Respiration in microbial cells is generally an energy-yielding type of metabolism in which an energy substrate (usually the electron donor) is oxidized using an exogenous or externally derived electron acceptor or oxidizing agent. Examples of the energy or respiratory substrates include organic and inorganic compounds and ions while the oxidizing agent or electron acceptors include CO2, sulphate, oxygen and nitrate as the case may be. Respiration in microbial cells can either occur aerobically or anaerobically. Some microbial cells also exhibit facultative type of respiration which is a combination of aerobic and anaerobic respiration. Microorganisms that exhibit facultative type of respiration can live in environments with oxygen or no oxygen concentration.
Aerobic respiration is a type of respiration that occurs in the presence of oxygen; and organisms that exhibit this type of respiration are generally known as aerobes. Aerobic microorganisms have the ability to grow in the presence of oxygen (O2). Most microorganisms (including those that parasitize man, animals and plants) have the ability to live in a state or environmental condition in which O2 is present. Environmental conditions in which O2 is always present is generally known as aerobiosis. Oxygen is usually the final electron acceptor in aerobic respiration.
Anaerobic respiration occurs in the absence of oxygen. Microorganisms that thrive in the absence of oxygen are generally known as anaerobes. Anaerobes (inclusive of pathogenic and non-pathogenic organisms) live in habitats in which there is a complete absence of O2. Such a state with no O2 concentration is known as anaerobiosis. The final electron acceptor in anaerobic respiration is usually organic but the majority is inorganic, and they include CO2, Fe3+, SO42- and NO3– among others. Both aerobic and anaerobic respiration releases energy required for the formation of ATP, the energy currency of the cell. Microorganisms have many different metabolic reactions based on their energy sources; and they use either respiration (inclusive of aerobic and anaerobic respiration) or fermentation pathways to make their ATP according to their individual oxygen requirements. Glycolysis or the glycolytic pathway, TCA cycle and the electron transport chain (ETC) are all part of the respiration activities in microbial cells.
Fermentation is simply defined as the energy-yielding type of metabolism in which an energy-substrate is oxidized or metabolized to release free energy in the absence of an external electron acceptor (e.g., oxygen). It releases little functional energy for cellular/microbial metabolic activities compared to aerobic and anaerobic respiration which releases appreciable amount of energy in the form of ATP for cellular activities. Generally, fermentation mainly occurs in under anaerobic conditions or in conditions where there is infinitesimal amount of aerobiosis. The energy released during fermentation (about 4 molecules of ATP) is usually small because the energy substrate is oxidized partially unlike in aerobic and anaerobic respiration in which energy-substrate use is very efficient. Because the energy substrate is fully oxidized in respiration, the energy yield is thus high. (The net energy yield in cellular respiration is 38 molecules of ATP). When oxygen and other electron acceptors for either aerobic or anaerobic respiration are lacking, microorganisms generate their energy via the fermentation pathway. Typical examples of fermentative pathways employed by microorganisms include ethanol or alcohol fermentation, methane fermentation, butyric acid fermentation, amino acid fermentation and lactic acid fermentation among others.
Though the energy yield in fermentation processes is small, microorganisms that can carry out both fermentation and respiration type of metabolism often resorts to fermentation in scenarios where there is no other available exogenous or external electron acceptor (e.g. O2) for the cell to utilize. Fermentation pathways vary amongst microorganisms; and prokaryotic cells, eukaryotic cells and Archaea have different metabolic pathways for fermentative activities. Generally, oxidative phosphorylation cannot occur in fermentative biosynthetic pathways (which require no oxygen) as is the case for respiration; and the energy substrate is usually the electron donor while the electron acceptor for the fermentative process is usually a derivative of the energy substrate. However, ATP is formed in fermentative pathway by a substrate-level type of phosphorylation. Most microorganisms that utilize the fermentative pathway (inclusive of bacteria, fungi, algae and some protozoan) play tremendous roles in the industries where they are manipulated for the production of goods and services that are of domestic, economic and industrial purposes. For example, yeasts (e.g., Saccharomyces species) use the ethanol fermentative pathway to manufacture alcoholic drinks, wine and bread and other products that are of economic importance. Lactic acid bacteria are also employed for the manufacture of cheese and yoghurt.
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