Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

758 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
Figure 14–9 Biochemical fractionation of purified mitochondria into
separate components. Large numbers of mitochondria are isolated from
homogenized tissue by centrifugation and then suspended in a medium
of low osmotic strength. In such a medium, water flows into mitochondria
and greatly expands the matrix space (yellow). While the cristae of the inner
membrane unfold to accommodate the swelling, the outer membrane—
which has no folds—breaks, releasing structures composed of the inner
membrane surrounding the matrix. These techniques have made it possible
to study the protein composition of the inner membrane (comprising a
mixture of cristae, boundary membranes, and cristae junctions), the outer
membrane, and the matrix.
is because it contains many porin molecules, a special class of β-barrel-type
membrane protein that creates aqueous pores across the membrane (see Figure
10–23). As a consequence, the intermembrane space between the outer and inner
membrane has the same pH and ionic composition as the cytoplasm, and there is
no electrochemical gradient across the outer membrane.
If purified mitochondria are gently disrupted and then fractionated (Figure
14–9), the biochemical composition of membranes and mitochondrial compartments
can be determined.
The Inner Membrane Cristae Contain the Machinery for Electron
Transport and atp Synthesis
Unlike the outer mitochondrial membrane, the inner mitochondrial membrane
is a diffusion barrier to ions and small molecules, just like the bacterial inner
membrane. However, selected ions, most notably protons and phosphate, as well
as essential metabolites such as ATP and ADP, can pass through it by means of
special transport proteins.
The inner mitochondrial membrane is highly differentiated into functionally
distinct regions with different protein compositions. As discussed in Chapter
10, the lateral segregation of membrane regions with different protein and lipid
compositions is a key feature of cells. In the inner mitochondrial membrane,
the boundary membrane region is thought to contain the machinery for protein
import, new membrane insertion, and assembly of the respiratory-chain complexes.
The membranes of the cristae, which are continuous with the boundary
membrane, contain the ATP synthase enzyme that produces most of the cell’s
ATP; they also contain the large protein complexes of the respiratory chain—the
name given to the mitochondrion’s electron-transport chain.
At the cristae junctions, where the membranes of the cristae join the boundary
membrane, special protein complexes provide a diffusion barrier that segregates
the membrane proteins in the two regions of the inner membrane; these complexes
are also thought to anchor the cristae to the outer membrane, thus maintaining
the highly folded topology of the inner membrane. Cristae membranes
have one of the highest protein densities of all biological membranes, with a lipid
content of 25% and a protein content of 75% by weight. The folding of the inner
membrane into cristae greatly increases the membrane area available for oxidative
phosphorylation. In highly active cardiac muscle cells, for example, the total
area of cristae membranes can be up to 20 times larger than the area of the cell’s
plasma membrane. In total, the surface area of cristae membranes in each human
body adds up to roughly the size of a football field.
INNER
MEMBRANE
INTACT
MITOCHONDRION
matrix
outer membrane
inner membrane
intermembrane
space
in medium of low osmolarity
the influx of water causes the
mitochondrion to swell and the
outer membrane to rupture, releasing
the contents of the intermembrane
space; the inner membrane remains
intact
centrifugation leaves the contents
of the intermembrane space in the
nonsedimenting fraction
INTERMEMBRANE
SPACE
transfer to a medium of high
osmolarity causes shrinkage
+
density-gradient centrifugation
separates the outer membrane
from the dense matrix and its
surrounding inner membrane
disruption and centrifugation
separate inner membrane from
matrix components
MATRIX
OUTER
MEMBRANE
The Citric Acid Cycle in the Matrix Produces NADH
Together with the cristae that project into it, the matrix is the major working part
of the mitochondrion. Mitochondria can use both pyruvate and fatty acids as fuel.
Pyruvate is derived from glucose and other sugars, whereas fatty acids are derived
from fats. Both of these fuel molecules are transported across the inner mitochondrial
membrane by specialized transport proteins, and they are then converted
to the crucial metabolic intermediate acetyl CoA by enzymes located in the mitochondrial
matrix (see Chapter 2).
MBoC6 m14.07/14.09