Lecture 5: Bacterial Photosynthesis - Exam 4 Flashcards
What is phototrophy?
The harnessing of light energy and converting this into chemical energy to power metabolism and growth.
What is photosynthesis is based on?
Chlorophyll
Are archaea photosynthetic?
The Haloarchaea have?
What are bacteriorhodopsins?
Archaea are not photosynthetic.
The Haloarchaea have bacteriorhodopsin.
Bacteriorhodopsins are light-driven proton pumps, that power their proton-motive force.
Describe Bacteriorhodopsins.
They have 7 trans membrane helices.
-Surrounds a molecule of retinal
A photon is absorbed by _____, which?
What is pumped from the cytoplasm across the membrane?
What is generated and what does that drive?
Retinal, which shifts configuration of the protein.
-Pumps 1 H+ from the cytoplasm across the membrane.
The proton gradient generated drives ATP synthesis by a typical F1F0 ATP synthase.
Who else has rhodopsins?
Some proteobacteria. Their rhodopsins are called proteorhodopsins, and are found in marine bacteria.
Where does the energy for photosynthesis come from?
The energy for photosynthesis derives from the photoexcitation of a light-absorbing pigment.
What does photoexcitation lead to?
Leads to photolysis, the light-driven separation of an electron from an electron donor molecule such as H2O (oxygenic photosynthesis) or H2S (anoxygenic), or an organic molecule.
Where do the electrons from photolysis go?
What does this generate?
Each electron is then transferred to an Electron Transport System.
-The ETS generates a proton potential and the reduced coenzyme NADPH.
-The proton potential drives the synthesis of ATP through an F1F0 ATP synthase.
Discuss phototrophic bacterial diversity.
A wide diversity exists within phototrophic bacteria:
-Photosynthesis vs rhodopsin
-The types and structure of chlorophyll
-The types and structure of accessory pigments.
-The cellular structure for the photopigments
-The type of photosystem Reaction center
What do phototrophs have that maximize light energy absorption?
Photosynthetic membranes are associated with?
What are embedded within the membranes?
Phototrophs have increased membrane content to maximize light energy absorption.
Photosynthetic membranes are associated with, or invaginated from, the cytoplasmic membrane.
Embedded within the membranes are photosynthetic pigment molecules organized into one or more photosystems, where light energy is converted into chemical energy.
What is each photosystem composed of?
A light-harvesting complex (antenna complex) and a reaction center.
What do antenna complexes do?
What do the reaction centers connect?
Trap light and transfer this energy to the reaction center.
The reaction center connects the photosystems to the electron transport chain.
Photosynthetic pigments vary in the _________________ that they absorb.
Wavelengths of light
The longer the wavelength of light, the…?
Deeper it can penetrate in water, soil, etc.
Bacterial phototrophs have a combination of different photosystem pigments. What does this allow?
This allows many phototrophs to occupy the same environment without competing with one another.
What are the two major pigment classes based on location?
- Light Harvesting Pigments (Antenna pigments):
-Chlorophylls and carotenoids are important in plant and green algae.
-Phycobilin proteins in cyanobacteria and red algae. - Reaction Center Pigments:
-Primarily chlorophyll/bacteriochlorophyll with some carotenoids.
What are the primary pigments?
Chlorophylls and bacteriochlorophylls, which are tetrapyrrole derivatives with Mg+ in the middle
The differences between chlorophylls lies in…?
In different chemical side groups around the ring that affect light absorption properties.
Where is chlorophyll present?
In cyanobacteria and eukaryotes (present in all oxygenic phototrophs)
-Chlorophyll a is present in cyanobacteria and eukaryotes
-Other chlorophyll types also exist.
Chlorophyll absorbs what kind of light?
Shorter wavelengths of light
What is bacteriochlorophyll?
Structurally similar to chlorophyll, present in other photosynthetic bacteria (purple bacteria, green sulfur bacteria).
Bacteriochlorophylls differ how?
What kind of light do they absorb?
Differ in their side groups and wavelengths of light they absorb.
They absorb longer wavelengths not absorbed by cyanobacteria or eukaryotes.
What is an example of an accessory pigment?
Describe this accessory pigment.
Carotenoids are secondary pigments present in all oxygenic phototrophs (both eukaryotic and cyanobacteria).
These are the yellow-orange pigments.
They are long isoprenoids that absorb light energy and transfer the energy to the reaction center of chlorophyll.
What kind of light do carotenoids absorb?
Short wavelengths of light in the visible range, between 400nm-500nm
What is another example of an accessory pigment? Describe it.
Carotenes are carotenoid pigments with six-membered rings at both ends of the molecule.
