<P> A few groups of unicellular eukaryotes have only vestigial mitochondria or derived structures: the microsporidians, metamonads, and archamoebae . These groups appear as the most primitive eukaryotes on phylogenetic trees constructed using rRNA information, which once suggested that they appeared before the origin of mitochondria . However, this is now known to be an artifact of long - branch attraction--they are derived groups and retain genes or organelles derived from mitochondria (e.g., mitosomes and hydrogenosomes). </P> <P> Monocercomonoides appear to have lost their mitochondria completely and at least some of the mitochondrial functions seem to be carried out by cytoplasmic proteins now . </P> <P> A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and proteins . The two membranes have different properties . Because of this double - membraned organization, there are five distinct parts to a mitochondrion . They are: </P> <Ol> <Li> the outer mitochondrial membrane, </Li> <Li> the intermembrane space (the space between the outer and inner membranes), </Li> <Li> the inner mitochondrial membrane, </Li> <Li> the cristae space (formed by infoldings of the inner membrane), and </Li> <Li> the matrix (space within the inner membrane). </Li> </Ol>

The number of membranes surrounding a mitochondrion is