Fatty acids are unbranched hydrocarbons (carbon chains with hydrogens attached) with an organic carboxylic acid group at the end. The carbon chains in some hydrocarbons can be branched, but naturally occurring fatty acids are not branched. The carboxylic acid group consists of a carbon double-bonded to one oxygen, then single bonded to another oxygen. The latter oxygen has a hydrogen atom at the end, which can come off as a proton (H+) in water, which is the definition of an acid. The structure of a carboxylic acid group is shown below:
Naturally occurring fatty acids are usually between 12-24 carbons long (including the carbon of the carboxylic acid). The bonds between the carbons are usually single, with two hydrogen atoms attached to each carbon in the middle (the end will have three hydrogens, since every carbon can only have four bonds). Fatty acids with all single bonds between the carbons are called saturated, since they have the maximum number of hydrogen atoms per carbon atom. Saturated fatty acids are found in saturated fats, which are usually solid at room temperature.
Alternatively, some of the bonds between the carbons can be double bonds, which results in only one hydrogen per carbon on both sides of the double bond. These fatty acids are called unsaturated. In natural fatty acids, the double bonds are positioned so that the hydrogen atoms are on the same side of the bond. These fatty acids are called cis fatty acids, shown below.
Synthetic fatty acids found in margarine and other processed fats have the hydrogen atoms on the opposite side of the carbon-carbon double bond, and are called trans fatty acids, as shown below.
Trans fatty acids have a different three-dimensional structure and can be metabolized into toxic products.
Saturated fatty acids generally form solid fats at room temperature, whereas unsaturated fatty acids and polyunsaturated fatty acids are generally liquid (oils). Polyunsaturated oils are generally from plant sources, whereas saturated fats are usually from animals.
Fatty acids are non-polar, and do not dissolve in water. Three fatty acids can chemically combine with a glycerol molecule (a three carbon, three alcoholic molecule) to form what is called a triester, or fat. When only two fatty acids combine with the glycerol, and the third position gets a phosphate group with another water-soluble organic group on the other side of it, then this is called a phospholipid. Phospholipids are the main building blocks of the cell membrane, which separates the cell from the outside aqueous milieu. In addition to phospholipids, there are proteins in the cell membrane which control what goes into and out of the cell.