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Futile cycle
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<p>A futile cycle, also known as a substrate cycle, occurs when two <a href="/facts/Metabolic_pathway/8Py8HKJd">metabolic pathways</a> run simultaneously in opposite directions and have no overall effect other than to <a href="/facts/Dissipation/C134STy1">dissipate</a> energy in the form of <a href="/facts/Heat/FjDGssCs">heat</a>. The reason this cycle was called "futile" cycle was because it appeared that this cycle operated with no net utility for the organism. As such, it was thought of being a quirk of the metabolism and thus named a futile cycle. After further investigation it was seen that futile cycles are very important for regulating the concentrations of metabolites. For example, if <a href="/facts/Glycolysis/a5ApxLoG">glycolysis</a> and <a href="/facts/Gluconeogenesis/Jj5THIHh">gluconeogenesis</a> were to be active at the same time, <a href="/facts/Glucose/oUSkoLCD">glucose</a> would be converted to <a href="/facts/Pyruvate/qJTgl4J9">pyruvate</a> by glycolysis and then converted back to glucose by gluconeogenesis, with an overall consumption of <a href="/facts/Adenosine_triphosphate/nB9bwgcU">ATP</a>. Futile cycles may have a role in metabolic regulation, where a futile cycle would be a system oscillating between two states and very sensitive to small changes in the activity of any of the <a href="/facts/Enzyme/DSI1Fh7y">enzymes</a> involved. The cycle does generate heat, and may be used to maintain thermal <a href="/facts/Homeostasis/rloTGAHa">homeostasis</a>, for example in the <a href="/facts/Brown_adipose_tissue/ut8P9TLS">brown adipose tissue</a> of young <a href="/facts/Mammal/phSwTzBo">mammals</a>, or to generate heat rapidly, for example in <a href="/facts/Insect/IgDNvnUL">insect</a> flight muscles and in hibernating animals during periodical arousal from <a href="/facts/Torpor/IlGtqU22">torpor</a>. It has been reported that the glucose metabolism substrate cycle is not a futile cycle but a regulatory process. For example, when energy is suddenly needed, ATP is replaced by AMP, a much more reactive adenine. </p>