteria and their descendants the chloroplasts
How many diverse groups of photosynthetic prokaryotes are there? What is interesting regarding this? What does this mean?
There are six diverse groups of photosynthetic prokaryotes, yet Photosystem I and Photosystem II appear to be homologous in all species (so the pathways did not evolve multiple times).
How many types of evolutionary possibilities regarding the previous card are possible? Explain.
- There are two evolutionary possibilities:
- Both photosystems evolved early, but were lost numerous times
- Genes for photosynthesis were horizontally transferred.
Type I Photosynthesis
2 examples are?
green sulfur bacteria, heliobacteria; pigment antennae consist of chlorosomes in green sulfur, membrane-embedded protein-chlorophyll complex in heliobacteria; anoxygenic and strictly anaerobic
Type I photosynthesis name three things that are important?
- -Cyclic photophosphorylation in type I photosynthesis uses photosystem I
- -Non-cyclic photophosphorylation in type I photosynthesis utliizes photosystem I only
- -----Utilizes H2 or reduced sulfur compounds like H2S as electron donors
Type II Photosynthesis
- -Cyclic photophosphorylation in type II photosynthesis uses photosystem II
- -In type II photosynthesis, non-cyclic photophosphorylation requires reverse electron transport
- ---Utilizes H2 or reduced sulfur compounds like H2S as electron donors
- ---Like chemoautotrophy, this process requires reverse electron transport which is less efficient than in the type I
Next Few Cards are RE: Type I/II Photosynthesis
Cyclic photophosphorylation in cyanobacterial photosynthesis uses what?
photosystem I only
Non-cyclic photophosphorylation in cyanobacterial photosynthesis uses what?The proton pumping activity of the two systems is combined with what?
- -both photosystem I and II
- -Combined with noncyclic making more of a contribution to energy metabolism than in other systems
The oxygen-evolving complex, combined with a high-potential photosystem II, allows cyanobacteria and their descendants to use what?
water as electron donor
Ability to use water as electron donor is what?
unique and dependent on special protein complex
Cyanobacterial photosystem II have a very high reduction potential (higher than water) which makes what?
makes organisms with this system able to grow in environments lacking highly reduced compounds like H2S and H2
Oxygen evolution on the early earth may have been advantageous because?(a)
Oxygenesis must have evolved at about the same time as?(b)
Oxygenesis could have served as?
- a. of its toxicity
- b. methods of detoxification of oxygen
- c. weapon against competitors??
An alternative form of photophosphorylation is based on?
retinal instead of chlorophyll
An alternative form of photophosphorylation is based on retinal instead of chlorophyll. Give 3 examples.
- -Utilize bacteriorhodopsin (in archaea) and proteorhodopsin (in bacteria) with light absorbing pigment retinal (instead of chlorophyll)
- -In marine and halophilic environments where may serve as means of energy metabolism during anaerobic periods; photoheterotrophy may conserve energy in nutrient poor environments
- -Bacteriorhodopsin combines functions of reaction center AND ETC
The next card section is : Carbon Dioxide Fixation
What is the most common pathway of CO2 fixation?
The Calvin-Benson cycle is the most common pathway of CO2 fixation
Most of the enzymes are from central metabolism, interconverting what? Oxygen?
- 3-, 4-, 5-, 6-, and 7-carbon sugars; phosphoribulokinase and RuBisCO are unique to this pathway
- -Oxygen competes with CO2 for the active site of RuBisCO
The reverse TCA cycle is used by? Since a-oxaglutarate dehydroenase and citrate synthetase are irreversible what?
- -a diverse group of autotrophic bacteria and archaea
- -Used by green sulfur, sulfate-reducers, some archaea
- -Since α-oxoglutarate dehydrogenase and citrate synthetase are irreversible, alternate enzymes must catalyze these steps
What pathway is common in strictly anaerobic autotrophs? Also known as?
- -The Ljungdahl-Wood pathway.
- -Also know as acetyl-Co A pathway, utilized by some sulfate-reducers, acetogenic bacteria, methanogens
- -Non-cyclic pathway in which CO2 is reduced to CH3 and CO which form acetyl group
- -Process actually produces energy and is used for ALL energy needs by some bacteria
The hydroxypropionic acid cycle is used by who?
the green non-sulfur bacteria (utilizes some TCA enzymes + some unique ones)
Next card set: Nitrogen Fixation
Nitrogen fixation is a very what process?(a)Nitrogenase enzyme complex contains?(b) Catalyzes reduction of what? (c) What is needed for each electron transfered? (d)
- a-expensive process
- b- multiple subunits, Mo-Fe-S center
- c- 1 N2 and 2 H+ to NH3
- d-2 ATP needed for each electron transferred (total of 16 ATP) to overcome activation energy of breaking N-N triple bond
Nitrogen fixation is an what process?
an intrinsically anaerobic proces
Active sites of nitrogenase complex are readily what?
oxidized by O2
Some nitrogen fixation occurs in? some?
obligate and facultative anaerobes, some only fix under microaerophilic conditions where cytoplasm is anaerobic, azotobacter have less efficient ETC used when fixing N2
Physical methods of keeping N-fixation anaerobic include? (3)
- -heterocysts of cyanobacteria
- -thick capsular layers to reduce O2 diffusion
- -special proteins to bind and stabilize nitrogenase
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