-B-carotene
Carotenes are the light harvesting complexes of purple photosynthetic bacteria
Bacteriochlorophyll-carotenoid complexes absorb what kind of light?
Absorb much longer wavelengths of light. 800nm-1000nm
What is the third type of accessory pigments? Describe it!
Phycobilins and Phycobilisomes.
Phycobilins are major light harvesting pigments of cyanobacteria and red algae.
-absorb light at 520-670nm
Phycobilisomes are light harvesting complex containing phycobilins and phycobilinproteins covering the thylakoids and attached to the photosystem.
What kind of light do phycobilins absorb?
Short wavelengths (with more energy) at which chlorophylls or carotenoids don’t absorb light well.
What are the types of phycobilins in a phycobilisome?
-Phycoerythrin = absorb short wavelengths
-Phycocyanin = longer wavelengths
-Allophycocyanin = situated closest to the reaction center and probably acts mostly as an energy transducer
What are thylakoids?
What produces thylakoids?
Thylakoids are photosynthetically active membranes found in cyanobacteria and chloroplasts of green algae and plants.
Extensive folding of the photosynthetic membranes produces thylakoids.
Cyanobacterial thylakoid membranes are located where? Arranged how? Make contact with what? And have what attached?
Cyanobacterial thylakoid membranes are located directly into the cytosol, are arranged in layers, make contact with the cytoplasmic membrane, and have attached phycobilisomes as the antenna pigments (light harvesting pigments)
All forms of photolysis share a common design:
- Antenna system
- Reaction center complex
- Electron transport system
- Energy carriers
The light dependent reactions involved two what?
What are the units within these things?
Involve two photosystems called photosystem I and photosystem II.
Within the photosystems are units called antenna complexes which maximize light absorption.
What are antenna complexes composed of?
What is each antenna complex able to do?
Chlorophyll molecules and accessory pigments located in the thylakoid membrane.
Each antenna complex is able to trap light and transfer this energy to a complex of chlorophyll molecules and proteins called the reaction center.
Again, the reaction center is…?
The protein complex in which chlorophyll photoexcitation connects to the Electron Transport System.
Where is the reaction center compared to the antenna complexes?
The antenna complexes associate in a ring around the RC.
What is transferred to the reaction center?
In the RC, what specifically happens?
Harvested light energy is transferred to the reaction center.
In the RC, an excitable electron absorbs the light energy, causing the electron to move to a higher energy level.
Now, the excited electron is transferred to an electron acceptor molecule of an electron transport chain. In the ETC, the light energy is converted to chemical energy.
What does oxygenic photosynthesis use?
Uses water as the initial electron donor and produce oxygen as a byproduct.
Many photosynthetic bacteria carry out anoxygenic photosynthesis, using…?
These bacteria do not produce what?
Using alternative primary electron donors, such as H2S.
These bacteria do not produce oxygen as a photosynthetic byproduct.
What are the three types of phototrophs?
- Oxyphototrophs: Cyanobacteria
-Use photosystem I and II (Z-pathway)
-Have type I and type II reaction centers - Type I Anoxygenic phototrophs: Green sulfur bacteria
-Use photosystem I
-Have type I reaction centers: Terminal e- acceptors are iron-sulfur centers and reduce NADP+ instead of quinone. - Type II Anoxygenic phototrophs: Purple bacteria
-Use photosystem II
-Have Type II reaction centers: terminal e- acceptors are quinone complexes.
The steps of photolysis and electron transport occur in three different kinds of systems, in different classes of bacteria:
- Anaerobic photosystem I
-Requires and exogenous source of e- to make ATP - Anaerobic photosystem II
-Does not require an exogenous source of e- to make ATP - Oxygenic Z pathway
-Consists of PS I and PS2, which are coupled, as in eukaryotic photosynthesis.
Where is photosystem I found?
Found in green sulfur bacteria and other anaerobic photosynthetic bacteria.
What goes on in photosystem I?
-Separates electrons associated with hydrogens from H2S or another organic electron donor, such as succinate.
-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 proton motive force to drive ATP synthesis.
Where are green sulfur bacteria found?
In anaerobic habitats where there is some light availability, and where hydrogen sulfide accumulates.
Describe the reactions that go on in photosystem I.
Cytochrome c oxidizes hydrogen sulfide (H2S), releasing two protons across the membrane and two electrons flow to the type I reaction center.
- In the RC, light energizes bacteriochlorophyll (P840) which energizes the electrons.
-The energized electrons flow to a quinone acceptor (Phylloquinone) and then to ferredoxin.
-The enzyme ferredoxin-NADP+ reductase transfers the electron from ferredoxin to NADP+ to generate NADPH.
What is ferredoxin?
A soluble protein that facilitates reduction of NADP+ to NADPH.
