The production of energy from organic compounds, such as glucose, by oxidation using chemical (usually organic) compounds from within a cell as "electron acceptors" is called fermentation. This is an alternative to cellular respiration, in which electrons from glucose and other compounds being oxidized are transferred to an acceptor brought from outside the cell, typically oxygen. This is an alternative to cellular respiration (without oxygen, cellular respiration cannot occur).
Fermentation vs. Cellular Respiration
While fermentation can take place under anaerobic (lack of oxygen) conditions, it can happen when oxygen is abundant too. Yeast, for instance, prefers fermentation to cellular respiration if enough glucose is available to support the process, even if plenty of oxygen is available.
Glycolysis: The Breakdown of Sugar Prior to Fermentation
When energy-rich sugar--glucose in particular--enters a cell, it is broken down in a process called glycolysis. Glycolysis is a prerequisite step both for cellular respiration and fermentation. It is a common pathway for the breakdown of sugar, which can lead to either fermentation or cellular respiration.
Glycolysis Requires No Oxygen
Glycolysis is an ancient biochemical process, having emerged very early in evolutionary history. The core reactions for glycolysis were "invented" by microorganisms long before photosynthesis evolved, which emerged roughly 3.5 billion years ago, but which would take roughly 1.5 billion years to fill the seas and atmosphere with any appreciable amount of oxygen. Thus, even complex eukaryotes (the biological "domain" that includes the animal, plants, fungi, and protist kingdoms) are capable of producing energy without respiration, without oxygen, etc. In yeast, which belong to the fungi kingdom, the chemical products of glycolysis are fermented to produce energy for the cell.
From Glycolysis to Fermentation
At the end of glycolysis, the six-carbon structure of glucose will have been split into two molecules of the three-carbon compound called pyruvate. Also produced is the chemical NADH, from a more "oxidized" chemical called NAD+. In yeast, pyruvate undergoes "reduction," the gaining of electrons, which are then transferred from the NADH produced earlier in glycolysis to yield acetaldehyde and carbon dioxide.
Acetaldehyde then is reduced further to ethyl alcohol, the ultimate product of fermentation. In animals, including humans, pyruvate can be fermented when the availability of oxygen is low. This is especially true in muscle cells. When this happens, though tiny amounts of alcohol are produced, most of the pyruvate from glycolysis is reduced not to alcohol, but rather to lactic acid.
While lactic acid can leave animal cells and be used to produce energy in the heart, it can build up within muscles, causing pain and decreased athletic performance. This is the "burning" feeling you feel after lifting weights, running for a long period of time, sprinting, lifting heavy boxes, etc.
ATP and Energy Production Via Fermentation
The universal energy carrier in cells is a chemical known as ATP (adenosine triphosphate). If utilizing oxygen, cells can produce ATP through glycolysis followed by cellular respiration--such that one molecule of glucose sugar yields 36-38 molecules of ATP, depending on the cell type.
Out of these 36-38 molecules of ATP, only two are produced during the glycolysis phase. Thus, if using fermentation as an alternative to cellular respiration, cells make a great deal less energy than they do using respiration. However, in low oxygen or anaerobic conditions, fermentation can keep an organism living and surviving since they would otherwise have no respiration without oxygen.
Uses for Fermentation
Humans utilize the process of fermentation for our own benefit, especially when it comes to food and drink. Bread making, beer and wine production, pickles, yogurt, and kombucha all use the process of fermentation.