In eukaryotes, mitochondria are responsible for the production of ATP. ATP is used as the currency of the cell, or so says my molecular biology professor Dr. Niels Bohls; if you want to do anything as a cell, you must have ATP to power your cellular machinery. Mitochondria is therefore thought of as the "powerhouse" of the cell.

That is, if the organism is aerobic and uses oxygen as an electron acceptor like humans, cats, and fish, for example. If the eukaryote is anaerobic instead, such as the genera Trichomonas, hydrogenosomes are present in place of mitochondria.

Hydrogenosomes serve the same function as mitochondria, to convert ADP to ATP, but lacks cristae and citric acid cycle enzymes.

To convert ADP to ATP, the hydrogenosome must have access to pyruvate, which is obtained from glucose metabolism. The pyruvate is oxidized to produce hydrogen gas, carbon dioxide gas, and acetate. The energy liberated from this reaction is used by ATP Synthase for ATP production.

Methanogens can sometimes be found living in the cytoplasm of these eukaryotic cells in a symbiotic relationship. These methanogens will consume the hydrogen gas produced by the host organism, resulting in methane gas.

Anaerobic eukaryotes are relatively rare. Ciliates found in rumen are an example of such, and yeast can be considered facultatively aerobic, meaning they can live without oxygen. According to Roberts (1998), anaerobic eukaryotes evolved fairly recently.

Converted from Brock Biology of Microorganisms, 10th Edition. Pearson Education, Inc. Upper Saddle River, NJ 07558. Pp 475 - 476.

Roberts, Dave. "Eukaryotes in Extreme Environments."