How cells extract energy from glucose in the absence of oxygen. In yeast, anaerobic reactions produce alcohol, while in your muscles, they form lactic acid.
Both processes can happen thanks to alternative glucose degradation pathways that occur when normal cellular respiration that uses oxygen (aerobic) is not possible, that is, when there is no oxygen available that acts as an acceptor at the end of the transport chain of electrons. These fermentation pathways include glycolysis with a few extra reactions at the end. In yeast, the extra reactions produce alcohol; in the muscles, lactic acid. Fermentation is a widespread route, but it is not the only way to obtain energy from fuels anaerobically (in the absence of oxygen). Some living systems use a different inorganic molecule instead of OR 2 OR 2 I mean start text, O, end text, start subscript, 2, end subscript, like sulfate, as the final acceptor in an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaeas.
So where then did all the carbon that living organisms are built of come from? It turns out that most of the carbon we use today came from a collision with another smallish planet about 4.4 billion years ago.
