Lecture 22

• Use of sunlight to generate proton motive force
• Energy of PMF is used to make ATP and reduce NAD to NADH or NADP to NADPH
• The ATP and NAD(P)H are used to fix CO₂ into organic molecules with which make cellular material

• Obtain their energy for ATP synthesis from sunlight
• Photoautotroph- CO₂ as carbon source, H₂S or H₂O as the source of electrons and protons for reducing NAD
• Photoheterotroph- Organic molecules as carbon source and as source of electrons and protons for reducing NAD

H₂S (or other reduced compounds, but not H₂O) as the reductant; yields sulfur (S^o) or sulfate (SO₄^2-)

• H₂O as the reductant; yields oxygen
• Carried out by only cyanobacteria (and Eukarya phototrophs- plants, algae, and certain protists)

• Pigments that absorb light and release the energy as electrons
• Similar in structure to the heme portion of cytochromes, but with Mg in place of Fe
• Chl a (chlorophyll a)- in oxygenic phototrophs
• Bchl a (bacterial chlorophyll a)- in anoxygenic phototrophs

• Bchl in embedded in cytoplasmic membrane
• Many bacteria have extensive internal photosynthetic membrane systems
• These are extensions of the cytoplasmic membrane and are seen as vesicles or membrane stacks

• Reaction centers participate directly in conversion of light energy into ATP
• Antenna pigments collect light and channel it to the reaction center

• Carotenoids- give cells colors (red, purple, brown)
• 1. Help transfer light energy to RC
• 2. Protect photosynthetic apparatus by absorbing excess light
• Phycobilins- accessory pigments associated with proteins
• - Phycobiliproteins help collect different wavelengths of light and transfer that light energy to RC

• Bchl-based light-mediated ATP synthesis via a series of electron carriers in the membrane
• Main RC component is P870 (a complex of chlorophylls and protein)

• Two separate but linked reactions
• Each has a distinct form of reaction center chlorophyll a
• PSI- chl a absorbs longer wavelenghts of light
• PSII- chl a absorbs shorter wavelengths of light and releases oxygen

Assimilation of CO₂ as a carbon source (carbon fixation)- Calvin cycle

• Purple phototrophic bacteria- anoxygenic photosynthesis, bacteriochlorophyll; and various different carotenoid pigments
• Purple sulfur bacteria- use H₂S as an electron donor- produce granules of elemental sulfur in the cell, found in illuminated anoxic zones of lakes where H₂S accumulates (Chromatium)
• Purple nonsulfur bacteria- can use H₂S but at lower concentrations that purple sulfur bacteria, some can grow anaerobically in the dark using fermentation and anaerobic respiration (Rhodopseudomonas)

• Green sulfur bacteria- anoxygenic, obligately anaerobic, utilize H₂S as an electron donor, excrete sulfur as globules outside the cell (Chlorobium)
• Green nonsulfur bacteria- anoxygenic, filamentous, thermophilic, photoautotrophic or photoheterotrophic (Chloroflexus- can grow anaerobically in the dark as chemoorganotroph)
• Gram-positive bacteria- Heliobacteria, anoxygenic phototrophs; strictly anaerobic soil bacteria; fix nitrogen and can grow by fermentatively
• Cyanobacteria- oxygenic photosynthesis; very large and diverse group of microbes