It includes the hydrolysis products of simple and compound lipids and also various other compounds such as steroids, terpenes, fatty acids, alcohols, fatty aldehydes, ketones etc.
- Since they contain no fatty acids, they are non-saponifiable, i.e., cannot be hydrolyzed by heating with alkali to yield soaps of their fatty acid components.
- The steroids may be separated from the fat after the latter is saponified since they occur in ‘nonsaponifiable residue’.
- All steroids may be considered as derivatives of a fused and fully saturated ring system called cyclopentanoperhydrophenanthrene or sterane.
- This system consists of 3 cyclohexane rings (A, B and C) fused in nonlinear or phenanthrene manner and a terminal cyclopentane ring (D).
Some common steroids are:
- Cholesterol has a molecular formula, C27H45OH.
- In addition to an OH group at C3, there is a double bond at C5.
- The hydroxyl group constitutes its polar head, the rest of the molecule is hydrophobic.
- It is a white crystalline solid and is optically active, [α]D 39°.
- The crystals are rhombic plates with one of the angles broken.
- It has a melting point of 149°C.
- Cholesterol is generally believed to be notorious as a major cause of heart disease.
- There are 2 types of cholesterol, the lowdensity lipoprotein cholesterol (LDL-C) and the high-density lipoprotein cholesterol (HDL-C).
- It is the principal sterol of higher animals and is especially abundant in nerve tissues and in gallstones.
- It occurs either free or as fatty esters in all animal cells.
- It was first isolated in 1784, from human gallstones which consist almost entirely of cholesterol and hence so named (cholesterol literally means ‘solid alcohol from bile’).
- Its main sources are fish liver oils and the brain and spinal cord of cattle.
- Cholesterol is , however, not found in plant fats.
- It occurs in feces and is produced in the intestine as a result of bacterial action on the double bond of cholesterol.
- The A/B junction is -cis in coprostanol in contrast to -trans in cholesterol.
- It is present in ergot (hence its nomenclature), yeast and the mould Neurospora.
- Its parent hydrocarbon is ergostane, C28H50.
- Ergosterol has a molecular formula, C28H43OH with one OH group at C3 and 3 double bonds at C5, C7 and C22.
- It is also optically active, [α]D -135°.
- It is a major constituent of wool fat and is also present in minor quantities in liver and yeast.
- Lanosterol is a C 30 compound with twin methyl groups at C 4 and a third angular methyl group on C 14.
- There are 2 double bonds at C8 and C24.
- It is an intermediate in the biosynthesis of cholesterol.
- Among the non-saponifiable lipids found in plants are many hydrocarbons known as terpenes (from turpentine).
- In general, these hydrocarbons and their oxygenated derivatives have lesser than 40 carbon atoms.
- The simplest terpenes are called monoterpenes and conform to the formula C10H16 (equivalent to 2 isoprene units), those with the formula C15H24 are called as sesquiterpenes, with C20H32 as diterpenes and with C30H48 as triterpenes.
- Terpenes with 40 carbon atoms (or tetraterpenes) include compounds called carotenoids.
- The presence of long hydrocarbon chain in carotenoids makes them lipid-soluble; they are hence also called lipochromes or chromolipids.
- Some examples of terpenes are shown below: