Phototrophy

use light as energy source

use CO2 as carbon source

use organic carbon sources

light energy is conserved as chemical energy (ATP)

chemical energy is used to reduce CO2 to organic compounds (CO2 fixation)

• biological oxidation-reduction process
• CO₂ is the electron acceptor - reduced
• H₂A is the electron donor - oxidized

• oxidation of water to oxygen
• water is electon donor
• results in oxygen production
• production of reducing power (NADPH)

• H₂S or other is the electron donor
• bacteria including purple bacteria, green sulfur bacteria and heliobacteria
• no oxygen is produced
• use light or reverse electron flow to generate reducing power

• porphyrins like cytochromes but has Mg ion
• associated with photosynthetic membranes

• color of pigments is the color not being absorbed by the pigment
• pigment diversity has ecological significance because it allows light absorption at many different wavelengths

• diversity - from different R groups
• complimentary
• clues in genome
• absorption peaks reflect pigments dissolved in methanol
• actual bacteriochlorophylls are in the membranes

• always found in phototrophic organisms
• hydrophobic pigments embedded in the membrane
• absorb blue light
• transfer energy to the reaction centers
• have a photoprotective role by quenching toxic oxygenic species

• depends on pigment structure and associated pigment-binding proteins
• energy is inversely proportional to wavelength of light
• E= hc/wavelength

• the main light-harvesting pigments of cyanobacteria
• absorbs higher energy - shorter wavelengths
• evolution - get from opening of the porphyrin ring of chlorophyll

called thylakoids found inside chloroplasts

• integrated into the internal membrane systems from:
• invaginations of the cytoplasmic membrane - purple bacteria
• the cytoplasmic membrane - heliobacteria
• in both the cytoplasmic membrane and specialized structures called cholorsomes - green sulfur bacteria
• in thylakoid membranes - cyanobacteria

• within a membrane, pigment molecules are associated with pigment-binding proteins to form light harvesting antenna complexes
• complexes funnel energy to the reaction center where charge separation and electron transport occurs

• protein available in large quantities
• denaturation indicated by color change
• choice/use of detergent for solubilization of membrane protein
• molecular sieve (sizing column) used to separate and purify

• green sulfur bacteria and heliobacteria:
• pigments have more negative reduction potential
• FeS is first stable electron acceptor (more negative reduction potential than NADH)
• FD is direct electron donor for CO2 fixation
• light is source of energy and reducing power

• purple bacteria:
• first stable electron acceptor is quinone
• external electron donor and reverse electron flow is required to generate reducing power for CO2 fixation

use of the proton motive force to transfer electrons from molecules with more postive electrochemical potentials to molecules with more negative electrochemical potentials

• Z scheme
• light generates ATP and NADPH
• electrons to reduce NADP come from H2O
• non-cyclic
• when sufficient reducing power can have cyclic photophosphorylation with PSI

• PSI - more negative electrochemical potential, provides reductant
• PSII P680 - positive electrochemical potential, can accept electrons from splitting of water, involved in ATP generation