BIOL 2004 Lecture Notes - Lecture 6: Atp Synthase, Photosynthetic Reaction Centre, Electron Transport Chain
3 May 2018
School
Department
Course
Professor
Diversity of Bacteria II
March 15-17, 2016
Phototrophy
• Conversion of light into chemical energy
• + storage as organic molecules = photosynthesis
• microorganisms are globally important photosynthesizers (especially in water)
• much more varied in prokaryotes than eukaryotes
o several (bacterio)chlorophyll-based photosynthesis systems
o also Bactriorhodopsin-based phototrophy
▪ light driven proton pump in cell membrane *very light sensitive; changes
conformations act to move H+ from inside to outside the cell (against
concentration gradient)
▪ increase in PMF drives ATP synthesis (using ATP synthase)
▪ supplementary ATP synthesis for heterotrophs
▪ in some bacteria (some proteobacteria and archaea)
(Bacterio)chlorophyll-based systems in a nutshell
• Respiration
o Electron carrier: NADH *low reduction potential
o Electron acceptor: NAD+ *high reduction potential
o Generates ATP via PMF in electron transport system
• Cyclic Photophosphorylation (light energy ATP)
o Bad electron donor: absorbs energy from light, exciting electrons good
electron donor
o Returns back to low energy state when it passes through ETS
o PMF ATP via ATP synthase
• Non-cyclic Photophosphorylation (light energy new high energy electrons on carrier)
o Bad electron donor accepts light good electron donor
o Electron carrier (NADPH) transports electrons to from donor
o Electrons first move into the bad electron donor by external electron donor before
light is absorbed
• i.e. Photon captyre excites electron
o Cyclic
▪ Energy from ETS harnessed to generate PMF
▪ PMF is used to generate ATP
▪ Electrons eventually return to excitable molecule
o Non-cyclic
▪ E transferred to a carrier and used for biosynthesis
▪ Electrons are replaced from external electron donor (photolysis of donor)
What is the external electron donor in photosynthesis in plants? WATER! (oxidized to oxygen)
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Some generalities about photosynthesis
• Membrane localized process
• Light-harvesting antenna system:
o Chlorophylls/ bacteriophylls
o Accessory pigments
• Transfers the harvested energy to the reaction centre: includes reaction centre chlorophyll
or bacteriophyll
• ETS transfers excited electrons to energy carriers
• Model: Purple Bacterium (photosynthetic proteobacteria)
o Antenna absorbs light energy and passes it onto the reaction centre (from one
chlorophyll to the next)
o Cyclic photophosphorylation occurs in reaction centre P870 P870* Bph
Q
o Q moves into quinone pool QH2 bc1 Fe-S c2 REACTION
CENTRE
o Proteins inside the cell are released out to periplasm *to create PMF
o ATPase used to generate ATP across membrane (against gradient)
Phototrophs usually have many expanded photosynthetic membranes to hold light harvesting
systems.
Properties of Photosynthetic Bacteria
• Purple Bacteria (Proteobacteria)
o 1 photosystem (II)
o bacteriochlorophylls (Bchl)
o Main accessory pigment: carotenoids
o External Electron Donor: H2S, sulfur or organic **anoxygenic
• Green Sulfur Bacteria (Chlorobi)
o 1 photosystem (I)
o bacteriochloropylls (Bchl)
o Main accessory pigment: carotenoids
o EED: H2S, sulfur **anoxygenic
• Cyanobacteria
o 2 photosystems (I&II)
o chlorophyll a (Chl a)
o Main accessory pigment: phycobillins (& cartenoids)
o EED: H2O (O2) **oxygenic
*Bchl and Chl a have different absorption spectra
• Green Bacteria (Bchl c and a) = 350 nm – 1,100 nm
• Purple (Bchl b) = 350 nm – 1,100 nm
• Purple (Bchl a) = 350 nm – 1,000 nm
• Cyanobacteria (Chl a and phycobillins) = 350 – 750 nm *high O2; above Bchl
• Chlorophyll = absorbs red light
• Bacteriochlorophylls = cannot absorb as high as Chl
• Overall
o Bchl absorb longer wavelengths (red/infrared) than chlorophylls
o Accessory pigments (carotenoids) extend organisms absorption spectrum
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