32 Environmental Biotechnology
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 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 oxida-
tion–reduction reactions whose function is to transfer electrons from one complex
to the next. The ratios of the complexes one to another varies from cell type to
cell type. However, the concentration of thecytochrome acomplex per unit area
of inner membrane stays fairly constant. What changes from cell type to cell type
is the degree of infolding of the inner membrane, such that cells requiring a large
amount of energy have mitochondria which have a very large surface area of
inner membrane, which is highly convoluted thus providing a high capacity for
electron transport. The process which couples ATP synthesis to electron trans-
port in mitochondria and which still evades a complete description, is oxidative
phosphorylation or more accurately, respiratory-chain phosphorylation. There are
three sites within the mitochondrial chain which span the interaction between two
neighbouring complexes, which on the basis of energy calculations are thought
to witness a release of energy sufficient to synthesise almost one molecule of
Figure 2.9 Mitochondrial electron transport chain