Essential Cell Biology 5th edition

Mitochondria and Oxidative Phosphorylation
459
MITOCHONDRIA AND OXIDATIVE
PHOSPHORYLATION
Mitochondria are present in nearly all eukaryotic cells, where they produce
the bulk of the cell’s ATP. Without mitochondria, eukaryotes would
have to rely on the relatively inefficient process of glycolysis for all of
their ATP production. When glucose is converted to pyruvate by glycolysis
in the cytosol, the net result is that only two molecules of ATP are
produced per glucose molecule, which is less than 10% of the total free
energy potentially available from oxidizing the sugar. By contrast, about
30 molecules of ATP are produced when mitochondria are recruited to
complete the oxidation of glucose that begins in glycolysis. Had ancestral
cells not established the relationship with the bacteria that gave rise
to modern mitochondria, it seems unlikely that complex multicellular
organisms could have evolved.
The importance of mitochondria is further highlighted by the dire consequences
of mitochondrial dysfunction. Defects in the proteins required for
electron transport, for example, are responsible for an inherited disorder
called myoclonic epilepsy and ragged red fiber disease (MERRF). Because
muscle and nerve cells need large amounts of ATP to function normally,
individuals with this condition typically experience muscle weakness,
heart problems, epilepsy, and often dementia.
MERFF, like many of the disorders that affect mitochondrial function,
stems from mutations that disable genes present in mitochondrial DNA
(see Figure 14−5). Because mitochondria are passed down from mother
to child (sperm mitochondria are lost after fertilization), such mutations
are transmitted by the egg. To prevent the transmission of these lifethreatening
defects, reproductive biologists have developed methods for
removing the nucleus from an egg that carries faulty mitochondria and
transferring it to a donor egg that has healthy mitochondria. Although a
baby boy produced using this form of mitochondrial replacement therapy
was born in 2016, the approach remains controversial, in part because
the effects of having genetic material from three “parents”—mother,
father, and mitochondrial donor—are unknown.
In this section, we review the structure and function of mitochondria.
We outline how this organelle makes use of an electron-transport chain,
embedded in its inner membrane, to generate the proton gradient needed
to drive the synthesis of ATP. And we consider the overall efficiency with
which this membrane-based system converts the energy stored in food
molecules into the energy contained in the phosphate bonds of ATP.
Mitochondria Are Dynamic in Structure, Location, and
Number
Isolated mitochondria are generally similar in appearance to their bacterial
ancestors. Inside a cell, however, mitochondria are remarkably
adaptable and can adjust their location, shape, and number to suit that
particular cell’s needs. In some cell types, mitochondria remain fixed in
one location, where they supply ATP directly to a site of unusually high
energy consumption. In a heart muscle cell, for example, mitochondria
are located close to the contractile apparatus, whereas in a sperm they
are wrapped tightly around the motile flagellum (Figure 14–6). In other
cells, mitochondria fuse to form elongated, tubular networks, which are
diffusely distributed through the cytoplasm (Figure 14–7). These networks
are dynamic, continually breaking apart by fission (see Figure 14–4) and
fusing again (Movie 14.1 and Movie 14.2).