Fermentation is either an evolutionary bypass that some organisms use to keep glycolysis functioning under anaerobic conditions or is a biological remnant that involved very easily in the history of life, when the earth’s atmosphere contained little or no oxygen. As with glycolysis, the presence of fermentation is strong evidence for common descent of organisms from primitive cells in which glycolysis and fermentation first appeared and still persists.
In fermentation hydrogen atoms that glycolysis generates are donated to organic molecules and then reduced compound can be an organic acid like lactic acid, acetic acid, propionic acid or an alcohol in the form of ethanol, butanol. Fermentation regenerates NAD, which is needed to drive glycolysis to ultimately obtain ATP. During fermentation glucose is not completely degraded, so considerable unusable energy still remains in the products. Beyond two ATP molecules formed during glycolysis, no more ATP is produced. Fermentation serves only to regenerate NAD (oxidized from of NADH).
Types of Fermentation:
There are two types of fermentation depending upon the end product obtained. The fermentation in which the end product is an alcohol is known as alcoholic fermentation where as the fermentation in which some acid specially lactic acid is formed is known as Lactic acid fermentation.
Alcoholic fermentation: The pathway from private to ethanol is called alcoholic fermentation and is catalysed by specific microbial enzymes.
Lactic acid fermentation: The pathway in which lactate or lactic acid is produced as end product from private is known as lactic acid fermentation.
There are certain animals cells that are deprived of oxygen, temporarily carry out lactic acid fermentation.
Types of Fermenting Organisms:
Two types of organisms can carry out fermentation, obligative or obligate anaerobic and Facultative anaerobic.
Obligative anaerobic organism:
The organisms that survive only in complete absence of molecular oxygen are termed as obligative or obligate anaerobic organisms. These include certain types of bacteria.
Facultative anaerobic organisms: The organisms that survive only in the absence of molecular oxygen are termed as facultative anaerobic organisms. They are certain bacteria, yeasts, animal muscle cells which can ferment nutrients when oxygen is absent to generate some ATP by providing NAD for glycolysis. Such organisms and tissues carry out more efficient energy harvesting when oxygen is present.
Aerobic respiration: The major source of ATP:
Anaerobic generation of ATP through glycolysis and fermentation is inefficient. The end product of glycolysis (pyruvate) still contains great deal of potential bond energy that can be harvested by further oxidation. Evolution of aerobic respiration in micro-organisms and in the mitochondria of eukaryotic cells became possible only after free oxygen had accumulated in the earth’s atmosphere as a result of photosynthesis. Addition of oxygen requiring stage to energy harvesting mechanisms provided cells with more powerful and efficient way f extracting energy from nutrient molecules. Indeed without mitochondria’s large scale ATP production life would have to be at a “snail’s space” and most animals present on earth today would never have evolved.
In aerobic respiration pyruvate that glycolysis produces is shunted into a metabolic pathway called Kreb’s cycle or citric acid cycle; NADH goes to electron transport chain. During this aerobic metabolism free oxygen accepts electrons and reduces to water together with production of 34 molecules of ATP from each molecule of pyruvate consumed.
Aerobic respiration is organized into Kreb’s cycle and electron transport chain. Two electron carriers’ mitotinamide adenine di-nucleotide (NAD) and falvin adenine di-nucleotide (FAD) act as hydrogen acceptors and reduce to NADH and FADH2.
Most of the remaining energy is in the form of NADH and FADH2. These two molecules are shuttled into electron transport chain. In this chain reduced NADH and FADH2 are oxidized and their electrons are passed along a series of oxidation reduction reaction to the final acceptor oxygen. During this phase of the cycle, three molecules of Co2 are generated from each pyruvate molecule and some energy is harvested in the form of ATP.