Introduction
Aerobic respiration or oxidative phosphorylation is the normal pathway in which the body utilizes glucose in the presence of adequate oxygen to produce adenosine triphosphates (ATPs - the molecular form of energy). This is the path that the body usually prefers to produce energy but in certain situations like energy-taxing exercise or physical activities, the oxygen supply cannot keep up with the energy demand. In such dire needs, the body may turn to other paths of energy production, although for a short spur. Anaerobic respiration is the intercellular pathway in which glucose is broken down for energy in oxygen-deficient conditions.
What Is Anaerobic Respiration?
Anaerobic respiration or anaerobic glycolysis is an alternate pathway of ATP synthesis in multicellular organisms, including humans, to compensate for a sudden increase in energy demand. This pathway can sustain the demand for a short time as it replenishes very quickly. This is an extremely inefficient way of ATP production, yielding just two ATP per glucose molecule against 32 ATPs through the oxidative pathway. Yet, the speed of ATP yield is a hundred times faster with this pathway. Anaerobic respiration may be considered an ancient form of energy synthesis in primitive organisms when the earth lacked an oxygen-rich environment.
What Is the Anaerobic Respiration Pathway?
Glucose transported through the blood enters the cells in the presence of insulin. These glucose molecules undergo glycolysis in the cytosol or cytoplasm of the cell through the ten-step process called the Embden-Meyerhof (EMP) pathway, ultimately yielding a net of two ATPs and ending with two pyruvate molecules.
This pyruvate molecule enters the mitochondria (the powerhouse of the cell) and undergoes a decarboxylation step to form two acetyl coenzyme-A molecules (acetyl CoA) which is an important molecule of aerobic respiration. This molecule enters the Krebs cycle or the citric acid cycle and runs through ten steps to ultimately yields two ATPs, a few ATP precursors, and carbon dioxide. Each molecule of glucose yields about 38 ATPs segregated in various steps.
Alternatively, in an oxygen-starved situation like increased physical activity, high-intensity exercise, running, etc., the energy demand peaks well over the supply. So the pyruvate gets destined for a different path in the liver and muscle tissues. Glycogenolysis occurs in the liver when the glycogen is converted to glucose that enters the EMP cycle producing pyruvates. Generally, this may continue with the normal Krebs cycle in oxygenated states. Whereas in an oxygen-deficient state, the pyruvate gets converted to lactate in the cytoplasm of the muscle cells in the presence of the lactate dehydrogenase enzyme. This produces two ATPs per cycle in the muscle cells at a very fast rate. The end-product lactate load is removed from the muscle cells, and the burden is carried by the liver, which reverses the lactate into glucose via gluconeogenesis, consuming six ATPs. Overall, there is a net loss of four ATPs, borne by the liver; hence this cycle cannot be sustained for long and lasts for less than two minutes, usually between 10 and 30 seconds.
How to Check for Anaerobic Respiration?
To make a quantitative analysis of anaerobic respiration, the metabolic end-product levels of lactic acid can be measured in a clinical setup. When the body is poorly oxygenated, anaerobic respiration stands tall over aerobic, and the accumulated lactic acid levels may be diagnostic of sepsis, shock, blood loss, anemia, or cardiac failure. Hyperlactatemia and lactic acidosis are indicative of deficient cardiac output and have been associated with increased morbidity and mortality.
What Is the Clinical Significance of Anaerobic Respiration?
Certain parameters associated with anaerobic metabolism have been useful in establishing several diagnoses as they have been prime evidence of certain events. Some of them are:
Serum Lactic Acid: This is a very evident parameter in diagnosing events like anemia, heart failure, infections, sepsis, and shock. The lactic acid levels also provide guidelines for diagnosing as well as managing the said conditions.
Anaerobic Exercise: During intense training or high-intensity workouts, the oxygen demand may far exceed the availability due to a sudden increase in ATP requirement by the skeletal muscles. Although the normal pathway produces 15 times more energy than the anaerobic pathway, the latter produces energy molecules at roughly 100 times the aerobic rate. Hence, this is essential during extreme energy demands.
The Warburg Effect: The Warburg effect marks the shift of the energy production pathway from aerobic to anaerobic within the tumor cells. As the tumor proliferates beyond the capillaries’ ability to provide blood supply, the cells transit towards anaerobic glycolysis to ensure adequate ATP supply to the tumor tissues.
Fibromyalgia: Fibromyalgia refers to chronic pains characterized by tenderness in various parts of the body. Fibromyalgia has been associated with increased pyruvate and lactate in the body as compared to healthy peers. They also presented with decreased ATP production and lower concentration of lactate dehydrogenase.
What Is Alcoholic Fermentation?
Alcohol fermentation is another form of anaerobic respiration and is a common energy pathway in prokaryotes and unicellular organisms. In such organisms, pyruvate or pyruvic acid is formed with partial oxidation of glucose. The molecule gets converted to ethanol and carbon dioxide in the presence of the enzymes-pyruvic acid decarboxylase and alcohol dehydrogenase. This pathway is termed lactic acid fermentation.
The accumulation and concentration of these end products can be harmful to the organism itself. For example, 13 percent alcohol content is fatal to the producer yeast itself. The process yields just two ATP molecules, but the characteristic byproduct has several commercial applications. These organisms are bred, engineered, and grown on large scales for multiple uses in the food, beverage, and pharmaceutical industry. Researchers continue to experiment with engineering these bacteria to increase the byproduct output to be industrially used.
Conclusion
Anaerobic respiration is an alternate way to manage the energy demands of the human body in situations of dire need. But the developed lactic acid within the muscle tissue can cause muscle fatigue and soreness. The liver takes the burden of it all. The other anaerobic pathway has been commercialized to an extended range to derive benefits with minimal expense. Although utilized infrequently, anaerobic metabolism continues to be one of the most important energy pathways, forming the backbone of dire-energy demands.
