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Photosynthetic organisms

Photosynthetic organisms. Anoxygenic Photosynthesis Green and purple (sulfur and non-sulfur) bacteria and heliobacteria. Oxygenic Photosynthesis Cyanobacteria. Pigments. Light harvesting pigments Bacteriochlorophyll (bchl) for anoxygenic photosynthesis

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Photosynthetic organisms

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  1. Photosynthetic organisms • Anoxygenic Photosynthesis • Green and purple (sulfur and non-sulfur) bacteria and heliobacteria • Oxygenic Photosynthesis • Cyanobacteria

  2. Pigments • Light harvesting pigments • Bacteriochlorophyll (bchl) for anoxygenic photosynthesis • Chlorophyll for oxygenic photosynthesis • Different varieties that can absorb light at different wavelengths Harness energy from light to excite electrons to higher energy states (more reducing, more negative E0) – these high energy electrons go through the electron transport chains to ultimately produce energetic molecules like ATP and NADPH to drive biochemical reactions in the cell

  3. Other pigments can harvest light energy at wavelengths an organism’s chlorophyll cannot – includes caratenoids, phycobilins (blue), and phycoerythrin (red)

  4. Q cycle • Ubiquinone (Q) pool is critical to H+ shuttling to maintain the pmf and drive phosphorylation to produce ATP Quinone (Q) is reduced with an electron through cytochrome bc1 to form hydroquinone (QH2) – QH2 is then oxidized by another site on cytochrome bc1 back to Q – this shuttles H+ across the membrane, the H+ goes through ATPase back into the cell, generating ATP http://www.geocities.com/awjmuller/anims_images/bacterialPSfast.gif for animation...

  5. Anoxygenic photosynthesis

  6. Anoxygenic photosynthetic organisms • Purple and green bacteria utilize bchl and H2S, S8, S2O32-, H2, Fe2+, and organics as electron donor • Many can deposit elemental sulfur intracellularly (and sometime outside the cell (epicellularly) to store that when H2S is unavailable • The term non-sulfur does not necessarily mean they can’t use sulfur - they are generally adapted to environments with less sulfur • Heliobacteria – Nitrogen-fixing, heterotrophic organisms found in soils and rice paddies Beggiatoa spp.

  7. Oxygenic Photosynthesis Chlorphyll a (P680) is very oxidized (E0=+1.1V), enough to oxidize H2O. BUT e- excitation takes it to E0=-0.7V, not enough to reduce NADP+ to NADPH. Thus a need for 2 photosystems…. Water-oxidizing complex is key – Mn4Ca-complex that oxidizes H2O to O2 in 4 steps (S0 through S4)

  8. Non-cyclic phosphorylation • Coupled photosystems linked by plastocyanin (pc), which takes the e- excited by P680, after it goes through the Q pool, and puts it into a similar chlorophyll a (P700) which is excited by a photon to a very reduced potential (-1.3V) that can reduce NADP+ to NADPH • If there is enough NADPH, PS I can act independently and function for cyclic phosphorylation

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