Is O2 always required to “burn” food for energy?
Many organisms, even humans, are capable of anaerobic energy production in a process known as fermentation. Fermentation processes do not produce as much energy as aerobic reactions and often generate harmful byproducts. An example in humans is the production of lactic acid by lactic acid fermentation in muscle cells during exercise. This occurs when O2 supplies in muscles are insufficient and the body adjusts by using fermentation as an energy reaction. This leads to the familiar burning sensation in overworked muscles. Fermentation is the process that creates many food products: yogurt, wine, and cheese, for example. In addition, there are certain bacteria, archaea, and protists that are strictly anaerobic, meaning oxygen is poisonous. Their only energy reaction is fermentation. These organisms live in habitats such as sediments or lakes that totally lack oxygen.
