Metabolism and Energy Production
The body requires energy constantly to produce Adenosine Triphosphate (ATP), which is the energy currency for all living things. The body produces the ATP needed from a combination of three metabolic pathways, and depending on the demand of the specific physical activity, one energy pathway may become more predominant for producing the ATP required.
ATP- CrP Pathway (Creatine Phosphate Pathway)
The ATP-CrP pathway produces ATP by transferring the phosphate from CrP (creatine phosphate), to ADP (Adenosine Diphosphate) by the action of the key enzyme creatine phosphokinase (CPK) to form ATP. The ATP-CrP pathway produces large amount of energy rapidly, but just for a few seconds. This because the amount of CrP stored within the body is limited, and is depleted quickly during very high intensity exercise. The ATP-CrP pathway is predominant during high-intensity, short-duration physical activity, requiring immediate release of high amount of energy, such as weightlifting and 100 meters sprint. The glycolysis anaerobic pathway becomes more predominant to meet the demand of medium-duration physical activity.
Glycolysis Anaerobic Pathway
Glycolysis anaerobic occurs within the skeletal muscles, and begins with the entry of glucose into the muscle fibres. Glucose entry from the blood stream into the muscle fibres is facilitated by specialised transport proteins called GLUT proteins, which are located within the sarcolemma (the plasma membrane of muscle fibres). The term glycolysis refers to glucose breakdown to pyruvic acid to generate energy, without the use of oxygen. Glycolysis anaerobic is far more complex than the ATP-CrP pathway, and requires a number of enzymatic reactions, which occur within the cell cytoplasm. Glycolysis can also use muscle glycogen, which is the stored form of glucose. When required, glycogen is broken down to form glucose through hydrolysis, which can then be used to produce ATP. The amount of ATP that can be produced during glycolysis anaerobic is limited. However, the combined action of the ATP-CrP and glycolitic system, allows the muscles to generate force even when the oxygen supply is limited. The glycolysis anaerobic pathway is more predominant and provides most of the energy for medium-duration activities, like wrestling, sprint swimming or bodybuilding. This pathway produces relatively quickly a significant amount of energy to fuel muscular contraction, and partially compensating when the ATP-CrP system begins to phase out.
Aerobic Pathways or Cellular Respiration
The aerobic pathway is the most complex of the three energetic pathways. The aerobic pathway or cellular respiration, is the process by which the body breaks down the substrate with the aid of oxygen to generate ATP. The oxidative production of ATP occurs within special cell organelles, termed mitochondria. These are scattered throughout the sarcoplasm, which is the cytoplasm of muscle fibres. Muscles need a steady supply of energy to continuously produce the force required during long-term physical activity. Unlike the anaerobic ATP production, the oxidative system has a tremendous energy-yielding capacity, and its activity is predominant for energy production at rest and during endurance events. The aerobic pathway places considerable demand on the body’s ability to deliver oxygen to the working muscles. The breakdown of glucose to pyruvic acid by the aerobic system is the same as glycolysis anaerobic, and if sufficient amount of oxygen is supplied, pyruvic acid is converted into Aceyl CoA. Once Acetyl CoA is formed, it enters the Krebs cycle, also known as citric acid cycle, which is a complex series of chemical reactions that allow the complete oxidation of Acetyl CoA. At the end of the Krebs cycle, 2 moles of ATP have been formed, and glucose has been broken-down into carbon dioxide and hydrogen. During glycolysis and Krebs cycle, hydrogen is released in the form of cations, which then combine with two coenzymes NAD (nicotinamide adenine dinucleotide), and FAD (flavin adenine dinucleotide), respectively. These carry the hydrogen cations to the electron transport chain, where they combine with oxygen to form water, and thus preventing acidification (decrease in pH). This process provides energy for the phospholarysation of the ADP to form ATP. During aerobic respiration, triglycerides (fats) can also be used as a substrate. Triglycerides are broken-down into 1 molecule of glycerol and 3 molecules of free fatty acid (FFA), this process is known as lipolysis. Once freed from glycerol, FFA can enter the blood stream, and can be transported throughout the body, entering the muscles by diffusion. Their rate of entry depends on the concentration gradient. Increasing the FFA concentration in the blood steam, drives them into the muscles, where are used to from ATP. The cellular respiration is able of producing ATP for a prolonged period of time, but not in large amount. Thus, cellular respiration is predominant in producing ATP during endurance activities, such as long distance running or cycling.
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