ABSTRACT
Comparisons of DNA sequences indicate that the energy-converting organelles in present-day eukaryotes originated from prokaryotic cells that were endocytosed during the evolution of eukaryotes. The common evolutionary origin of the energy-converting machinery in mitochondria, chloroplasts, and prokaryotes is reflected in the fundamental mechanism that they share for harnessing energy. Electrons move through protein complexes in biological systems via tightly bound metal ions or other carriers that take up and release electrons easily, or by special small molecules that pick electrons up at one location and deliver them to another. Mitochondria occupy up to 20" of the cytoplasmic volume of a eukaryotic cell. Mitochondria also interact with other membrane systems in the cell, most notably the endoplasmic reticulum (ER). Contacts between mitochondria and ER define specialized domains thought to facilitate the exchange of lipids between the two membrane systems. The acquisition of mitochondria was a prerequisite for the evolution of complex animals.
