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

Chemotrophs Phototrophs

Chemo-lithotrophsChemo-organotrophs

Energy

Carbon metabolism

Heterotroph

Autotroph (CO2)

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 bacteriumRhodopseudomonas viridis.

Cyclic and noncyclic electron flow in purple nonsulfur bacteriaRhodobacter 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 bacteriaThe 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

Synechococcus

Synechocystis

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.

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.