Let us make an in-depth study of the meaning, mechanism and energy yield in fermentation.
What is Fermentation?
When enough oxygen is available, NADH, formed in glycolysis transfers its hydrogen to oxygen via electron transport chain, and changes to NAD for reuse.
Lack of oxygen blocks the path of hydrogen to oxygen, and soon the entire NAD of the cell is converted to NADH. This will stop glycolysis as NAD is essential for this process. Under these conditions, organisms can respire without oxygen by using an alternative path for electron transfer. This anaerobic (without air) respiration (or glycolysis) is called fermentation. It serves as a substitute or auxiliary source of energy.
Mechanism of Fermentation:
Pyruvic acid formed at the end of glycolysis has the property of reacting readily with hydrogen. If the normal path of hydrogen to oxygen is blocked, pyruvic acid ceases to be a fuel and becomes a hydrogen acceptor. Use of pyruvic acid as hydrogen acceptor produces ethyl alcohol in plants and most microorganisms, and forms lactic acid in animals and certain bacteria. Ethyl alcohol is the active ingredient in alcoholic beverages.
(i) Alcoholic Fermentation (Fig. 7.11):
Pyruvic acid first undergoes decarboxylation (removal of a carboxyl group in the form of carbon dioxide with the help of an enzyme pyruvate decarboxylase. This produces acetaldehyde and carbon dioxide from pyruvic acid. Acetaldehyde then accepts hydrogen from NADH2, and is reduced to ethyl alcohol, (ethanol), producing oxidized NAD+. The process is catalyzed by enzyme alcohol dehydrogenase.
The alcohol is excreted by the cell. Thus, most of the chemical energy of glucose contained in ethyl alcohol is wasted. One glucose molecule produces two molecules of ethyl alcohol. Accumulation of ethyl alcohol by fermentation in a culture of yeast (unicellular fungus) may stop multiplication and kill the cells. In the presence of oxygen, yeast can respire aerobically, breaking sugar down fully to CO2 and H2O.
Utility of Alcoholic Fermentation:
The type of fermentation described above is typical of yeast cells grown under anaerobic conditions, and is the basic process underlying the production beers and wines. In fact, it has been used by man since the beginning of history for preparing alcohol. The carbon dioxide released by yeast cells in alcoholic fermentation raises the dough (flour kneaded with water) used in baking breads.
(ii) Lactic Acid Fermentation (Fig. 7.12):
Pyruvic acid receives hydrogen from NADH2, producing lactic acid and oxidized NAD+. The enzyme lactic dehydrogenase catalyzes the reaction. NAD+ is reused in glycolysis. One glucose molecule gives two lactate molecules (Fig. 7.12).
In vertebrates, the lactic acid formed in muscles in anaerobic respiration is carried by the blood to the liver where it is used to resynthesize glucose. Thus, these animals save the chemical energy of glucose contained in lactic acid. Microorganisms, which use this type of fermentation, excrete lactic acid. They, thus, lose the energy of glucose contained in lactic acid.
Energy Yield in Fermentation:
Fermentation yields only about 5% of the energy obtained by aerobic respiration. This small amount of energy is sufficient to maintain the life of organisms such as yeasts, many bacteria and other anaerobes (organisms that normally live or can live in the absence of oxygen).
Vast majority of organisms are, however, aerobes, i.e., need oxygen for respiration. Fermentation energy is too little to maintain the life of these organisms. They die within minutes in the total absence of oxygen. Fermentation can supplement the aerobic energy in them.
Anaerobic respiration yields much less energy than aerobic respiration because:
(i) The end products of anaerobic respiration still contain energy which remains untapped and is wasted.
(ii) NAD is not regenerated from NADH in the absence of O2. This deprives the cell of energy likely to be produced by transfer of hydrogen electrons from NADH2 over ETS.