Prokaryotes are single celled organisms that do not contain membrane bound organelles or a nucleus. Eukaryotes have these organelles and cellular respiration takes place in the mitochondria to convert nutrients into ATP, the main energy storage unit for a cell. Prokaryotes also produce ATP but the enzymes required for its production are attached to the cellular membrane that surrounds the cell. The cellular membrane can adjust and fold itself to promote ATP production through these enzymes when necessary. Prokaryotes can be either autotrophic or heterotrophic. Autotrophs can synthesize their own food for energy production whereas heterotrophs must consume food to use for energy production.