Ch. 6 Flashcards
Depending upon the photosynthetic system, light energy has one or both of the following properties:
- In ALL: light energy can drive the phosphorylation of ADP to make ATP (photophosphorylation)
- In SOME: light can also drive the transfer of electrons from H2O to NADP+
How are phototrophic prokaryotes categorized?
Physiological differences
- Whether of not they produce oxygen
- Source of electrons for biosynthesis
List the photosynthetic microorganisms we went over in class.
Oxygenic phototrophs
- Cyanobacteria
- Procholorophytes
Anoxygenic phototrophs
- Purple bacteria (sulfur and nonsulfur)
- Green bacteria (sulfur and nonsulfur)
- Heliobacteria
What are photosystems?
Organization of photosynthetic pigments within the membrane
The _____ consists of multiple proteins and associated _____ that absorb light energy and become excited.
- light-harvesting complex (antenna)
- pigments (accessory pigments)
The _____ contains a pigment molecule that can undergo oxidation, giving up an electron.
reaction center
What replaces the electron after the pigment in the reaction center gives it up?
Electron donor
What are the two outcomes of prokaryotic photosynthesis?
-
Proton motive force forms
- Used to generate ATP - The electron is used to produce NADH or NADPH
What type of reaction center do purple bacteria and green non-sulfur bacteria have? What does it use as terminal electron acceptors?
- Type II RC
- Uses quinones as terminal electron acceptors
What are the eight steps of cyclic phosphorylation?
- Dimer of bacteriochlorophyll (P870) absorbs light energy
- 1 electron is excited to a higher energy level (P*870) [has very low reduction potential (E’m)] - Electron donated to acceptor: Bpheo
- Bpheo transfers electron to ubiquinone A (UQA)
- UQA transfers electron to UQB –> UQ-B
- Steps 1-4 repeated to add second electron –> UQ2-B - UQ2-B picks up 2 protons from the
cytoplasm and is released from the reaction center as UQH2 - UQH2 transfers the electrons to a bc1 complex
- bc1 complex reduces cytochrome c2 –> protons translocated –> generates ∆p
- cytochrome c2 returns electron to oxidized bacteriochlorophyll molecule in RC
What is happening in steps 5-6 of cyclic phosphorylation?
Q cycle
Why is UQB referred to as a two-electron gate?
Because 2 electrons are required before reduced quinone leaves the RC
List the components of the reaction center of purple bacteria.
-
RC protein: serves as scaffolding for Bchl and Bpheo
- Quinones and nonheme iron are bound -
4 bacteriochlorophyll (Bchl) molecules bound
- 2 dimers, 2 monomers - 2 Bpheo (A and B)
- 2 UQ (A and B)
- 1 nonheme ferrous iron
- 1 carotenoid molecule (protect the RC pigments from photodestruction)
Describe the redox reactions in the RC of purple bacteria.
- The RC and bc1 cooperate to translocate protons to the outside
- The RC generates a ∆Ψ and ∆pH
- Electron moves from outside to inside by cyt c
- 2 protons are taken from the cytoplasm and released in periplasm
_____ and _____ are important sources of electrons for growth but serve separate functions.
- NADH
- NADPH
NADH donates electrons to _____, resulting in _____. NADPH donates electrons to _____.
- the respiratory chain
- ATP synthesis
- biosynthesis reactions
What must purple bacteria do to generate NADPH?
Use the proton motive force (∆p) created by light energy to drive reverse electron transport
What type of reaction center does green sulfur bacteria have?
Type I
How do the RCs in green sulfur bacteria and heliobacteria different from the RCs in purple bacteria?
- Possess iron–sulfur centers as terminal electron acceptors
- Reduce NAD(P)+ instead of quinone
What are the prosthetic groups in green sulfur bacteria?
- Primary electron acceptor (A0) = bacteriochlorophyll 663
- A1 = quinone-like molecule
- 2-3 Fe-S centers
- Fd is first reduced product outside RC
What results in cyclic vs. non-cyclic flow in green sulfur bacteria?
