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– 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

Pigment Chl a Chl b BChl a BChl b BChl c BChl d BChl e

Color in graph above

black red magenta orange cyan blue green

Absorption maxima (nm)

430, 663 463, 648 364, 770 373, 795 434,666 427,655 469, 654

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

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...

Anoxygenic photosynthesis

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.

Oxygenic Photosynthesis

Water-oxidizing complex is key – Mn4Ca-complex that oxidizes H2O to O2 in 4 steps (S0 through S4)

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….

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