Eukaryotes Without Mitochondria
In contrast to the prokaryotes, eukaryotes have a more complex layout, including membrane-bound organelles like mitochondria. Most eukaryotes have mitochondria, and all multi-cellular eukaryotes do. However, a few one-celled eukaryotes lack mitochondria. All of this type of eukaryote live as parasites. These particular eukaryotes are believed to be descended from primitive eukaryotes that never had mitochondria, or ones with a "secondary loss," meaning that, at one point, their ancestors had mitochondria, but later lost them. Additionally, some multicellular eukaryotes lack mitochondria in specific cells. For example, human red blood cells lack mitochondria, an adaptation that either reduces the cells' size or prevents them from using the oxygen they carry.