Introduction
Cellular respiration is of two types: aerobic and anaerobic. While aerobic respiration involves the use of oxygen, anaerobic respiration can take place in the complete absence of oxygen. Yeast, fungi, and certain bacteria are examples of anaerobes. In humans, anaerobiosis occurs in the skeletal muscle.
One of the main differences between aerobic and anaerobic respiration is that in anaerobic respiration, there is only a partial breakdown of sugar. Therefore, if carried out continuously, anaerobic respiration is not an efficient process, but in the short term, it is useful. There are many commercial applications of anaerobic respiration.
Anaerobic respiration
Anaerobic respiration or anaerobiosis is a process by which certain organisms obtain energy by breaking down sugar in the complete absence of oxygen (Vaughan, 1986). Such an organism is called an anaerobe (Vaughan, 1986). Some examples of anaerobic organisms include yeasts, fungi, certain bacteria, and certain worms living in mud, which is deficient in oxygen (Vaughan, 1986). Anaerobic respiration is also a feature of vertebrate skeletal muscle during heavy muscular activity (Vaughan, 1986.)
The term fermentation is frequently used interchangeably with anaerobic respiration, especially when the glycolytic pathway is used for energy production in the cell. However, these terms are not synonymous because “certain anaerobic prokaryotes can generate all of their ATP using an electron transport system and ATP synthase”(Vaughan, 1986, p.108.)
Anaerobic respiration of glucose can be depicted by the following word and symbol equation (2007): Glucose → Lactic acid + Energy (ATP)
Anaerobes are of 2 types:
- Complete anaerobes: these organisms live in permanent oxygen-deficient conditions, and are completely independent of oxygen for respiration (Vaughan, 1986.)
- Partial anaerobes: these organisms thrive in the presence of oxygen but even if oxygen is not available or is deficient they can change to anaerobic respiration. Most of the anaerobes fall into this group (Vaughan, 1986.)
Another classification of anaerobes is:
- Obligate anaerobes-bacteria, which carry out anaerobic respiration exclusively, and are killed if oxygen is present (Hill & Scheckler, 2003.)
- Facultative organisms (aerobic or anaerobic)- carry out aerobic respiration if oxygen is present, but if oxygen is absent or not sufficient, they can change to anaerobic respiration (Hill & Scheckler, 2003.)
Some examples of obligate anaerobes are clostridium tetani, which causes tetanus, and c. perfringens, which causes gangrene.
The presence of oxygen kills these organisms because of the production of singlet oxygen, superperoxide ion, hydrogen peroxide, hydroxyl ion, and other toxic byproducts.
One of the main differences between aerobic and anaerobic respiration is that in anaerobic respiration, there is only a partial breakdown of sugar. This is converted into either ethanol (ethyl alcohol) or lactic acid, instead of being oxidized to carbon dioxide and water (Vaughan, 1986.)
In plants and yeast (alcoholic fermentation) ethanol is the end product of anaerobiosis, while in animals, lactic acid is the end product (Vaughan, 1986). This is depicted below.
Anaerobiosis in plants and yeast (alcoholic fermentation): C6H12O6 → 2CH3CH2OH + 2CO2 ↑ + 210 kJ
Anaerobiosis in animals: C6H12O6 → 2CH3CH (OH) COOH + 150 kJ
In aerobic respiration, about 2880 kJ is produced, whereas, in anaerobiosis, less energy is released because the breakdown of the sugar is incomplete (Vaughan, 1986). In anaerobic respiration, a lot of energy is held up in the ethanol or lactic acid molecules (Vaughan, 1986). In animals, the subsequent conversion of the lactic acid back into pyruvic acid liberates this energy, which is then oxidized in the usual manner.
Oxygen is required for this process, and if it is not available, then the lactic acid is excreted.
Therefore, if carried out continuously, anaerobic respiration is not an efficient process, but in the short term, it is useful. As an example, during muscular exercise in humans, the lactic acid that accumulates can be later broken down or reconverted to carbohydrates when oxygen becomes available.
