<P> Fatty acid metabolism consists of catabolic processes that generate energy, and anabolic processes that create biologically important molecules (triglycerides, phospholipids, second messengers, local hormones and ketone bodies). Fatty acids are a family of molecules classified within the lipid macronutrient class . One role of fatty acids in animal metabolism is energy production, captured in the form of adenosine triphosphate (ATP). When compared to other macronutrient classes (carbohydrates and protein), fatty acids yield the most ATP on an energy per gram basis, when they are completely oxidized to CO and water by beta oxidation and the citric acid cycle . Fatty acids (mainly in the form of triglycerides) are therefore the foremost storage form of fuel in most animals, and to a lesser extent in plants . In addition, fatty acids are important components of the phospholipids that form the phospholipid bilayers out of which all the membranes of the cell are constructed (the cell wall, and the membranes that enclose all the organelles within the cells, such as the nucleus, the mitochondria, endoplasmic reticulum, and the Golgi apparatus). Fatty acids can also be cleaved, or partially cleaved, from their chemical attachments in the cell membrane to form second messengers within the cell, and local hormones in the immediate vicinity of the cell . The prostaglandins made from arachidonic acid stored in the cell membrane, are probably the most well known group of these local hormones . </P> <P> Fatty acids are released, between meals, from the fat depots in adipose tissue, where they are stored as triglycerides, as follows: </P> <Ul> <Li> Lipolysis, the removal of the fatty acid chains from the glycerol to which they are bound in their storage form as triglycerides (or fats), is carried out by lipases . These lipases are activated by high epinephrine and glucagon levels in the blood (or norepinephrine secreted by sympathetic nerves in adipose tissue), caused by declining blood glucose levels after meals, which simultaneously lowers the insulin level in the blood . </Li> <Li> Once freed from glycerol, the free fatty acids enter the blood, which transports them, attached to plasma albumin, throughout the body . </Li> <Li> Long chain free fatty acids enter the metabolizing cells (i.e. most living cells in the body except red blood cells and neurons in the central nervous system) through specific transport proteins, such as the SLC27 family fatty acid transport protein . Red blood cells do not contain mitochondria and are therefore incapable of metabolizing fatty acids; the tissues of the central nervous system cannot use fatty acids, despite containing mitochondria, because long chain fatty acids (as opposed to medium chain fatty acids) cannot cross the blood brain barrier into the interstitial fluids that bathe these cells . </Li> <Li> Once inside the cell long - chain - fatty - acid--CoA ligase catalyzes the reaction between a fatty acid molecule with ATP (which is broken down to AMP and inorganic pyrophosphate) to give a fatty acyl - adenylate, which then reacts with free coenzyme A to give a fatty acyl - CoA molecule . </Li> <Li> In order for the acyl - CoA to enter the mitochondrion the carnitine shuttle is used: </Li> </Ul> <Li> Lipolysis, the removal of the fatty acid chains from the glycerol to which they are bound in their storage form as triglycerides (or fats), is carried out by lipases . These lipases are activated by high epinephrine and glucagon levels in the blood (or norepinephrine secreted by sympathetic nerves in adipose tissue), caused by declining blood glucose levels after meals, which simultaneously lowers the insulin level in the blood . </Li>

In the metabolism (breakdown) of fats the fatty acids enter the reactions of the
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