This is an artist’s conception of what a mitochondrion looks like.
Almost always artists’ conceptions of these things have only vague
resemblance to the reality. But, in any case, you can begin to get a
feeling for what one thinks about their appearance. Here are
mitochondria sliced open by the hand of the artist. And, interestingly,
mitochondria have their own DNA in them. One now accepts the fact
that mitochondria are the descendents of bacteria which insinuated
themselves into the cytoplasms of larger cells, roughly 1.5 billion years
ago, and began to do a specialized job which increasingly became the
job of energy production within cells.