Fatty Acids

The structure of fatty acids

Fatty acids are, like the lipids that they build, organic molecules. This means they contain carbon and hydrogen. They contain oxygen as well.

They are often referred to as long molecules, as they consist of a long, straight chain of carbon and hydrogen atoms, with a carboxyl acid group (―COOH) at one end and a methyl group $-{\mathrm{CH}}_3$ at the other. The simple formula is RCOOH, where R is the hydrocarbon chain, including the methyl group. The R-group may be saturated or unsaturated.

In addition, fatty acids can vary in terms of the length of the hydrocarbon chain (12 to 20 carbon atoms).

The sheer length of the hydrocarbon chain means that there are many carbon and hydrogen atoms but far fewer oxygen atoms as a part of the COOH. This makes these fatty acids hydrophobic.

The two types of fatty acids

There are two different types of fatty acids: saturated and unsaturated.

Saturated fatty acids

Saturated fatty acids have only single bonds between carbon atoms in the hydrocarbon chain. As you already know, carbon can form four bonds with other atoms. You also know that the hydrocarbon chain is composed of carbon atoms, to which hydrogen atoms attach. They attach to two of the four bonds of carbon atoms. This means that there are a lot of hydrogen atoms attached to each carbon, so you can say that there is a maximum number of hydrogen atoms in the chain and that the fatty acid is saturated with hydrogen.

Saturated fatty acids are primarily present in food and products from animal sources, such as beef, lamb, butter, cream, etc., and oils derived from plant sources, such as coconut and palm oil.

They are solid at room temperature because the hydrocarbon chain is straight, which in turn allows molecules of saturated fatty acids to pack closely together.

Fig. 1 - Simplified chemical structure of isopalmitic acid, a saturated fatty acid. It consists of single bonds between carbon and hydrogen atoms in the long hydrocarbon chain

Unsaturated fatty acids

Unsaturated fatty acids have single bonds between carbon atoms but double bonds as well. As there are double bonds between some carbon atoms in the hydrocarbon chain, not as many hydrogen atoms can attach to carbon atoms. So you can say that these fatty acids are not saturated or unsaturated with hydrogen.

Double bonds make them liquid at room temperature. As they cause the molecule to bend, they prevent unsaturated fatty acids from packing together as closely as saturated fatty acids.

Depending on how many double bonds are present, there are two different types: mono-unsaturated and poly-unsaturated.

Mono-unsaturated fatty acids

Mono-unsaturated fatty acids have only one double bond between two carbon atoms in the hydrocarbon chain.

Fig. 2 - Simplified chemical structure of lumequeic acid, a mono-unsaturated fatty acid. It consists of single bonds between carbon and hydrogen atoms but one double bond as well

Poly-unsaturated fatty acids

Poly-unsaturated fatty acids have two or more double bonds between carbon atoms in the hydrocarbon chain.

Fig. 3 - Simplified structure of colneleic acid, a poly-unsaturated fatty acid. It consists of two or more double bonds in the hydrocarbon chain

Essential fatty acids

In your lipids and fatty acids studies, you will undoubtedly come across essential fatty acids. They are called essential because they are extremely important for our health, especially the heart and the brain, but they cannot be synthesized (made) by our body. Therefore, we need to consume food high in essential fats: fish and other seafood, seeds such as hemp and pumpkin seeds, leafy vegetables, etc.

In humans, there are two: alpha-linolenic acid or ALA and linoleic acid or LA.

ALA is an omega-3, while LA is an omega-6 fatty acid. Both are poly-unsaturated.

Two other omega-3 acids are of great importance for our bodies: eicosapentaenoic (EPA) and docosahexaenoic (DHA). Our bodies can produce these by converting the ALA but in a very limited amount.

You may wonder why these acids are called omega-3 and omega-6. This is because, in the long hydrocarbon chain, the carbon atoms are numbered 1, 2, 3, etc. They are numbered starting from the end that has the methyl group. The last letter of the Greek alphabet, the omega (ω), is used to indicate the double bonds in the chain.

The similarities and differences between saturated and unsaturated fatty acids