- Cyclic: if the electron is donated from ferredoxin to MQ
- Non-cyclic: if the electron is donated from ferredoxin to NAD(P)
How does photosynthesis in cyanobacteria and chloroplasts differ from photosynthesis in purple and green bacteria? (Hint: plants)
- H2O is the electron donor and oxygen is evolved
- There are two light reactions in series occurring in two different RCs
- Electron flow is primarily noncyclic, producing both ATP and NADPH
How many reaction centers do cyanobacteria/chloroplasts have? What are they?
2 RCs
1. Photosystem I (RC I or P700)
- Similar to Type I RC of green bacteria
2. Photosystem II (RC II or P680)
- Similar to Type II RC of purple bacteria
Diagram and explain the Z-scheme.
- PS II is connected to PS I by a short ETC that contains the b6f complex (analogous to bc1 complex)
- Electrons move from water –> PS II –> pheophytin –> electron carriers -> PS I
- ∆p is created by the b6f complex via a Q cycle
What are the different types of photosynthetic pigments? What are their functions?
- Light-harvesting pigments
- Absorb light at wavelengths different from RC chlorophyll
- Transfer energy to the RC - Chlorophylls and bacteriocholorophylls
- Harvest energy from light - Carotenoids
- Absorb light energy and transfer it to the RC chlorophyll
How are photosynthetic pigments regulated?
- Transcriptional regulation based on:
- Oxygen levels
- Redox potentials
- Light - Regulated according to Earth’s rotation (circadian clock)
Describe chlorophylls and pheophytin (general structure and what organisms have them).
- Chlorophylls
- General structure: 4 pyrrole derivatives with covalently bound Mg2+
- Different side chains from the pyrrole rings
- Photosynthetic bacteria have Bchl
- Note: narrow wavelength range - Pheophytin
- Same structure as chlorophyll but with no Mg or Zn
- In eukaryotes and cyanobacteria
Describe carotenoids.
- Absorb light over wavelengths of 400-600 nm
- Transfer light energy to chlorophylls
- Expand wavelength range of light that is able to drive photosynthesis
- Protect biological materials from photooxidation caused by reactive oxygen derivatives
- Over 30 different types known
Describe phycobiliproteins.
1. Are they soluble or insoluble?
2. General structure?
3. Found in what organisms (general)?
- Soluble
- Contain bilin (4 pyrroles in linear form)
- Found in eukaryotes and cyanobacteria (not in other photosynthetic bacteria)
Describe bacteriopheophytin (what organisms have it, type of RC).
- Found in photosynthetic bacteria
- Use pheophytin-quinone type RC (but green sulfur bacteria and heliobacteria Fe-S type RC)
Describe the photosynthetic apparatus in cyanobacteria.
1. What is it called?
2. Antenna molecules
3. RCs
- Thylakoid
- Similar to eukaryotes
- Found in cytoplasm
- Bilayer membrane - Antenna molecules
- Found in cytoplasmic side
- Chlorophyll a
- Phycobiliproteins
- Carotenoids - RCs
- Chlorophyll a
Describe the photosynthetic apparatus in green bacteria.
1. What is it called?
2. Antenna molecules?
3. RCs?
- Chlorosome
- Found on cytoplasmic membrane
- Monolayered galactosyl diglyceride - Antenna molecules
- Rod-shaped
- Bchl c, d, or e
- Carotenoids
- Have baseplates bound to a RC - RCs
- In cytoplasmic membrane
- Contain Bchls and Fe-S proteins or Bpheo
Describe the photosynthetic apparatus in purple bacteria.
A. Less well-developed structure
B. Intracellular membrane structure:
- Continuous with cytoplasmic membrane
- Antenna molecules and RCs
- Shape varies with strain
C. Membrane is produced under anaerobic light conditions when pigments are produced
