Prokaryotes obtain energy by performing chemical reactions within the cell. Some prokaryotes are also capable of additional reactions that produce more energy, similar to what happens in the mitochondria of eukaryotes.
Prokaryotes can obtain energy from a number of sources, including:
Phototrophs: Obtain energy from sunlight
Chemotrophs: Obtain energy from chemical compounds
Autotrophs: Fix carbon from carbon dioxide
Heterotrophs: Get carbon from other living things
Most prokaryotes are chemoheterotrophs, meaning they depend on other organisms for both energy and carbon.
Prokaryotic cells use the ETC (electron transport chain) system on their cell membrane to produce ATP. In the absence of mitochondria, cellular respiration takes place in the cytoplasm or surface of the plasma membrane.
Scientists believe that prokaryotes do not have a nucleus because they evolved prior to the development of more complex cells with internal membranes. An internal nuclear membrane is necessary for the formation of a nucleus.
Prokaryotes only have one loop of DNA, so they don't need a nucleus to contain their chromosomes. Additionally, the evolution of the nuclear membrane allowed for the separation of transcription and translation processes, which is helpful for eukaryotic cells but not prokaryotes.