Two cross-sections of mitochondria. You can see the cristae.

A diagram of a mitochondrion's inner parts.

Schematic of typical animal cell, showing subcellular components. Organelles:
(1) Nucleolus
(2) Nucleus
(3) Ribosomes
(4) Vesicle
(5) Rough endoplasmic reticulum (ER)
(6) Golgi apparatus
(7) Cytoskeleton
(8) Smooth ER
(9) Mitochondria
(10) Vacuole
(11) Cytoplasm
(12) Lysosome
(13) Centrioles within centrosome

Mitochondria (sing. mitochondrion) are organelles, or parts of a eukaryote cell. They are in the cytoplasm, not the nucleus.

They make most of the cell's supply of adenosine triphosphate (ATP), a molecule that cells use as a source of energy. Their main job is to convert energy. They oxidise glucose to provide energy for the cell. The process makes ATP, and is called cellular respiration. This means mitochondria are known as "the powerhouse of the cell".