Where is photosystem II found?
Found in alpha-proteobacteria, “purple sulfur and non-sulfur” bacteria and Chloroflexi “green non-sulfur bacteria”
What goes on in photosystem II?
-Separates an electron directly from the bacteriochlorophyll
-These electrons are then transferred to an electron transport system.
Ultimately, an electron is return to bacteriochlorophyll.
-This process, which generates ATP, is called cyclic photophosphorylation**
PSII, unlike PSI, provides what?
Provides no direct way to make NADP or NADPH for reductive biosynthesis.
Describe the reactions that go on in photosystem II.
Light energizes bacteriochlorophyll (P870) in the reaction center, which energizes an electron from bacteriochlorophyll.
The energized electron is transferred to a series of quinone acceptors and then to the cytochrome bc complex.
The cytochrome bc complex reduces cytochrome c, and translocates protons across the membrane, thereby generating PMF.
The reduced cytochrome c returns the electron to the reaction center. The PMF is used to generate ATP via ATP synthase.
Anoxygenic bacteria using photosystem II, such as purple bacteria and green non-sulfur bacteria, use a process called ___________ to make ATP.
Cyclic photophosphorylation
What does the “cycle” in cyclic photophosphorylation involve?
Involves an electron being displaced directly from bacteriochlorophyll and is moved through a membrane photosynthetic electron transport system, being successively passed from an iron-sulfur protein to a quinone to a cytochrome, finally back to bacteriochlorophyll.
-As the electron is transported, a proton motive force is established on the membrane, and ATP is synthesized by an ATPase enzyme.
So, the manner of converting light energy into chemical energy is called?
cyclic photophosphorylation
During cyclic photophosphorylation, what is not being generated?
During cyclic photophosphorylation, no reduced NADH or NADPH is generated because the electron is being taken from and returned to the RC bacteriochlorophyll.
The normally cyclical flow of electrons during bacterial photosynthesis must be opened up in order to obtain electrons for CO2 fixation. What is often the source of electrons?
H2S
Light energy boosts an electron, derived from H2S to the level of ferredoxin, which __________ to provide electrons for autotrophic CO2 fixation (in the calvin cycle or reductive TCA)
reduces NADP
What is chloroflexi?
A phylum of “green non sulfur bacteria”
-The bacteria uses photosystem II and so do not need an external source of electrons.
-The chloroflexus species contain photosynthetic apparatus within chlorosomes, a complex that can contain up to 250,000 chlorophyll molecules.
Describe purple non sulfur bacteria.
Considered to be among the most metabolically versatile groups of bacteria.
Where is the oxygenic Z pathway found?
Found in cyanobacteria and chloroplasts.
-Includes homologs of bacterial PSI and PSII.
-It produces O2
-Oxygenic photosynthesis forms 3 ATP + 2 NADPH per H2O photolyzed.
The Z pathway has how many reactions centers? What does it normally start with?
It has two reaction centers. It normally starts with a photosystem II homolog.
What goes on in the Z pathway?
Light energizes chlorophyll in the reaction center, which causes photolysis of H2O, producing O2, and protons as by-products, with the electrons passed to quinone acceptors.
-The electrons are passed to cytochrome bf complex, which translocates protons across the membrane, thereby generating PMF and ATP via ATP synthase.
-The electrons are then passed through cytochrome c and to the Plastocyanin protein, which links the two photosystems.
-The electron is passed to the photosystem I reaction center and is again energized by the light.
Ferredoxin-NADP+ Reductase transfers the electrons from ferredoxin to NADP+ to generate NADPH
Describe cyanobacteria “blue-green algae.”
Oxygenic photosynthetic bacteria
-Contain chlorophyll a and accessory pigments the phycobilins.
-Conduct photosynthesis in thylakoids using Z pathway.
-Fix CO2 in calvin cycle.
Many species of cyanobacteria play an important role in …?
Nitrogen fixation
Cyanobacteria can fix nitrogen even in the presence of oxygen, thanks to a specialized cell type called?
Heterocyst
What are nitrogenases? What are they responsible for? What is the nitrogenase complex highly sensitive to? Who is this a problem for? What mechanism has evolved to help this problem?
Nitrogenases are enzymes that are produced by certain bacteria that are responsible for the reduction of nitrogen to ammonia.
The nitrogenase enzyme complex is highly sensitive to oxygen, since the iron containing protein is inactivated by even low O2 levels.
This is a problem for aerobic bacteria.
Cyanobacteria have evolved a mechanism to protect the nitrogenase from oxygen inhibition: Nitrogen fixation occurs in special cells called heterocysts that possess only photosystem I, which can generate ATP but will not produce O2.
