Mitochondria (MlET-oh-KAHN-dree-uh) (singular, mitochondrion) are tiny organelles that transfer energy from organic molecules to adenosine triphosphate (ATP). ATP ultimately powers most of the cell's chemical reactions. Highly active cells, such as muscle cells, can have hundreds of mitochondria. Cells that are not very active, such as fat-storage cells, have few mitochondria.

Like a nucleus, a mitochondrion has an inner and an outer phospholipid membrane, as shown in Figure 4-13. The outer membrane separates the mitochondrion from the cytosol. The inner membrane has many folds, called cristae (KRlS-tee). Cristae contain proteins that carry out energy-harvesting chemical reactions.

Mitochondrial DNA

Mitochondria have their own DNA and can reproduce only by the division of preexisting mitochondria. Scientists think that mitochondria originated from prokaryotic cells that were incorporated into ancient eukaryotic cells. This symbiotic relationship provided the prokaryotic invaders with a protected place to live and provided the eukaryotic cell with an increased supply of ATP.

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