Mitochondria are found in nearly all eukaryotic cells, including
those of plants, animals, fungi, and most unicellular
eukaryotes. Some cells have a single large mitochondrion, but more often a cell has hundreds or even thousands of
mitochondria; the number correlates with the cell’s level of
metabolic activity. For example, cells that move or contract
have proportionally more mitochondria per volume than
less active cells.
Each of the two membranes enclosing the mitochondrion
is a phospholipid bilayer with a unique collection of
embedded proteins. The outer membrane
is smooth, but the inner membrane is convoluted, with
infoldings called cristae. The inner membrane divides the
mitochondrion into two internal compartments. The first
is the intermembrane space, the narrow region between the
inner and outer membranes. The second compartment, the
mitochondrial matrix, is enclosed by the inner membrane.
The matrix contains many different enzymes as well as the
mitochondrial DNA and ribosomes. Enzymes in the matrix
catalyze some of the steps of cellular respiration. Other proteins
that function in respiration, including the enzyme that
makes ATP, are built into the inner membrane. As highly
folded surfaces, the cristae give the inner mitochondrial
membrane a large surface area, thus enhancing the productivity
of cellular respiration.