Energy metabolism is generally defined as the entirety of an organism's chemical processes. These chemical processes typically take the form of complex metabolic pathways within the cell, generally categorized as being either catabolic or anabolic. In humans, the study of how energy flows and is processed in the body is termed bioenergetics, and is principally concerned with how macromolecules such as fats, proteins, and carbohydrates break down to provide usable energy for growth, repair, and physical activity.
Anabolic pathways use chemical energy in the form of adenosine triphosphate (ATP) to power cellular work. The building of macromolecules out of smaller components, such as the synthesis of proteins from amino acids, and the use of ATP to power muscular contraction are examples of anabolic pathways. To power anabolic processes, ATP donates a single phosphate molecule, releasing stored energy in the process. Once a working cell's supply of ATP is depleted, more must be generated by catabolic energy metabolism 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 able to synthesize and store its own ATP through both anaerobic and aerobic energy metabolism. Anaerobic metabolism takes place 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 lower intensity exercise, as well as the daily work of the cell.
Anaerobic energy metabolism occurs in two forms, the ATP-creatine phosphate system and fast glycolysis. The ATP-creatine phosphate system uses stored creatine phosphate molecules to regenerate ATP that has been depleted and degraded to its low-energy form, adenosine diphosphate (ADP). The creatine phosphate donates a high-energy phosphate molecule to the ADP, thereby replacing spent ATP and re-energizing the cell. Muscle cells typically contain enough free-floating ATP and creatine phosphate to power approximately ten seconds of intense activity, after which the cell must switch to the fast glycolysis process.
Fast glycolysis synthesizes ATP from glucose in the blood and glycogen in the muscle, with lactic acid produced as a byproduct. This form of energy metabolism is associated with brief, intense bursts of activity &mash; such as power lifting or sprinting — when the cardio-respiratory system does not have time to deliver adequate oxygen to the working cells. As fast glycolysis progresses, lactic acid accumulates on the muscle, causing a condition known as lactic acidosis or, more informally, muscle burn. Fast glycolysis produces the majority of ATP that is used from ten seconds to two minutes of exercise, after which time the cardio-respiratory system has had opportunity to deliver oxygen to the working muscles and aerobic metabolism begins.
Aerobic metabolism takes place in one of two ways, fast glycolysis or fatty acid oxidation. Fast glycolysis, like slow glycolysis, breaks down glucose and glycogen to produce ATP. Since it does so in the presence of oxygen, however, the process is a complete chemical reaction. While fast gycolysis produces two molecules of ATP for every glucose molecule metabolized, slow gycolysis is able to produce 38 ATP molecules from the same amount of fuel. As there is no lactic acid accumulation during the reaction, fast glycolysis has no associated muscle burn or fatigue.
Finally, the slowest and most efficient form of energy metabolism is fatty acid oxidation. This is the process used to power activities such as digestion and cellular repair and growth, as well as long-duration exercise activities, such as marathon running or swimming. Rather than using glucose or glycogen as fuel, this process burns fatty acids that are stored in the body, and is capable of producing as many as 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.