3 Ways to Train
Bioenergetics: ATP Production
An ATP molecular consists of adenosine combined with three inorganic phosphate (Pi) groups. When acted on by the enzymes ATPase, the last phosphate group splits away from the ATP molecule, rapidly releasing a large amount of energy. This reduces the ATP to adenosine disphosphate (ADP) and Pi. But how was that energy originally stored?
The process of storing energy by forming ATP from other chemical sources is called phosphorylation. Through various chemical reactions, a phosphate group is added to a relatively low energy compound, adenosine diphosphate, converting it to adenosine triphosphate (ATP). When these reactions occur without oxygen, the process is called anaerobic metabolism. When these reactions occur with the aid of oxygen, the overall process is called aerobic metabolism, and the aerobic conversion of ADP to ATP is oxidative phosphoylation.
Cells generate ATP by three methods:
The ATP-PCr System
The Glycolytic System
The Oxidative System
The ATP-PCr System
The simplest of energy systems is the ATP-PCr system. In addition to ATP, your cells have another high-energy phosphate molecule that stores energy. This molecule is called phosphocreatine, or PCr. Unlike ATP, energy released by the break down of PCr is not used directly to accomplish cellular work. Instead, it rebuilds ATP to maintain a relatively constant supply. With this system, as energy is released from ATP by the splitting of a phosphate group, your cells can prevent ATP depletion by reducing PCr, providing energy to form more ATP. This process is rapid and can be accomplished without any special structures within the cell. Although it can occur in the presence of oxygen, this process does not require oxygen, so the ATP-PCr system is said to be anaerobic. During the first few seconds of intense muscular activity, such as sprinting, ATP is maintained at a relatively constant level, but the PCr level declines steadily as it is used to replenish the depleted ATP. Thus, your capacity to maintain ATP levels with energy from PCr is limited. Your ATP and PCr stores can sustain your muscles energy needs for only 3 to 15 seconds during an all out sprint. Beyond that point, the muscles must rely on other processes for ATP formation; the glycolytic and oxidative combustion of fuels.
The Glycolytic System
Another method of ATP production involves the liberation of energy through the breakdown of glucose. This system is called the glycolytic system because it involves glycolysis, which is the breakdown of glucose via special glycolytic enzymes. Glucose accounts for about 99% of all sugars circulating in the blood. It comes from the digestion of carbohydrate and the breakdown of liver glycogen. It is stored in the liver or in muscle until needed. At that time, the glycogen is broken down to glucose-1-phosphate through the process of glycogenolysis. We are referring to the process of anaerobic glycolysis, without the need for oxygen. Glycolsis requires 12 enzymatic reactions for the breakdown of glycogen to lactic acid. This energy system does not produce large amounts of ATP. Despite this limitation, the combined actions of the ATP-PCr and glycolytic systems allow the muscles to generate force even when the oxygen supply is limited. These two systems predominate during the early minutes of high intensity exercise. Another major limitation of anaerobic glycolysis is that it causes an accumulation of lactic acid in the muscles and body fluids. In an all out sprint lasting 1-2 minutes, the demands on the glycolytic system are high. A muscle fibers rate of energy use during exercise can be 200 times greater than at rest. The ATP-PCr and glycolytic systems alone cannot supply all the needed energy.
The Oxidative System
This is the most complex of the three-energy system. Because oxygen is used, this is an aerobic process. The oxidative production of ATP occurs within special cell organelles; the mitochondria. In muscles, these are adjacent to the myofibrils and are also scattered throughout the sarcoplasm. Muscles need a steady supply of energy to continuously produce the force needed during long-term activity. Unlike anaerobic ATP production, the oxidative system has a tremendous energy yielding capacity, so aerobic metabolism is the primary method of energy production during endurance events.
Oxidation of Carbohydrates
In carbohydrate metabolism, glycolysis plays a role in both anaerobic and aerobic ATP production. The process of glycolysis is the same whether or not oxygen is present.
The Krebs Cycle
Once formed, acetyl CoA enters the krebs cycle (citric acid cycle), a complex series of chemical reactions that permit the completion of oxidation of acetyl CoA. At the end of the Krebs cycle, 2 mol of ATP have been formed, and the substrate has been broken down into carbon dioxide and hydrogen.
The Electron Transport Chain
During glycolysis, hydrogen is released as glucose is metabolized to pyruvic acid. More hydrogen is released during the krebs cycle. If it remains in the system, the inside of the cell becomes too acidic. The krebs cycle is coupled to a series of reactions known as the electron transport chain. The hydrogen released during glycolysis and during the krebs cycle combines with two coenzymes: NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). These carry the hydrogen atoms to the electron transport chain, where they are split into protons and electrons. At the end of the chain, the H combines with oxygen to form water, thus preventing acidification.
Energy Yield from Carbohydrate
The oxidative system of energy production can generate up to 39 molecules of ATP from one molecule of glycogen. If the process begins with glucose, the net gain is 38 ATP molecules (one is used for conversion).
Oxidation of Fat
Fat also contributes to muscles energy needs. Muscle and liver glycogen stores may be able to provide only 1,200 to 2,000 kcal of energy, but the fat stored inside muscle fibers and in fat cells can supply at least 70,000 to 75,000 kcal, even in a lean adult. Even though chemical compounds such as triglycerides, phospholipids and cholesterol are classified as fats, only triglycerides are major energy sources. They are stored in fat cells and between and within skeletal muscle fibers. To be used as energy, it must be broken down to its basic units; one molecule of glycerol and three molecules of free fatty acids (FFA). This process is called lipolysis.
Although fat provides more kilocalories of energy per gram than carbs, fat oxidation requires more oxygen than carbohydrate oxidation. Oxygen delivery is limited by the oxygen transport system, so carbohydrate is the preferred fuel during high intensity exercise.
As noted earlier, carbs and fatty acids are our bodies preferred fuels. But proteins, or father the amino acids that form them, are also used. Some amino acids can be converted into glucose. Alternatively, some can be converted into various intermediates of oxidative metabolism such as pyruvate or acetyl to enter the oxidative process. Proteins energy yield is not as easily determined as that of carbs or fat because protein also contains nitrogen. When amino acids are catabolized, some of the released nitrogen is used to form new amino acids, but the body cannot oxidize the remaining nitrogen. Instead it is converted into urea then excreted as urine.
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