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.