Sometimes the cell can control this process and other times it cannot. The energy involved in this process has both chemical and physical aspects and is far from completely understood. Fats are esters formed from glycerol and fatty acids. Fatty acids are carboxylic acids with 12 to 20 carbon atoms in the chain. In general, animal fats are more saturated and plant oils are more unsaturated.
Steroids are another class of lipids.
All steroids contain a tetracyclic ring system. One steroid produced by your body is cholesterol. Some important vitamins are lipids. Vitamins are substances used by living cells to aid enzyme reactions. Your body uses vitamins in ways ranging from the synthesis of pigments needed for good vision to the red pigment in hemoglobin that carries oxygen in the blood.