Transcript Notes 8 - Photosynthesis

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
Color in graph
above
Absorption
maxima
(nm)
Chl a
Chl b
BChl a
BChl b
BChl c
BChl d
BChl e
black
red
magenta
orange
cyan
blue
green
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
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)
Non-cyclic phosphorylation
• Coupled photosystems
linked by plastocyanin
(pc), which takes the eexcited 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