About Energy Systems 1

ATP - the primary energy source

The ATP molecule consists of adenosine and three (tri) phosphate groups. Energy is released when ATP releases a phosphate to become adenosine diphosphate (ADP). This energy lasts for a very short time only, 5s.
To replenish the limited stores of ATP, a biochemical process called phosphorylation adds a phosphate group back to ADP to create ATP. If this occurs in the presence of oxygen it is labelled aerobic metabolism. If it occurs without oxygen it is labelled anaerobic metabolism (2).

Energy Substrates
Several energy sources, also known as substrates, are available to regenerate ATP. One of these substrates is called phosphoCreatine (PCr).

PhosphoCreatine (PCr) : is readily available in the muscle cells and rapidly regenerates ATP. However there is only about 100g of ATP and about 120g of PCr stored in the body, mostly within the muscles.

Fat : Fat is stored predominantly as adipose tissue throughout the body and is a vast energy reservoir. Fat is not quickly accessible for cellular metabolism because it must first be biochemically converted from its complex form, triglyceride, to the simpler components of glycerol and free fatty acids. So although fat acts as a vast stockpile of fuel, its energy release is too slow for very intense activity.

Carbohydrate : Unlike fat, carbohydrate is not stored in peripheral deposits throughout the body. At rest, carbohydrate is taken up by the muscles and liver and converted into glycogen, as blood glycogen and muscle glycogen. Glycogen can be used to form ATP and in the liver it can be converted into glucose and transported to the muscles via the blood. A heavy training session can deplete carbohydrate stores in the muscles and liver, as can a restriction in dietary intake. Carbohydrate can release energy much more quickly than fat.

Protein : Protein is used as a source of energy, particularly during prolonged activity, however it must first be broken down into amino acids and then converted into glucose. Similar to fat, protein cannot supply energy at the same rate as carbohydrate. The rate at which energy is released from the substrates is determined by a number of factors, including the diet, the physical and physiological condition of the individual.

Glucogenesis is the process of synthesizing glycogen from glucose. When the body chooses to store ingested carbohydrates in the muscles or liver, it must first be converted to glycogen.

Glycogenelysis is the process of glucose formation from glycogen. Before stored glycogen can be used as an energy substrate, it must first be converted to glucose.

Gluconeogenesis is the process of converting a non-carbohydrate substrate such as protein or fat into glucose.

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