An Introduction to Lipids

Hello :-) In this post, we'll be looking at the structure and function of the major lipid groups, we'll discuss how they're classified as well as the nomenclature used to name non esterified fatty acids (NEFAs). We'll also talk about the functions of fatty acids.

Classification of Lipids

Lipids can be broken down into four categories:

• Non esterified fatty acids (NEFAs) which consist of:
• volatile fatty acids (VFAs)
• long chain fatty acids
• eicosanoids
• Glycerol Lipids, which consist of:
• Triacylglycerols
• Phospholipids
• Sphingolipids
• Cholesterol

Non Esterified Fatty Acids

The general structure of a NEFA is shown below:

R-COOH

Volatile Fatty Acids (which are also known as short chain fatty acids) are fatty acids that have 1 to 6 carbon atoms in their structure. VFAs are water soluble and smell at room temperature. Acetate, propionate and butyrate are the most common and they are used for fuel and the biosynthesis of fat and glucose in animals. They are very important for ruminants. 

Medium chain fatty acids also exist, they have 7-10 carbon atoms in their structure, however, they are not very common. 

Long chain fatty acids have 12-16 carbon atoms in their structure and this number is always even-numbered in biology. Long chain fatty acids make up oils and fats and their properties depend on their chain length, hydrophobicity and the position and number of double bonds present. Long chain fatty acids are used as fuel in the body, particularly during prolonged exercise and fasting, as well as for the biosynthesis of fats and phospholipids. 

Saturated Long Chain Fatty Acids:

These fatty acids have no carbon-carbon double bonds and are straight chained molecules that pack well, allowing for a higher melting point. As a result, these FAs form solids at room temperature. They are used as storage fats in animals. Examples include palmitic acid (C16:0; which means there are 16 carbon atoms and no double bonds) found in palm oil, and stearic acid (C18:0) which can be found in beef fat.

Monosaturated Long Chain Fatty Acids  

These contain one carbon-carbon double bond and are cis isomers that don't pack well. This results in a lower melting point and thus these fatty acids are liquids at room temperature. Examples include palmitoleic acid (C16:1) and oleic acid (C18:1) which are shown below. 

Oleic Acid

Palmitoleic Acid

There are two systems used to describe the position of the double bond in these molecules:

• The ω (omega) or n system: this method counts from the methyl end. For example, Palmitoleic Acid would be ω7C16:1 or n7C16:1. This is because the double bond occurs on the seventh carbon atom from the methyl end. 
• The Δ (delta) system: this method counts from the carboxyl (COOH) end and is more rare than the n system. For example, Palmitoleic Acid would be C16:1Δ9 because the double bond occurs on the ninth carbon atom from the carboxyl end.

Polyunsaturated Long Chain Fatty Acids:

These have two or more carbon-carbon double bonds. Examples include linoleic acid (n6 C18:2), which is rich in plant oils, and linolenic acid (23 C18:3) which is found in some plant oils and animal membrane tissues. 

 

Linoleic Acid

Linolenic Acid

That's it for this post, if you have any questions please feel free to ask :)