Glycolysis (it literally means glucose degradation) is a metabolic pathway which changes glucose C6H12O6 into pyruvate CH3COCOO + H. The free energy which is generated in this process is utilized to make high energy compounds namely ATP (adenosine triphophate) and NADH (reduced nicotinamide adenine dinucleotide).
It is a confirmed process of ten reactions with ten intermediate compounds (one step has 2 intermediates). The intermediate steps supply the point of entry for glycolysis. Now let’s consider one example, a lot of monosaccharide like fructose, glucose and galactose, could be turned in to one of the intermediates. The intermediate in itself may directly be of use. Consider an example the intermediate dihydroxyacetone phosphate; it is a source of glycerol which gets together with fatty acids to create fat.
Glycolysis is recognized as the archetype of a universal metabolic pathway. This process happens with some degree of variation in all the organisms both aerobic and anaerobic. The frequent occurrence of glycolysis shows that it is one of the older known metabolic pathways. The most commonly occurring glycolysis is the Embden-Meyerhof pathway which was found out by Gustav Embden and Otto Meyerhof.
One way of doing this is to just get the pyruvate to do oxidation; in this procedure the pyruvate gets converted in to lactate (this is the conjugate base of lactic acid) in a process which is called lactic acid fermentation. This process can be represented in a word equation as:
pyruvate + NADH + H -> lactate + NAD
This reaction happens in the bacteria which are involved in making yogurt (lactic acid makes the milk to curdle). This reaction also happens in animals which are under hypoxic (or partially anaerobic) conditions, found for example in overused muscles which are lacking oxygen, or in infracted heart muscle cells. In most tissues for cells this is the final resort for energy; most of the animal tissues cannot maintain the anaerobic respiration over an extended period of time.
Some organisms like yeast turn NADH to NAD in a reaction called as ethanol fermentation. In the reaction the pyruvate is turned first into acetaldehyde and CO2, after this into ethanol.
The lactic acid fermentation and ethanol fermentation can happen in the lack of oxygen presence. The anaerobic fermentation lets a lot of single celled organisms to use glycolysis as their only source of energy. From the two examples above regarding the fermentation, NADH is oxidized by sending 2 electrons to pyruvate. But anaerobic bacteria use a big range of compounds as the terminal electron acceptors in the process of cellular respiration.
Remember oxygen is not an essential for the glycolysis to occur. In many organisms such as C. tetani (this causes tetanus) or C. Perfringence (this causes gangrine) called as obligate anaerobes, the oxygen presence will be lethal. In the organisms that use glycolysis, absence of oxygen stops pyruvate from being metabolized to CO2 and H2O through the citric acid cycle and the electron transport chain (which relies on oxygen) doesn’t work. Fermentation will not generate energy more that already generated from glycolysis (2 ATP’s) but serves to re obtain NAD so the glycolysis can go on. There are useful end products created such as lactate or ethanol.