In order to understand the mechanism by which the energy released
during
respiration is conserved as ATP, it is necessary to appreciate the
structural features of mitochondria. These are organelles in animal and
plant cells in which oxidative phosphorylation takes place. There are
many
mitochondria in animal tissues; for example, in heart and skeletal
muscle,
which require large amounts of energy for mechanical work, in the
pancreas,
where there is biosynthesis, and in the kidney, where the process of
excretion begins. Mitochondria have an outer membrane, which allows the
passage of most small molecules and ions, and a highly folded inner
membrane (cristae), which does not even allow the passage of small ions
and
so maintains a closed space within the cell. The electron-transferring
molecules of the respiratory chain and the enzymes responsible for ATP
synthesis are located in and on this inner membrane, while the space
inside
(matrix) contains the enzymes of the TCA cycle. The enzyme systems
primarily responsible for the release and subsequent oxidation of
reducing equivalents are thus closely related so that the reduced
coenzymes formed during catabolism (NADH and FADH) are available as
substrates for respiration.
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Images of mitochondria Three-dimensional model was reconstructed by electron tomography. The outer membrane and cristae are shown in purple and yellow, respectively. [work by T. Frey (SDSU) and G. Perkins (UCSD); with the permission of Professor Frey] |
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