Biomedical Engineering Reference
In-Depth Information
electrons flow down from high to low energy using that energy to drive protons
across a membrane against a high concentration, thus developing the proton gra-
dient. When the electron flow stops, the protons migrate down the concentration
gradient, simultaneously releasing energy to drive the synthesis of ATP through
membrane associated proteins. The model system described first is that of mito-
chondria and later in this chapter, comparisons with bacterial systems associated
with oxidative phosphorylation and those systems associated with methanogene-
sis will be made.
Electron transport chains comprise cytochrome molecules which trap electrons,
and enzymes which transfer electrons from a cytochrome to its neighbour. The
quantity of energy released during this transfer is sufficient to drive the synthesis
of approximately one ATP molecule by the enzyme ATP synthetase . The whole
system is located in a membrane which is an essential requirement of any electron
transport chain because of the need to organise it topographically, and to allow
the establishment of a pH gradient. Also there is evidence that during active
electron transport, the morphology of the membrane changes and is believed to
store energy in some way yet to be fully elucidated. Consequently, an intact
membrane is essential. Any toxic substance which damages the integrity of a
membrane has the potential to interrupt the functioning of the electron transport
chain thereby reducing the facility for ATP synthesis and potentially killing the
organism. The chain may also be disrupted by interference with the electron
carriers. Such a chemical is cyanide, which complexes with cytochrome oxidase,
and for which research into a biological remediation route is underway.
The mitochondrial electron transport system and oxidative phosphorylation
The electron transport system in eukaryotes is located in the inner membrane of
mitochondria. A representation of the system is given in Figure 2.9.
The chain is a series of complexes comprising cytochromes, and enzymes
involved in oxidation-reduction reactions whose function is to transfer elec-
trons from one complex to the next. The ratios of the complexes one to another
Figure2.9 Mitochondrialelectrontransportchain
Search WWH ::




Custom Search