constantly change shape and intracellular position; they multiply by sequent closure of channels selectively permeable to sodium and
growth and fission, and may undergo fusion. potassium).
The mitochondrial matrix is an aqueous environment. It contains a Carrier proteins bind their specific solutes, such as amino acids, and
variety of enzymes, and strands of mitochondrial DNA with the capacity transport them across the membrane through a series of conforma-
for transcription and translation of a unique set of mitochondrial genes tional changes. This latter process is slower than ion transport through
(mitochondrial mRNAs and transfer RNAs, mitochondrial ribosomes membrane channels. Transport by carrier proteins can occur either pas-
with rRNAs). The DNA forms a closed loop, about 5 µm across; several sively by simple diffusion, or actively against the electrochemical gradi-
identical copies are present in each mitochondrion. The ratio between ent of the solute. Active transport must therefore be coupled to a source
its bases differs from that of nuclear DNA, and the RNA sequences also of energy, such as ATP generation, or energy released by the coordinate