Biological sources

Fatty acids are found in biological systems either as free molecules or as components of more-complex lipids. They are derived from dietary sources or produced by metabolism, as described below.

Digestion of dietary fatty acids

The main source of fatty acids in the diet is triglycerides, generically called fats. In humans, fat constitutes an important part of the diet, and in some countries it can contribute as much as 45 percent of energy intake. Triglycerides consist of three fatty acid molecules, each linked by an ester bond to one of the three OH groups of a glycerol molecule. After ingested triglycerides pass through the stomach and into the small intestine, detergents called bile salts are secreted by the liver via the gall bladder and disperse the fat as micelles. Pancreatic enzymes called lipases then hydrolyze the dispersed fats to give monoglycerides and free fatty acids. These products are absorbed into the cells lining the small intestine, where they are resynthesized into triglycerides. The triglycerides, together with other types of lipids, are then secreted by these cells in lipoproteins, large molecular complexes that are transported in the lymph and blood to recipient organs. In detail, the process of triglyceride or fat absorption from dietary sources is quite complex and differs somewhat depending upon the animal species.

Storage

After transport through the circulation, triglycerides are hydrolyzed yet again to fatty acids in the adipose tissue. There they are transported into adipose cells, where once again they are resynthesized into triglycerides and stored as droplets. Fat or adipose tissue essentially consists of cells, whereby the interior of each cell is largely occupied by a fat droplet. The triglyceride in these droplets is available to the body on demand as communicated to adipose tissue by hormone messengers.

Various animals store triglycerides in different ways. In sharks, for example, fat is stored in the liver, but in bony fish it is deposited in and around muscle fibres. Insects store fat in a special organ called the fat body. The development of true adipose tissue is found only in higher animals.

Biosynthesis

In mammals, fatty acids are synthesized in adipose and liver cells from glucose via a fairly complex pathway. In essence, the six carbons of a glucose molecule are oxidized to a pair of two-carbon carboxylic acid fragments called acetate. The starting point for biosynthesis is an acetate group chemically linked to a molecule of CoA (coenzyme A). The process of building up the acyl chain of a fatty acid then begins, basically through the sequential chemical addition of two-carbon fragments from CoA-acetate to generate, for example, the 16-carbon saturated fatty acid palmitate. This process is catalyzed by a complex enzyme known as fatty acid synthase. Elongation of the palmitate carbon chain and the introduction of carbon-carbon double bonds are carried out subsequently by other enzyme systems. The overall process is basically the same in organisms ranging from bacteria to humans.