Photosynthesis

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Photosynthesis

• The conversion of light energy to chemical energy

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Basic energy considerations
The possible fates of an excited electron
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Energy and Carbon Metabolism An overview
Energy
Chemotrophs
Phototrophs
Chemo-lithotrophs Chemo-organotrophs
Carbon metabolism
Heterotroph
Autotroph (CO2)

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Rhodopseudomonas palustris (Bacteria) Commonly
found in soil and water. A remarkably versatile
microbe, it derives energy from sunlight and from
other sources, and can live with or without
oxygen
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Structures of chlorophyll a and
bacteriochlorophyll a. The chlorophylls are
structurally related to heme, but the Fe2 of
heme is replaced by Mg2 in the chlorophylls.
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Diagram of a photosynthetic unit, showing the
pathway of exciton transfer from antenna
molecules to the reaction center (orange)
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Note that cyanobacteria photosystem I resembles
that of the green sulphur bacteria and
cyanobacteria photosystem II resembles that of
the purple bacteria.
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The purple non-sulphur bacteria can have
different lifestyles
Light and anaerobic (no oxygen present) In the
light under anaerobic condition they can grow
using photophosphorylation. If they come into
contact with oxygen photosynthesis is
stopped. Dark and aerobic (oxygen present) In the
dark, in the presence of oxygen the purple
non-sulphur bacteria carry out C-degradation in
which the reducing equivalents NADH H act as
an electron donor in respiration. Oxygen is the
terminal electron acceptor. In this respect they
are very similar to E. coli
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Reaction center of purple nonsulfur
bacteriumRhodopseudomonas viridis.
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Cyclic and noncyclic electron flow in purple
nonsulfur bacteriaRhodobacter sphaeroides during
photosynthetic (anaerobic) growth (black arrows)
and chemoheterotrophic aerobic growth (red arrows)
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The KEGG database
Kyoto encyclopedia of genes and
genomes. http//www.genome.jp/kegg/ This is a
complex and extensive database. Complete genomes
sequences (DNA sequence) are automatically
translated into genes. These are in turn compared
to all known genes and a function, if possible is
assigned to each gene. These results are used to
predict the metabolism of the organism in
question. There are over 170 bacteria and archeae
sequences in the KEGG database. Have a look, but
remember that these are computer generated and
most of the predicted pathways have nver been
confirmed by laboratory experiments.
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Escherichia coli
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Rhodopseudomonas palustris
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Green sulphur bacteria
Phototrophic autotrophs Electron donors that can
be used are hydrogen, hydrogen sulphide and
thiosulphate. Found in water at a depth where
there is still light and a source of, lets say
hydrogen sulphide. Strictly anaerobic. Specialized
light harvesting system called chlorosomes.
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Cyclic and noncyclic electron flow in green
sulfur bacteriaThe P840 of these organisms has
a sufficiently high reduction potential to
directly reduce pyridine nucleotid.
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Organization of a chlorosome from a green sulfur
bacterium
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An electron micrograph of Chlorobium
tepidum. Chlorobium tepidum, has for years been a
model species for researchers studying
green-sulfur bacteria.
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Cyanobacteria
The cyanobacteria are a very large group of
ecologically diverse bacteria. They are
photoautotrophs. They have, complex internal
membrane systems, specialized light harvesting
systems and two photosystems. Water is used as an
electron donor and the oxidized product is oxygen.
Synechococcus
Synechocystis
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Electron flow in reaction center of a
cyanobacterium
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Phycobilisome of cyanobacteria
The antenna pigments of cyanobacteria are
arranged in phycobilisomes. These knoblike
structures project from the outer surface of the
cell membrane. Shown here is the phycobilisome of
Synechococcus sp.
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Photolysis reaction of photosystem II
Evolution of one molecule of oxygen requires the
stepwise accumulation of four oxidizing
equivalents in photosystem II.
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Chromophores of phycobilisomes
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Synechocystis sp.
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Halophilic archeae
The halophilic (salt loving) archeae live in salt
rich environments. This is a so called extreme
environment. Very few other microorganisms are
found in these environments. The halophilic
archeae are heterotrophic and have an aerobic
respiration system in which amino acids or sugars
are oxidized to CO2 and H2O. They contain a
membrane bound bacteriorhodopsin which is a light
driven H pump. The proton gradient so produced
can be used in the synthesis of ATP from ADP and
phosphate.
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Light-driven proton pump of halophilic bacteria
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Light-driven proton pump of halophilic bacteria
The chemical reactions of retinal underlying the
pumping mechanism. No electron transport is
involved in this system
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Demonstration that a proton gradient drives ATP
synthesis.