Chapter 8A - Microbial Metabolism: Photosynthesis, Autotrophic Growth, & Nitrogen Fixation Flashcards
____________________________ is mediated by membrane-embedded reactionc enters.
Photochemistry
What is present in all reaction centers?
Chlorophyll
In almost all prokaryotes, reaction centers are located in the cell membrane or invaginations of it. The only exception is the __________________, which have intracellular membrane vesicles called __________________, in which reaction centers are embedded.
Cyanobacteria
Thylakoids
_________________________ is a tetrapyrrole with a magnesium atom in the center and is present in all reaction centers.
Chlorophyll
R groups vary in different chlorophylls; however, ___ is always a long hydrocarbon chain.
R6
Some reaction centers also contain ___________________ or chlorophyll molecules that lack the central magnesium atom and are tightly bound quinones.
Pheophytins
__________________________ are bacterial compartments consisting of polyhedral protein shells filled with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase
Carboxysomes
Reaction centers have prosthetic groups that undergo irreversible redox reactions. True or false?
False; these reactions are reversible
Prosthetic groups of reaction centers may include ________________, pheophytin, and/or _____________.
Chlorophyll
Quinones
What are reaction centers associated with?
Pigment antennas
What is a pigment antenna?
A large group of pigment molecules that absorbs light
What is resonance energy transfer?
Absorption of a photon by a molecule takes place leading to electronic excitation when the energy of the captured photon matches that of an electronic transition. The fate of such excitation can be a return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another.
In resonance electron transfer, are electrons transferred?
No, only the energy absorbed is transferred from one pigment molecule to another through the antenna complex until it reaches the reaction center
Usually the pigment molecules are chlorophylls, but in the cyanobacteria, several linear tetrapyyroles called ______________________ are used.
Phycobilins
Three groups of bacter have distinctive anatomical structures to house their pigment antennas. What are these three bacteria?
Cyanobacteria
Green-sulfur bacteria
Acidobacteria
Phycobillins in the cyanobacteria are contained in ______________________.
Phycobilisomes
Chlorophyll in green-sulfur bacteria and Acidobacteria are contained in ___________________.
Chlorosomes
____________ _______________________ generates energy in the light with no material input.
Cyclic photophosphorylation
What is the general scheme of cyclic photophosphorylation?
The reaction center, energized by two photons of light, transfers two electrons to an electron transport system. As electrons pass through the electron transport chain, protons are pumped, maintaining a protonic potential. The electrons are transferred back to the oxidized reaction center, reducing it back to the ground state
In cyclic photophosphorylation, what acts as both the electron donor and acceptor?
The reaction center
Do all phototrophs employ cyclic photophosphorylation for maintaining the protonic potential?
Yes, at least all known phototrophs do
While energy is transferred from one chlorophyll molecule to the next in the pigment antenna, ___________________ are transferred in the ________________ ______________.
Electrons
Reaction center
Unlike phototrophs, autotrophs require an exogenous electron donor. Why?
Because they need to reduce carbon dioxide to organic material
What two things are needed to fix carbon?
NADPH
Reduced ferredoxin
Electron donors and ___________ ___________________ are needed for autotrophic growth only.
Noncyclic photophosphorylation
The challenge of phototrophs is to use their electron donor, often a weak reducing agent like water or H2S, to reduce NADP+ or ferredoxin. Because this is a strongly endergonic reaction, an input of energy is necessary. How do autotrophic growth gain the energy to do so?
The mechanism for putting energy into this reaction is to route the electrons through the reaction center; thus, light provides some or all of the necessary energy
What is the general scheme for noncyclic photophosphorylation?
The reaction center, excited by two photons of light, transfers electrons to form NADPH or reduced ferredoxin, which are then used to reduce carbon dioxide. The oxidized reaction center is reduced back to the ground state by electrons from the electron donor
How is the oxidized reaction center reduced in cyclic photophosphorylation? Noncyclic phosphorylation?
Cyclic photophosphorylation: the electrons are cycled back to the reaction center via cytochrome c
Noncyclic photophosphorylation: the electrons are accepted from an electron donor
Why does photoheterotrophic growth not require an electron donor?
Because they assimilate organic materials from their environments for biomass
Cells that grow photoheterotrophically do not need an electron donor. They use cyclic photophosphorylation to generate energy, and they assimilate organic compounds from the enviornment as their source of ___________________. The organic material is rearranged to produce the necessary precursor metabolites and then all of the molecules of the cell.
Carbon
When the organic substrate is at the same oxidation level as cell material, there is no net reduction or oxidation because it is unnecessary, and thus no electron donor or acceptor is needed - for instance, the assimilation of ____________ _________.
Lactic acid
However, when the organic material is more oxidized then the cell material, it must be reduced before assimiliation - as in the case of _________________. Under these conditions, typicalyl some of the substrate is oxidized to ___________ _____________ with the electrons generated being used to reduce the rest of the substrate to cell material.
Pyruvate
Carbon dioxide
If the organic substrate is more reduced than the cell material, it has to be oxidized before assimilation - for example, _________________. In this case, cells typically fix sufficient carbon dioxide to provide an electron acceptor for the oxidation of the organic substrate. Thus, their cell carbon comes partially from the organic compound and partly from carbon dioxide.
Butyrate
There are ____ fundamentally different types of reaction center.
Two
What are the two different types of reaction center?
