"JUST BREATH"...
The third metabolic pathway in order of dominance of energy production is the oxidative phosphorylation metabolic pathway, also known as the aerobic pathway. This metabolic pathway is dedicated to the breakdown (catabolism) of carbohydrates (carbs), triglycerides (fat), and amino acids (proteins) via the use of oxygen in the process of energy creation.
It is the only metabolic pathway that is tightly reliant on the environment as well as the cardiovascular system and the respiratory system. Oxygen in the cells is almost the end of the story, not the beginning. In the beginning, oxygen is part of the air we breath, accounting for approximately 20.95% of every litter of air under normal conditions. The air that we breath must make its way into the body, through the airways, diffuse from the lungs and their alveoli to our blood stream, bind to a hemoglobin protein carries within a red blood cell, and circulate nearby a cell in need of oxygen.
Advertisement
Upon arrival nearby a needing cell, it must diffuse once more into the cell (the process known as oxygen extraction), be transported by myoglobin and supplied to the mitochondria of the cell. Within the mitochondria, oxygen partakes in several chemical reaction that influence aerobic energy production. Utilizing oxygen diversifies energy production options within the cells by allowing aerobic glycolysis - the breakdown of carbs with oxygen; aerobic lipolysis - the breakdown of fat with oxygen; aerobic proteolysis - the breakdown of amino acids with oxygen.
As a continuous physical activity or exercise goes on, more and more oxygen finds itself within the active cells. After about 120 seconds, the amount of oxygen within the cells becomes enough to create most of the energy aerobically. Since molecules are commonly and naturally catabolized from smallest to biggest, and anaerobically to aerobically; and since we have been catabolizing carbs anaerobically just before oxygen is sufficiently supplied to the cells, the natural next metabolic process used to continue to create energy will be aerobic glycolysis.
Aerobic glycolysis consists of three stages, one of which occurs in the cytoplasm of the cell, and two occur within the mitochondrion of the cell. Initially, glucose is split to its chemical components of two molecules of pyruvate and four hydrogen atoms (as described previously also in part III). If there is sufficient enough oxygen, both pyruvate molecules (being defined as a carb itself), are catabolized by the oxygen resulting in the existence of acetyl-coA molecules and carbon monoxide molecules in the cytoplasm. Since pyruvate meets the criteria of being defined as a carb, and since it has been catabolized by oxygen, this specific process meets the definition of aerobic glycolysis (the breakdown of a carb via oxygen).
Thus, the first stage of the oxidative phosphorylation metabolic pathway occurs in the cytoplasm, and is the aerobic catabolism of the carb pyruvate by means of aerobic glycolysis. The aerobic catabolism of pyruvate within the cytoplasm is crucial since pyruvate is too big in size to enter the mitochondrion, yet acetyle-coA is small enough to enter the cell's mitochondria. Within the mitochondria, each acetyl-coA molecule will continue to the second stage called kreb's cycle.
Simplified, kreb's cycle via multiple reactions organized as a circle, creates an additional ATP molecule for every acetyl-coA that enters it, and a flow of hydrogen atoms, carried by NAD+ and FAD+ to the third stage called the electron transport chain (ETC). The electron transport chain is a relatively massive enzymatic complex that uses several sub-complexes to channel electrons through it and create an electrical gradient used to create water (termed metabolic water), and help energize the enzymatic complex ATP synthase (ATPase).
As NAD+ and FAD+ constantly supply hydrogen atoms to the ATPase complex, they flow through it from the upper end to the lower end, supplying it with the required energy to operate a shaft-like structure within it, that combine ADP (Adenosine Di Phosphate) with Pi (inorganic phosphate).
The number of ATP created during the splitting of the glucose molecule within the cytoplasm is the same for both anaerobic and aerobic glycolysis - two of three ATP. The number of ATP created in the kreb's cycle per glucose molecule catabolized is an additional two (four to five in total). The number of ATP created as a result of the work of the ETC is an additional thirty-two or thirty-three ATP molecules.
Thus, in comparison, while anaerobic glycolysis results in two or three ATP molecules, aerobic glycolysis results in thirty-six to thirty-eight ATP molecules for the catabolism of the same glucose. Therefor, aerobic glycolysis is much more energetically efficient than anaerobic glycolysis. Once even greater amounts of oxygen are supplied to the cells, aerobic catabolism of bigger substrate becomes possible, causing aerobic lipolysis to become the next dominant process of energy production in the cells.
Triglycerides are broken apart being split via water molecules in a process known as hydrolysis, resulting in glycerol and fatty acids. The fatty acids are oxidized into acetyl-coA which enters the mitochondria, repeating the processes previously explained regarding energy production within the mitochondrion. Each fatty acid creates at least forty-three ATP molecules, meaning that every triglyceride will create at least one hundred and twenty nine ATP molecules. This is about three and a half times more energy than the most energy created in aerobic glycolysis, and forty-three times more energy produced in anaerobic glycolysis.
As a fact, aerobic proteolysis creates even greater amounts of energy since the average protein is bigger than the average fat molecule, and the average fat molecule is bigger than the average carb. Yet, the catabolism of proteins (proteolysis; amino acids) is problematic and limited to the creation of three percent at most of the total energy per activity or exercise. Proteolysis demands far more oxygen than lipolysis and glycolysis, bears very problematic and dangerous implications.
The source of amino acids for the proteolysis process during exercise are skeletal muscles themselves, meaning that we are breaking down skeletal muscles as we task them with the creation of skeletal movement at the same time. This by itself is stupid as a matter of principle. Yet this happens for a reason, where proteolysis becomes a dominant process for energy production if carbs levels plunge to pre-dangerous levels or dangerous levels.
Advertisement
Critically low levels of carbs in the body risk the function of our nerves, skeletal muscles, and red blood cells (RBCs), crucial to normal function and maintaining life. While to be explained in-depth in a future post, our body can create from within glucose from an amino acid called alanine, which is a prominent component in skeletal muscle fibers' proteins. Since the creation of glucose from alanine is faster than that of fats, proteolysis is accelerated rather than continuing lipolysis.
Another danger of the proteolysis process is the release of poisonous gases that make up amino acids. Thus, while producing extremely large amounts of ATP, overall, the proteolysis process is very problematic, slow, and dangerous, thus being dominant last, and used the least possible. Last, anaerobic lipolysis and anaerobic proteolysis are possible and occur to an insignificant extent, sine these two processes would flood the cells with acids created and paralyze them swiftly.
Comments