A chain of chemical reactions in which all living organisms obtain the energy required to live is called respiration. Respiration is, perhaps, the essential function of a living organism to perform flawlessly. The respiration that happens at the minute level of our body viz. in the cell is called the cellular respiration. It could occur with (aerobically) or without the presence of oxygen. Any type of cellular respiration initiates with glycolysis where a 3-C molecule, pyruvic acid is formed as the end product. Different cells handle this pyruvate in two major ways as fermentation is one of them. Let us have a glance at fermentation and its types in the coming lines.
Fermentation: An Anaerobic Pathway
Anaerobic respiration is a type of cellular respiration where respiration takes place in the absence of oxygen. Fermentation is an anaerobic pathway- a common pathway in the majority of prokaryotes and unicellular eukaryotes. During the process, partial oxidation of glucose leads to the formation of acids, gases or alcohol. Depending on the end products, there are two types of fermentations viz. alcoholic or ethanol fermentation and lactic acid fermentation.
In organisms, yeast, for example, the pyruvic acid formed by partial oxidation of glucose is converted to ethanol and carbon dioxide (CO2). This anaerobic condition is called alcoholic or ethanol fermentation. The whole reaction is catalyzed by the enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyze. In certain bacteria and animal muscle cells, under anaerobic conditions, the pyruvic acid is reduced to lactic acid by lactate dehydrogenase. This is called lactic acid fermentation. The end products of these anaerobic pathways make them hazardous processes. For example, a concentration of alcohol above 13 percent produced by yeast cells could kill themselves.
In the alcoholic and lactic acid fermentation, NADH+H+ is the reducing agent which is deoxidized to NAD+. The energy released in both the processes is not much and the total sum of ATP molecules produced during fermentation is two, which is very less as compared to aerobic respiration. However, this is a commercially employed process in food and beverage industries, and pharmaceutical industries etc.
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