Type I reaction centers
Type II reaction centers
Type I reaction centers are _____________ _____________ for which the electron acceptor is the very electronegative iron-sulfur protein _______________.
Ferredoxin reductases
Ferredoxin
Type II reaction centers are __________________ _________________ that reduce ______________ to quinols.
Quinone reductases
Quinones
How may distinct types of photosynthesis are there?
Three
What are the three types of photosynthesis?
Type I
Type II
Type I/II
__________________ ____ photosynthesis uses only ferredoxin, a __________________ __ reaction center.
Type I
__________________ ___ photosynthesis uses only _________________ __ reaction centers, quinones.
Type II
What two types of photosynthesis are anoxygenic, meaning that they don’t use water and don’t generate oxygen?
Type I
Type II
__________________ ___ photosynthesis uses both types of reaction centers and is _______________, meaning that it uses water and produces oxygen.
Type I/II
Oxygenic
Photosynthesis using reaction centers is confined to the _______________ and their descendents the chloroplasts.
Bacteria
There are six different groupings regarding distribution of photosynthesis within the bacterial tree. Which bacteria use Type I photosynthesis?
Green sulfur bacteria
Heliobacteria
Which bacteria use type II photosynthesis?
Green nonsulfur bacteria
Purple sulfur bacteria
Purple nonsulfur bacteria
Which bacteria use type I/II photosynthesis?
Cyanobacteria
Cyclic photophosphorylation in type I photosynthesis uses _______________________ __.
Photosystem I
The green sulfur bacteria are ____________________, meaning that they receive their energy from light and use inorganic electron donors to fuel biosynthetic reactions.
Photolithotrophs
What is a lithotroph?
An organism that uses inorganic electron donors for energy or biosynthesis
______________________, like green sulfur bacteria, can use carbon dioxide as the sole carbon source but also can use _____________________ as a carbon source.
Photolithotrophs
Acetate
Photolithotrophs oxidize ______________ with ultimate electron donors generally _________________.
Sulfur
Sulfide
The ultimate electron donor in photolithotrophs is generally sulfide, which is oxidized to _____________ and accumulates outside of cells. This compound is further oxidized to ____________.
Sulfur
Sulfate
Are green sulfur bacteria aerobes or anaerobes?
They are strict anaerobes
Do green suflur bacteria contain RuBisCO?
No
In type I photosynthesis, photosystem I is used. The reaction center reduces _____________, which reduces in turn _____________________ and then the cytochrome b/c complex. This complex then reduces periplasmic __________________ __, which diffuses through the periplasm to the oxidized reaction center and reduces it back to the ground state.
Ferredoxin
Quinone
Cytochrome c
What acts as the proton pump in type I photosynthesis?
Cytochrome b/c complex
Noncyclic photophosphorylation in type I photosynthesis uses photosystem I only. To generate NADPH needed for autotrophic carbon dioxide fixation, green sulfur bacteria use ______ or reduced _____ compounds as electron donors.
H2
Sulfur
In noncylic photophosphorylation in type I photosynthesis, electrons from H2 or H2S reduce the periplasmic _______________ ___, which then passes electrons on to the oxidized reaction center. From the excited reaction center, they are transferred to ____________________, which has a reduction potential more negative NADPH. Thus the electrons pass from _________________ to NADP+ - a reaction catalyzed by the membrane _____________________________.
Cytochrome c
Ferredoxin
Ferredoxin
Ferredoxin-NADP+ reductase
Which bacteria use cyclic phosphorylation type II photosynthesis?
Purple bacteria
Green non-sulfur bacteria
The purple bacteria include the _________________ and _________________ bacteria.
Purple sulfur
Purple non-sulfur
What is the primary difference between the purple sulfur and purple non-sulfur bacteria?
The purple sulfur bacteria use sulfur or sulfides as electron donors while the purple non-sulfur bacteria use hydrogen or other molecules as electron donors
Most green non-sulfur bacteria are __________________________.
Facultative aerobes
Under aerobic growth, green non-sulfur do not engage in __________________________.
Photosynthesis
Cyclic photophosphorylation type II photosynthesis uses ____________________ __.
Photosystem II
The cyclic system in type II photosynthesis is basically the same as in type I photosynthesis except that the reaction center reduces ______________, not ferredoxin. From there, the system is the same as in type I photosynthesis: _________________ reduces the periplasmic _____________ __ by passing its electrons through the cytochrome b/c complex, simultaneously pumping protons into the periplasm. The reduced _______________ ___ diffuses through the periplasm to the oxidized reaction center.
Quinone
Quinol
Cytochrome c
Cyctochrome c
Noncyclic photophosphorylation type II photosynthesis requires ________________ _____________ ____________ to make NADPH.
Reverse electron transport
Why must a reverse electron transport system be utilized in noncyclic photophosphorylation type II photosynthesis?
Because NADPH is a better reducing agent than quinol
In type II photosynthesis, noncyclic photophosphorylation requires reverse electron transport. Oxidation of the excited photosystem reduces ________________, which reduces ______________ to ___________ in a two step reverse electron transport driven by the entry of protons. _______________ is used as reductant in the fixation of CO2. Oxidation of a chemical electron donor reduces the periplasmic cytochrome c, which diffuses through the periplasm to the oxidized photosystem and reduces it back to the ground state.
Quinone
NADP+
NADPH
NADPH
