Fermentation

Production of ATP through the Krebs Cycle and Electron Transport Chain require the presence of oxygen. The cell's ability to produce ATP in the absence of oxygen can be traced back to the first cell, when there wasn't molecular oxygen available to the cells. Therefore, anerobic metabolism (a.k.a. fermentation) was evolved. Today, many anerobic organisms and special types of cells such as muscle tissue (which occasionally function without oxygen) use fermentation to derive their energy. Since glycolysis occurs without oxygen, it is able to convert glucose into pyruvate. Subsequently, the pyruvate is converted to either ethanol + CO₂ (in yeast) or lactate (in muscle) via fermentation. This further reduction of pyruvate gives off a little more energy for the cell to utilize. A buildup of lactate, such as when you exercise hard, causes the soreness you feel in the muscle.

The fermentation reactions can be described as follows: