Approximately 25% of the inner-membrane protein is made up of the
flavoproteins, cytochromes, and enzymes of the respiratory chain
and ATP synthesis. The other 75% is structural protein, in association
with lipids. The inner membrane is more like a bacterial membrane
than that of a eucaryotic cell, both in chemical composition and
in thickness and structure. Electron micrographs of comparable preparations
of bacterial membranes and mitochondrial inner membranes show the
same spheres on stalks. It has been proposed that the respiratory
chain in mitochondria is located at the base of these stalks, and
that the spheres contain the coupling factors for ATP synthesis.
The enzymes of the citric acid cycle float freely in the matrix,
like the enzymes of glycolysis in the cytoplasm.
The reactions of glucose metabolism are shared between the cell
and the mitochondria. (This separate-but-equal language is appropriate
since a mitochondrion is topologically outside the cell.) Degradation
of glucose to pyruvate via the glycolytic pathway is carried out
in the cell cytoplasm. In anaerobic metabolism by yeast, pyruvate
is reduced to ethanol, no net NADH is produced, and the story ends.
In oxygen-starved human muscles, the same process is followed with
lactate as the end product.