What are lipids?
Lipids are naturally-occurring organic compounds that are readily soluble in nonpolar solvents but insoluble in polar solvents.
The main biological functions of lipids include storing energy, signalling, and acting as structural components of cell membranes.
Since lipids are defined by their solubilities, they have varying structures, but they all have one thing in common: a polar, hydrophilic "head" and a nonpolar, hydrophobic, hydrocarbon "tail".
Lipids include the following classes.
Fats are esters of glycerol with long-chain fatty acids.
The ester groups form the polar head of the molecule.
In phospholipids, one of the fatty acids has been replaced by a phosphate group and a simple molecule such as choline.
Sphingolipids are based on sphingosine rather than glycerol. Sphingomyelin is a typical example.
Sterols have a tetracyclic hydrocarbon ring system with an attached hydrocarbon chain. Cholesterol is a typical example.
Vitamin A (retinol) is a typical example.
The OH group is the polar head of the molecule.