Mitochondria are enclosed by a double-membrane system made up of an inner and an outer membrane. The space between these two membranes is called (surprise!) the intermembrane space. The inner membrane encloses the interior of the mitochondrion, which is known as the mitochondrial matrix.The inner membrane also forms a large number of folds, called cristae, that extend into the matrix.See Fig.he space between these two membranes is called (surprise!) the intermembrane space. The inner membrane encloses the interior of the mitochondrion, which is known as the mitochondrial matrix. The inner membrane also forms a large number of folds, called cristae, that extend into the matrix. See Fig.
1 The strict definition of relative atomic mass (Ar) is that it equals the average mass of all the isotopic atoms present in the element compared to 1/12th the mass of a carbon-12 atom (relative isotopic mass of 12.0000).
Atomic masses expressed in unified atomic mass units (i.e. relative isotopic masses) are always close to whole-number values, but never (except in the case of carbon-12) exactly a whole number, for two reasons: 1  protons and neutrons have different masses, and different nuclides have different ratios of protons and neutrons.