Fatty acids are predominately nonpolar molecules consisting of a long chain of carbons with an oxygen and a hydroxyl group at one end.
A fatty acid is said to be saturated if each carbon is joined to its neighboring carbons by a single bond. If one or more double bonds is present, the fatty acid is said to be unsaturated. (The term "saturation" here means that all possible hydrogens are present.)
Unsaturated fatty acids are classified based on the position of the first double bond from the end of the chain. The counting starts at the end of the fatty acid molecule without the oxygens. For example, if the first double bond is in the third position, the fatty acid is classified as n-3 (also called omega-3).
While a saturated fatty acid is a straight molecule on the average, the double bond in an unsaturated fatty acid produces a kink in the molecule. This because a double bond cannot rotate. The bend in the carbon chain, however, is much more pronounced in the cis isomer compared to the trans isomer. For this reason, cis fatty acids (and triacylglycerols made from them) do not solidify as readily as trans fatty acids. Due to the larger bend, the cis isomers cannot line up next to one another in as ordered a fashion as the trans isomers.
While trans fatty acids are uncommon in natural fatty acids, they form readily when polyunsaturated fatty acids from plants are "partially hydrogenated" chemically. This is done commercially to make plant fatty acids more solid and to improve self-life. Epidemiological studies correlate consumption of trans fatty acids with increased risk of heart disease.
If a fatty acid has more than one double bond in the carbon chain, the fatty acid is termed polyunsaturated. Two types of polyunsaturated fatty acids are of particular importance in the diet, because they are used in the body, but cannot be synthesized by our enzymes or, indeed, by those of any mammal. These two types of essential fatty acids are n-3 and n-6. The absolute requirement for these fatty acids is normally easily met except in unusual circumstances, such as very poor absorption of fatty acids. But let's look more closely at these fatty acids, since the optimal level is probably much higher than the "essential" amount.
Linoleic acid is the important n-6 fatty acid in the diet, while alpha-linoleic acid is the important n-3 fatty acid. Plants and plant oils contain both of these, although most of the alpha-linoleic acid is in plant oils, notably from soybean, canola, flaxseed and walnut oils. But linoleic and alpha-linoleic acid do not perform the same physiological functions in the cell. Indeed, the two fatty acids can serve competing roles.
Linoleic acid ultimately can be be converted to arachidonic acid, an important precursor of certain regulatory molecules we will soon discuss.
On the other hand, alpha-linoleic acid can be converted in small amounts to two somewhat longer n-3 fatty acids, EPA and DHA, which both play physiological roles. Both EPA and DHA are found in far higher concentrations in fish and marine mammals eating the fish. Thus, fish, especially oily fish (anchovies, sardines, mackerel, salmon, herring), are the main dietary source of these n-3 fatty acids.
The n-6 or n-3 polyunsaturated fatty acids often affect the regulatory physiology following their incorporation into phospholipids in plasma membranes. Following an appropriate stimulus, the fatty acid is freed and then converted into a certain paracrine, which is the category of regulatory molecule that acts locally on nearby cells. You are going to encounter this type of paracrine frequently, and they are discussed in the webpage following the next two.
(These fatty acids also have other effects that we will not study. For example, within membranes they can directly affect opening and closing of ion channels and thus the electrical excitability of membranes. Sometimes they also bind to membrane receptors.)
Triacylglycerols are synthesized from three fatty acids joined together by one glycerol molecule. Glycerol by itself is a small carbohydrate molecule containing three carbons.
Triacylglycerols are the form in which fat energy is stored in adipose tissue. The various dietary plant oils, such as olive oil and corn oil, are also triacylglycerols.
The presence of unsaturated fatty acids in a triacylglycerol molecule makes it more fluid. This is due to the presence of the kinks in unsaturated fatty acids, which keeps the fatty acid chains from aligning uniformly. This is why plant oils, which contain primarily unsaturated fatty acids, are more liquid than animal fats. As noted above, when trans fatty acids are made, the trans configuration of the double bond also produces a straighter chain of carbons and thus the resulting triacylglycerols tend to be more solid.
Triacylglycerols are sometimes referred to as triglycerides.