Mitochondria are small compartments partitioned by membranes and found exclusively in complex cells. These organelles are often called the "power plants" of the cell because their main job is to make energy. Mitochondria are highly unusual--they contain their own genetic material and protein-making machinery enwrapped in a double membrane. Many scientists believe mitochondria were once free-living bacteria that colonized complex cells sometime during evolution. Besides their role in energy production, mitochondria participate in a natural process called programmed cell death--or PCD--during development. Scientists do not completely understand PCD, or how obsolete cells self-destruct. Researchers hope to answer that question by studying PCD in a number of creatures, including the fruit fly, Drosophila melanogaster, and the mustard plant, Arabidopsis thaliana.