In a plant cell, glycolysis leads to the formation of pyruvic acid. Further, it is broken down based on the availability of oxygen. In the absence of oxygen, the cell undergoes fermentation. The term ‘fermentation’ is derived from the Latin word ‘fevere,’ which means ‘to boil’. In it, complex substances like carbohydrates undergo incomplete oxidation. Based on the end product, the process can classified as either alcohol fermentation or lactic acid fermentation. The fermentation of alcohol takes place in yeast, which are facultative anaerobes. In them, pyruvic acid is reduced to carbon dioxide and ethanol with the help of enzymes. In this process, first the pyruvate is decarboxylated by pyruvate decarboxylase to acetaldehyde. The enzyme, alcohol dehydrogenase, uses NADH to reduce the acetaldehyde molecule to ethanol. The NAD+ so formed is re-used in the glycolysis process. Yeast is widely used in the production of alcohol beverages like wine and beer and to bake of bread. The fermentation of lactic acid is carried out by bacteria like Lactobacillus. In them, pyruvic acid is reduced to lactic acid. The NADH formed in the glycolysis process is re-oxidised to NAD+ by alcohol dehydrogenase to be re-used in the glycolysis process.
During physical exercise, muscle cells undergo anaerobic respiration. The enzyme, lactate dehydrogenase, uses NADH to reduce the pyruvate molecule to lactate. In this process, NADH is re-oxidised to NAD and used in the glycolysis pathway. The accumulation of lactate is the reason for the stiffness in the muscles. Both alcohol and lactic acid fermentation yield less energy. Not even 7% of the energy present in the glucose is released, and of that, only some amount of energy is trapped in the form of ATP. Thus, anaerobic respiration yields a net gain of 2 ATPs after deducting the ATPs required for the preparatory phase of glycolysis. In anaerobic respiration, the NADH formed is re-oxidised. Thus, the synthesis of ATP from NADH molecules similar to aerobic respiration is also not possible. Also, the products formed are either alcohol or lactic acid, which is hazardous in nature. In fact, yeast is poisoned to death when the concentration of alcohol reaches fourteen percent. Fermentation releases less energy than the process followed in the presence of oxygen.