The Fermentation Process
Fermentation refers to a metabolic process of energy generation that uses an organic compound as a terminal electron acceptor. Organisms obtain energy necessary to fuel biological work from the rearrangement of chemical bond energy, the movement of protons or the movement of electrons. In the case of the fermentation of sugars, the energy of light captured by the plant during photosynthesis and used to create sugar is recaptured in the catabolic process that breaks down sugars to create the high energy molecule adenosine triphosphate (ATP). The terminal phosphate bond energy released in the hydrolysis of ATP can be coupled to energetically unfavorable reactions to drive those reactions, producing adenosine diphosphate (ADP) in the process, along with the desired product or metabolite. ATP serves as the major source of energy used for biological work within cells.
In respiration, an inorganic compound is used as terminal electron acceptor. The most commonly used compound is molecular oxygen which, during the process of respiratory electron transfer, becomes converted to water. In this case, energy is captured from the movement of electrons from a high energy state to a lower energy state using a series of enzymes and co-factors housed in the mitochondria of the cell. Bacteria can use other inorganic compounds as terminal electron acceptors. ATP is also produced from this process. Finally, energy can be generated from the movement of protons from a region of high concentration (low pH) to a region of lower concentration (high pH) across a membrane creating what is called a “proton motive force”. The energy released in energetically-favorable proton movements can also be captured for generation of ATP. The lactic acid bacteria found in wine can use this mode of ATP synthesis. The production of ethanol from grape sugar by yeast is a fermentation process in which the intermediate, acetaldehyde, serves as the terminal electron acceptor, becoming reduced to the two-carbon alcohol, ethanol.