Energy metabolism is the chemical processes in an organism that create usable energy for growth, repair, and physical activity. Anabolic pathways use ATP to drive cellular work, while catabolic pathways break down large molecules into constituent parts, releasing energy. Anaerobic and aerobic metabolism occur in the absence or presence of oxygen, respectively. Rapid glycolysis and fatty acid oxidation are the most efficient forms of energy metabolism.
Energy metabolism is generally defined as the totality of chemical processes in an organism. These chemical processes generally take the form of complex metabolic pathways within the cell, generally categorized as either catabolic or anabolic. In humans, the study of how energy flows and is processed in the body is called bioenergetics, and is primarily concerned with how macromolecules such as fats, proteins and carbohydrates are broken down to provide usable energy for growth, repair and physical activity.
Anabolic pathways use chemical energy in the form of adenosine triphosphate (ATP) to drive cellular work. The construction of macromolecules from smaller components, such as the synthesis of proteins from amino acids, and the use of ATP to enhance muscle contraction are examples of anabolic pathways. To power anabolic processes, ATP donates a single phosphate molecule, releasing stored energy in the process. Once an active cell’s supply of ATP is depleted, catabolic energy metabolism must generate more for cellular work to continue.
Catabolic pathways are those that break down large molecules into their constituent parts, releasing energy in the process. The human body is capable of synthesizing and storing its own ATP through anaerobic and aerobic energy metabolism. Anaerobic metabolism occurs in the absence of oxygen and is associated with short, intense bursts of energy. Aerobic metabolism is the breakdown of macromolecules in the presence of oxygen, and is associated with less intense exercise, as well as the daily work of the cell.
Anaerobic energy metabolism occurs in two ways, the creatine ATP-phosphate system and rapid glycolysis. The creatine phosphate ATP system uses stored creatine phosphate molecules to regenerate ATP that has been depleted and broken down to its low-energy form, adenosine diphosphate (ADP). Creatine phosphate donates a high-energy phosphate molecule to ADP, thus replacing spent ATP and energizing the cell. Muscle cells typically contain enough free floating ATP and creatine phosphate to fuel approximately ten seconds of intense activity, after which the cell must switch to the rapid glycolysis process.
Rapid glycolysis synthesizes ATP from blood glucose and muscle glycogen, with lactic acid produced as a byproduct. This form of energy metabolism is associated with brief, intense bursts of activity and mash; such as powerlifting or sprinting, when the cardiorespiratory system does not have time to deliver adequate oxygen to functioning cells. As rapid glycolysis progresses, lactic acid builds up in the muscle, causing a condition known as lactic acidosis, or more informally, muscle burn. Rapid glycolysis produces most of the ATP that is used from ten seconds to two minutes of exercise, after which the cardiorespiratory system has had a chance to supply oxygen to working muscles and aerobic metabolism begins.
Aerobic metabolism occurs in one of two ways: rapid glycolysis or fatty acid oxidation. Fast glycolysis, like slow glycolysis, breaks down glucose and glycogen to produce ATP. However, since it does so in the presence of oxygen, the process is a complete chemical reaction. While fast glycolysis produces two ATP molecules for every glucose molecule metabolized, slow glycolysis can produce 38 ATP molecules from the same amount of fuel. Since there is no buildup of lactic acid during the reaction, rapid glycolysis has no associated fatigue or muscle burn.
Finally, the slowest and most efficient form of energy metabolism is the oxidation of fatty acids. This is the process used to fuel activities such as digestion and cell growth and repair, as well as long-duration exercise activities such as marathon running or swimming. Instead of using glucose or glycogen for fuel, this process burns fatty acids that are stored in the body and is capable of producing up to 100 ATP molecules per unit of fatty acids. While this is a highly efficient, high-energy process, it requires large amounts of oxygen and only occurs after 30 to 45 minutes of low-intensity activity.
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