Lipid – an organic compound consisting of carbon, hydrogen and oxygen atoms. They are hydrophobic and insoluble in water, however they are soluble in organic solvents.
There are different types of lipids including triglycerides, glycolipids, cholesterol and phospholipids. However in this post I mainly focus on triglycerides and phospholipids.
Triglycerides
Triglycerides are a type of lipid that are stored in fat cells. They provide insulation as well as protection for delicate organs.
Structure of Triglycerides
They consist of one molecule of glycerol bonded to 3 fatty acids. The below image shows the structure of glycerol and a fatty acid:
The R group on the fatty acid represents a variable hydrocarbon tail (a chain of carbon atoms bonded to hydrogen atoms). When bonded together we get the following structure:
Formation and Breakdown of Triglycerides
Triglycerides are formed from condensation reactions between glycerol and fatty acids and they can be broken by hydrolysis reactions.
A hydrogen atom on the glycerol molecule bonds with the hydroxyl (-OH) molecule on the fatty acid to release a molecule of water. This forms an ester bond between the fatty acid and glycerol which can be broken with the addition of water.
Phospholipids
Phospholipids are one of the core components of the cellular membrane, forming the bilayer.
Structure of Phospholipids
They contain glycerol, two fatty acids and a phosphate group. The fatty acids are hydrophobic (water hating), whilst the phosphate group is hydrophilic (water loving). Having a hydrophobic head and hydrophilic tail allows them to form the phospholipid bilayer of the cellular membrane.
The R group in this image does not represent a fatty acid.
Saturated and Unsaturated Lipids
Saturated Lipids
Saturated Lipids don’t have any double (carbon-carbon) bonds in their hydrocarbon tails (fatty acids). They are saturated with hydrogen, meaning that each carbon atom is bonded to at least 2 hydrogen atoms:
Unsaturated Lipids
Unsaturated lipids contain double (carbon-carbon) bonds in their hydrocarbon tails (fatty acids). Each double (carbon-carbon) bond causes a kink in the hydrocarbon tail:
Key Differences Between Saturated and Unsaturated Lipids
- Unsaturated lipids contain double bonds, saturated lipids don’t
- Saturated lipids are often found in animal fats, unsaturated lipids are often found in plants
- Saturated lipids have a higher melting point than unsaturated lipids
Key Definitions
Lipid – an organic compound consisting of carbon, hydrogen and oxygen atoms. They are hydrophobic and insoluble in water, however they are soluble in organic solvents.
Condensation Reactions – A reaction that involves the release of a water molecule when two molecules combine.
Hydrolysis Reaction – a reaction in which a molecule of water is used to break a bond (the opposite of a condensation reaction).
Hydrocarbon – an organic compound that consists of carbon and hydrogen atoms.
Hydrophobic – the property of a molecule that is repelled by water.
Hydrophilic – the property of a molecule that is attracted to water.
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