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Mitochondrial structure and
Mitochondria are rod-shaped or filamentous
organelles about 0.5-1.0 mm in diameter.
Number of mitochondria in a cell depends on
its activity. Mammalian liver cells contain
between 1000 and 2000 mitochondria,
occupying 20% of the cell volume.
Mitochondrion is surrounded by an envelope
of two phospholipid membranes. The outer
membrane is smooth, but the inner is much
folded inwards to form cristae. These give the
inner membrane a large total surface area
The two membranes have different
composition and properties. The outer
membrane is relatively permeable to small
molecules, whilst the inner membrane is less
Mitochondrion structure and
The inner membrane is more complex in structure
than the outer membrane as it contains the
complexes of the electron transport chain and the
ATP synthetase complex. It is permeable only to
oxygen, carbon dioxide and water. It is made up of
a large number of proteins that play an important
role in producing ATP, and also helps in regulating
transfer of metabolites across the membrane. The
inner membrane has infoldings called the cristae
that increase the surface area for the complexes
and proteins that aid in the production of ATP, the
energy rich molecules.
Mitochondrion structure and
The matrix of the mitochondrion is the site of
the lonk reaction and Krebs cycle, and
contains the enzymes needed for reactions.
It also contains ribosomes and several
identical copies of looped mitochondrial DNA.
ATP is formed in the matrix by the activity of
ATP synthase on the cristae. The energy for
the production of ATP comes from the
hydrogen ion gradient between the
intermembrane space and the matrix.
If oxygen is unavailable the Krebs cycle and
electron transfer chain cannot operate. This
is because without oxygen there would be no
way of disposing of the hydrogen at, for
example, the end of the electron transfer
chain. However, even in anaerobic conditions,
glycolysis occurs so reduced NAD still forms. If
glycolysis is to continue, the reduced NAD
must be reoxidized, that is, the hydrogen must
be removed and disposed of. Anaerobic
organisms have developed two ways of doing
1). In yeast, pyruvate is decarboxylated to
produce ethenal. Ethenal then accepts the
hydrogen from NAD and forms ethanol. This
releases the NAD to be reused
in glycolysis. The conversion of pyruvic acid to
ethanol with the release of carbon dioxide is
called alcoholic fermentation.
2). In mammals, the pyruvate accepts the
hydrogen from NAD and is reduced to
lactate. The NAD is then available for further
use in glycolysis. If oxygen later
becomes available, the lactate is reoxidised.
Although glucose is the essential respiratory
substrate for some cells, such as neurones in
the brain, red blood cells and lymphocytes,
other cells can oxidise lipids and amino acids.
When lipids are respired, carbon atoms are
removed in pairs, as acetyl CoA, from the fatty
acid chains and fed into the Krebs cycle. The
carbon-hydrogen skeletons of amino acids are
converted into pyruvate or into acetyl CoA.
Energy density (kJ/g)
The greater the number of hydrogens in the
structure of the substrate molecule, the greater
the energy value.
The energy value of a substrate is determined by
burning a known mass of the substance in
oxygen in a calorimeter.
The respiratory quotient (RQ) is defined as the
ratio of carbon dioxide produced to oxygen
consumed per unit time by an organism.
Oxygen uptake during respiration
can be measured using a