Fermentation refers to the metabolic reaction that converts glucose and other simple sugars into acids and gases without the use of oxygen. The process occurs when muscle tissues are deprived of oxygen supply hence the cells become incapable of metabolizing the sugars into water and carbon dioxide. This anaerobic breakdown of sugars in body cells is referred to as lactic fermentation. The process of fermentation occurs when the electron conveyance chain becomes inoperative. This is commonly associated with absence of electron receptors such as oxygen. The process of fermentation can be summarized by the equation below.
C 6 H 12 O 6 + 2 NAD + + 2 ADP + 2 Pi → 2 CH 3 COCO 2 − + 2 NADH + 2 ATP + 2 H2O + 2H +
CH3COCO2 − denotes pyruvate acid while Pi depicts a phosphate. The ADP molecules and inorganic phosphates are metabolized to form ATP through a process known as substrate-level phosphorylation.
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The primary input during the fermentation process is glucose or simple sugars. On the other hand, the main outputs are 2 ATP molecules, two molecules of pyruvate acid, and NADH. The pyruvate acid produced during this process enters the Krebs cycle in which energy is transferred to molecules such as ATP, NADH, and FADH2 which store energy. CO2 is released as the waste product.
There are various similarities that exist between lactic and alcoholic fermentation. To start with, the two processes begin with glucose and generates only 2 ATP molecules. Moreover, the processes require a huge supply of NAD+ to ensure that they convert glucose to pyruvate. Finally, the reactions generate pyruvate as a waste product; this ensures that NADH is recycled into the reaction chain as NAD+. Contrary, lactic acid does not produce waste CO2. The final waste product of lactic acid fermentation is made of 3 carbon molecules whereas alcoholic fermentation releases two carbon molecules. Moreover, in lactic fermentation, pyruvate is converted directly into waste product while in alcoholic fermentation an intermediate molecule, acetaldehyde, is produced.
The initial stage of aerobic respiration occurs in the absence of oxygen and yields 2 ATP molecules. In the second phase, the cell metabolizes the pyruvate molecules in the presence of oxygen during which 18 molecules of ATP are released. In total 38 ATP molecules are released from aerobic metabolism of one glucose molecule.