However, in plants, the ethanol, which is produced, cannot be used. It can neither be reconverted to carbohydrate nor broken down further in the presence of oxygen (Vaughan, 1986). Ethanol is toxic, and must not be allowed to accumulate. This is the main reason why very few plants can be complete anaerobes (Vaughan, 1986). Nevertheless, many plants (or parts of plants) can perform anaerobic respiration for a short period (e.g. germinating seeds, roots in waterlogged soil) but before the concentration of ethanol reaches a toxic level, they must switch back to aerobic respiration.
Even yeasts are not complete anaerobes (Vaughan, 1986.). They grow much better in aerobic conditions (Vaughan, 1986). If the oxygen concentration is too less, the concentration of ethanol rises and kills the yeast cells.
In anaerobic conditions, as the hydrogen atoms come off the carrier system, there is no oxygen available to accept them (Vaughan, 1986). This means that without oxygen, the carrier systems cannot work, and hence, there can be no Kreb’s cycle or any of the reactions of Kreb’s cycle.
Faster glycolysis does occur in the usual way, but instead of being converted into acetyl-CoA and diverted into the Kreb’s cycle, sugar is converted into lactic acid or ethanol.
Usually, the first step in the conversion of the phosphorylated triose sugar to pyruvic acid involves dehydrogenation: two hydrogen atoms are removed and taken up by NAD. In anaerobic conditions, the hydrogen atoms are taken up by the initial hydrogen acceptor NAD, and then handed to pyruvic acid, which is thereby converted into lactic acid (see figure below).
This reaction is important because it prevents the hydrogen atoms from accumulating.
In plants and yeast, the pyruvic acid, instead of being directly converted into ethanol, is first converted into acetaldehyde, which is then reduced by the H2 atoms to form ethanol. Carbon dioxide is liberated during the conversion of pyruvic acid into acetaldehyde.
The Krebs cycle, therefore, is omitted in anaerobic respiration, but glycolysis continues at a greatly accelerated rate. Since the Krebs cycle is omitted, lesser ATP molecules (only 2 molecules of ATP) are produced in anaerobic respiration, when compared to aerobic respiration (38 molecules) (Vaughan, 1986).
Anaerobic respiration in humans
During short, intense bursts of strenuous activity (like sprinting), “muscle cells use anaerobic respiration to supplement the ATP production from the slower aerobic respiration”.
Commercial applications of anaerobic respiration
Anaerobic digestion-the process of anaerobic respiration is used by a system called an anaerobic digester, which anaerobically digests waste (sewage and animal waste), to produce biogas. Biogas, in turn, is used to power electric generators, provide heat, and produce soil-improving material.
By using organisms like yeast, many products like buttermilk, yogurt, and cheese are produced. In addition, the process of anaerobiosis is also used in wine-making, baking, and brewing industries (Hunter, 2004).
Conclusion
Anaerobic respiration or anaerobiosis is a process by which certain organisms obtain energy by breaking down sugar in the complete absence of oxygen. Some examples of anaerobic organisms include yeasts, fungi, and certain bacteria.
Anaerobic respiration is also a feature of human skeletal muscle during heavy muscular activity and in sprinting.
In plants and yeast (alcoholic fermentation) ethanol is the end product of anaerobiosis, while in animals, lactic acid is the end product. Anaerobic respiration is not an efficient process, but in the short term, it is useful.
The commercial applications of anaerobic respiration include the production of biogas, buttermilk, yogurt, and cheese, as well as in wine-making, baking, and brewing industries.
References
Hill SA, Scheckler SE, 2003. Botany: An Introduction to Plant Biology. Jones and Bartlett Publishers.
Hunter GS, 2004. Let’s Review: Biology. 4th ed. Barron’s Educational Series.
Vaughan MB, 1986. Biology: A Functional Approach. Nelson Thornes.
Anaerobic respiration. 2007. Web.