A fatty acid

 A fatty acid is an organic acid (carboxylic acid -COOH), attached to a long aliphatic chain of either –CH₂- units (saturated) or –CH=CH- units (unsaturated). Natural fatty acids, those found in organic tissues, usually have an even number of carbon atoms in the structure because of the way they are made or synthesized in the specialized adipose cells.

Saturated fatty acids

Saturated Fatty Acids

Formula : Name : Melting point

Although the overall structure of unsaturated fatty acids is similar to saturated fatty acids, there is one important difference; the long aliphatic tail contains one or more alkene functional groups.

Part of the chain with the structure -CH₂-CH₂- becomes -CH=CH-; two carbon atoms are attached to each other with a double bond instead of a single bond.

There are several ways of illustrating where these double bonds are located in the fatty acid molecule.

Delta-9,12 : Counting from the –COOH group, the two double bonds join the ninth-tenth and twelfth-thirteenth carbon atoms.

Omega-3 : The first double bond starts at the third carbon atom.  This time the counting begins at the end of the chain furthest from the -COOH group.

Molecular sequences can also be drawn out in long form, thus;

A typical Omega-3 fatty acid is alpha linolenic and is drawn

Whereas an Omega-6 fatty acid such as linoleic acid would be drawn

Formula : Name : Melting point

Saturated fatty acids have a uniform rod-like shape and are often solids at room temperature, having higher melting points than similar sized unsaturated fatty acids.

The double bonds in the unsaturated fatty acids cause a kinked, or bent shape.  This makes it difficult to pack or stack lots of these molecules together in a regular array or crystalline lattice.

Free fatty acids are those not joined with any other kind of molecules and are usually formed from the breakdown of a larger, more complex lipid molecule such as a di- or triglyceride.  This gives the free fatty acids and a molecule of glycerol.

In the animal body, free fatty acids are a source of of energy and fuel reactions in lots of tissues and organs.  The breakdown of a fatty acid produces large quantities of ATP, which is then used to promote everything from muscular contraction to pumping water across cell membranes.

The heart and skeletal muscle prefer to use fatty acids as a source of energy, but the brain cannot obtain fatty acids and so must use the carbohydrate glucose as a source of fuel.

Some fatty acids are essential in the human diet.  These are usually polyunsaturated fatty acids that cannot be made in the human body.  They have to be obtained from food that is eaten.   The cells in the human body do not have all the enzyme machinery necessary to make more than on double bond in a fatty acid molecule (monounsaturated fatty acids).

Our cells can make molecules with one double bond, but not more.  However, polyunsaturated fatty acids are essential to our immune systems, are parts of substances such as prostoglandins, and have a role to play in blood pressure regulation.