Secondly, the chemoautotrophic organisms: their
ETC utilises reduced then oxidised inorganic
compounds (these can range from hydrogen
sulphide to uranium!).
Another group, the phototrophs, use relatively
oxidised compounds as electron donators; this
should not be favourable, but they overcome this by
capturing light to promote those donated electrons,
so that they are capable of favourably reducing the
terminal electron acceptor; this acceptor is then used
to fix CO2.
A sub-group of these organisms use water as an
electron donator; this is so unfavourable (as water’s
oxidation potential is very negative) that it requires
two photons of light to harness enough free energy
to reduce the terminal electron acceptor. This sub-
group include the cyanobacteria and the chloroplasts
found in all photosynthetic eukaryotes.
Many prokaryotic organisms retain the ability to use
various mechanisms of energy production, and so
can produce various forms of the ETC (a mix of the
ones discussed above). They simply up regulate
enzymes specific to certain substrates upon contact
with these substrates. This is possible because of the
quinone pool, which allows for modularity in the
electron transport train.
The fermentation process led to the production of organic
acids, which gradually lowered the pH of the
environment around the cells. In order to maintain their
internal pH (a consistent pH is necessary for enzyme
optimisation) an ATP driven H+ pump evolved to pump
protons outside the primitive cells.
Oxidation of reduced organic molecules was still required
to produce the ATP to pump these protons. However,
these reduced organic molecules take long periods of
geological time to produce and so began to be depleted.
This led to a shortage of ATP, and so ATP needed to be
directed towards more fundamental metabolic reactions.
This favoured the production of H+ pumps that could use
the difference in redox potentials between compounds to
produce free energy, rather than ATP hydrolysis. Initially
these used reduced organic molecules too and transferred
their electrons to oxidised molecules, but some developed
the ability to use inorganic compounds instead, which
was selected for as there was less substrate limitation.
These pumps were the precursors of the cytochrome
complexes in future ETCs.
The efficiency of these pumps improved, and eventually
were able to not just maintain constant cellular pH but
instead establish a strong artificial proton concentration
gradient; this is where the old ATP H+ pumps came in
useful. Due to the concentration gradient, H+ flowed the
opposite direction through the pumps into the cells,
How did these driving ATP production from ADP and Pi (essentially
the function of the pump). This was the origin
methods evolve? reversing
of the ETCs of chemoheterotrophs and
The evolutionary pathway and its specifics are very chemoautotrophs.
much disputed, particularly because of the difficulty
A group of these organisms then developed the ability to
of using genetic analysis to work out the linage of
fix CO2, which was in abundant supply. However, high
prokaryotes. However, the following process is the
levels of reducing power is needed to fix this CO2. The
most widely accepted theory that has been
only way that this much reducing power could be
proposed.
produced is though the use of a photon to promote
Initially, fermentation was the only method of energy electrons to higher energy levels at the beginning of the
generation in primitive prokaryotic cells, when there ETCs. The electrons were provided by reduced inorganic
was an abundance of geologically produced reduced compounds like hydrogen sulphide and some organic
organic molecules. compounds; this marks the origin of phototrophs.
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