Mitochondria (singular: mitochondrion)
are usually about 1 μm in diameter and can be various
They are surrounded by two membranes (an envelope).
The inner of these is folded to form finger-like cristae which project
into the interior solution, or matrix. The space between the
two membranes is called the intermembrane space.
The outer membrane contains a transport protein called porin,
which forms wide aqueous channels allowing easy access
of small, water-soluble molecules from the surrounding
cytoplasm into the intermembrane space. The inner
membrane is a far more selective barrier and controls
precisely what ions and molecules can enter the matrix.
BIOLOGY, tenth edition, Solomon, Martin, Martin, Berg, 2015, 2011 CENGAGE Learning
BIOLOGY, Eleventh Edition, Raven, Johnson, Mason, Losos, Singer, McGraw Hill Education