Our bodies require adenine triphosphate (ATP) for muscular contraction, but we do not eat ATP, rather, we consume food. Thus, our bodies need to take the food that we eat and convert it into ATP, which is required for movement. The processes that make up this conversion are collectively referred to as energy metabolism. The three main macronutrients carbohydrates, protein, and fat are found in varying quantities in the foods we eat. Macronutrients contain energy, or calories, that can only be used or stored after digestion and absorption. If energy, in the form of ATP, is needed immediately, the body can use the metabolites from the breakdown of macronutrients to meet these needs. The three different energy systems that include the phosphagen system, glycolysis, and mitochondrial respiration provide ATP as an energy source for movement and muscular contraction. Thus, ATP is considered the energy currency of the cell.
The phosphagen system is anaerobic, meaning it does not need oxygen and provides rapid energy for maximal intensity exercise lasting 10 to 15 seconds. This system rapidly rephosphorylates ADP and Pi into ATP but can only be used for a very short period of time. Glycolysis is also an anaerobic energy system that can provide ATP quickly, for a slightly longer period than the phosphagen system, about 30 seconds. Mitochondrial respiration is an aerobic energy system, meaning it requires the presence of oxygen, and is the energy system used at rest and during exercise of low-to-moderate intensity.
While these are three distinct energy systems, they are interrelated in their contribution to exercise. Exercise duration, intensity, and an athlete’s nutritional status will determine the contribution of each of these systems. Understanding the complex dynamic relationship between the three energy systems is essential for providing effective nutritional strategies for athletes to achieve optimal performance.
ATP: The Currency of the Cell
Recall the primary tenet of thermodynamics energy cannot be created or destroyed. We consume energy from food in the form of macronutrients, which have chemical bonds that, when broken, release energy. Carbohydrates, fats, and, to a small degree, proteins, are broken down to provide energy that can be used immediately or stored as glycogen in the liver and muscle, and triglycerides in adipose tissues and intramuscular triglycerides. This stored energy is used to supply ATP, which takes energy in chemical form and transforms it into mechanical energy used for muscular contraction.
ATP is the basic unit of energy for all cells, and therefore is considered the currency of the cell (see Figure 3.1). ATP, a nucleoside triphosphate, contains a protein atom (adenine) and a sugar molecule (ribose), which are attached to three phosphate groups. The bonds connecting the last two phosphate groups are high-energy bonds. When these bonds are hydrolyzed through the action of the enzyme ATPase, energy is released and transferred to muscle cells energizing muscular activity.
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Here you see the breakdown of ATP to ADP + Pi, with the cleavage of the bond between the ADP molecule and the last Pi releasing energy that powers muscular contraction. In order for work to continue, however, ADP needs to be rephosphorylated to ATP; that is, a phosphate group needs to be added to ADP to replenish ATP stores. The body only stores a limited amount of ATP (80 to 100 g, or, 3 oz), and only a percentage of ATP will be depleted during exercise in order to maintain basic cellular functioning (Baker, McCormick, and Robergs 2010). In fact, while total ATP stores remain relatively stable during exercise, there can
Glycolysis is the primary source of the ATP that is required for a maximal bout lasting around 30 seconds, with the phosphagen system contributing smaller amounts ofATP over this period (Baker, McCormick, and Robergs 2010). Sport examples relying primarily upon glycolysis include a 50 m sprint in swimming, a 200 to 400 m running sprint, or soccer and basketball players engaging in repeated, high-intensity intervals up and down the soccer field or basketball court.
While the production of ATP is rapid via “incomplete” or “fast” glycolysis, and storage of glycogen has not yet been depleted, this energy system cannot be used as the primary source of ATP for sustained activity. First, incomplete or fast glycolysis that converts pyruvate to lactate can produce ATP more rapidly but will only yield two ATPs. However, when pyruvate is converted to lactic acid, it quickly dissociates into lactate and an H+ ion. Lactate can be used by the liver as a fuel source or converted into glucose (Cori cycle); but the H+ ion that is produced from ATP hydrolysis reactions decreases the pH of the cell and slows enzymatic reactions. Athletes often “feel” this acidification of the body and refer to it as the “burn” or “build-up of lactic acid.” Thus, it is not lactic acid itself that causes fatigue but rather the acidosis that results from the hydrogen ions that are produced from a number of reactions, most notably the hydrolysis of ATP When pH drops, respiratory rate increases exponentially in an effort to buffer the hydrogen being produced. However, exhaled CO2 is not nearly as acidic as hydrogen and eventually exercise intensity must decrease precipitously because hydrogen directly impairs the muscle’s ability to contract.
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