So what is it that they have in place of mitochondiria?

A fascinating book "Power, Sex, Suicide: Mitochondria and the Meaning of Life" (http://www.amazon.com/Power-Sex-Suicide-Mitochondria-Meaning...) suggested that the multicellullar organisms on Earth evolved only once, when the symbiotic relationship between two types of bacteria was formed - one of them became "host", and the other became mitochondria and specialized for energy production.

The book suggests that the conditions for this to happen were very unusual and are unlikely to ever happen again. It does even suggest that this may mean that life in the Universe, more complicated that single-cell organisms, is very unlikely to evolve.

If eukaryotes evolved more than once, then there's some more hope for extraterrestrial life I suppose ...