proteins and some peroxisomal membrane proteins are synthesized by ate ATP; and the mitochondrial matrix, a space enclosed by the inner
cytosolic ribosomes and contain a peroxisome targeting signal that membrane and numerous cristae. The permeability of the two mito-
enables them to be imported by proteins called peroxins (Braverman chondrial membranes differs considerably: the outer membrane is
et al 2013, Theodoulou et al 2013). Mature peroxisomes divide by freely permeable to many substances because of the presence of large
small daughter peroxisomes pinching off from a large parental non-specific channels formed by proteins (porins), whereas the inner
peroxisome. membrane is permeable to only a narrow range of molecules. The pres-
Peroxisomes often contain crystalline inclusions composed mainly ence of cardiolipin, a phospholipid, in the inner membrane may con-
of high concentrations of the enzyme urate oxidase. Oxidases use tribute to this relative impermeability.
molecular oxygen to oxidize specific organic substrates (such as L-amino Mitochondria are the principal source of chemical energy in most