Transcript Cyclic and noncyclic electron flow in green sulfur bacteria

Photosynthesis
The conversion of light energy to chemical
energy
Basic energy considerations
The possible fates of
an excited electron
Energy and Carbon Metabolism: An
overview
Energy
Chemotrophs
Chemo-lithotrophs
Chemo-organotrophs
Carbon
metabolism
Heterotroph
Autotroph
(CO2)
Phototrophs
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
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.
Diagram of a photosynthetic unit, showing the pathway of exciton transfer
from antenna molecules to the reaction center (orange)
Note that cyanobacteria photosystem I resembles
that of the green sulphur bacteria and cyanobacteria
photosystem II resembles that of the purple bacteria.
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
Reaction center of purple nonsulfur bacterium
Rhodopseudomonas viridis.
Cyclic and noncyclic electron flow in purple nonsulfur bacteria
Rhodobacter sphaeroides during photosynthetic (anaerobic) growth (black
arrows) and chemoheterotrophic aerobic growth (red arrows)
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.
Escherichia coli
Rhodopseudomonas palustris
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.
Cyclic and noncyclic electron flow in green sulfur bacteria
The P840* of these organisms has a sufficiently high reduction potential to
directly reduce pyridine nucleotid.
Organization of a chlorosome from a green
sulfur bacterium
An electron micrograph of Chlorobium
tepidum.
Chlorobium tepidum, has for years
been a model species for researchers
studying green-sulfur bacteria.
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
Electron flow in reaction center of a cyanobacterium
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.
Photolysis reaction of photosystem II
Evolution of one
molecule of oxygen
requires the stepwise
accumulation of four
oxidizing equivalents
in photosystem II.
Chromophores of phycobilisomes
Synechocystis sp.
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.
Light-driven proton pump of halophilic bacteria
Light-driven proton pump of halophilic bacteria
The chemical reactions
of retinal underlying the
pumping mechanism.
No electron transport is
involved in this system
Demonstration that a proton gradient drives
ATP synthesis.