Lipids are apolar compounds that can be extracted from tissue using apolar solvents. The first group, the triglycerides, consists of triesters formed from glycerol and fatty acids. The most common fatty acids are carbonic acids with a chain length of 16 - 18 C atoms. These carbonic acids can be either saturated or unsaturated.

The saturated fatty acids are linear molecules that can stack easily and therefore solidify easily. As a result they have higher melting points. Unsaturated fatty acids have a cis-configuration around the double bond. This inserts a bend into the molecule which makes stacking more difficult, resulting in lower melting points.

The molecules illustrated are octadecanoic acid,
trans - 9 - octadecenoic acid and
cis - 9 - octadecenoic acid.