Broadly into two main classes the lipid classification may be done. Those are
1) Simple lipids
2) Compound lipids.
These are composed of fatty acid and alcohol only and do not contain any other substituent group. They may again be divided into three subgroups, those are:
Fats and oils or glycerides
Fats and oils are esters of fatty acids with glycerol and hence are also called glycerides. Those glycerides that remain solid at room temperature are called fats whereas low melting point glycerides that remain liquid at room temperature are called oils. Depending on the number of fatty acid molecules present in the glycerides, they may be of three types:
i) Monoglycerides: Here only one -OH group of glycerol is esterified by a single fatty acid
ii) Diglycerides: It is containing two fatty acids esterified with two -OH groups of glycerol.
iii) Triglycerides: In it all the three basic hydroxyl groups of glycerol remain esterified by three fatty acids. Triglycerides are also called neutral fats.
In lipid classification most of the natural fats and oils are triglycerides. Monoglycerides and diglycerides present in the body are mainly produced as intermediate compounds during synthesis and breakdown of triglycerides. Often the names neutral fat, triglyceride and fat are used synonymously.
Glycerides of each variety vary due to the kind of fatty acids present in them. Triglycerides may be divided into two groups-simple and mixed. Simple triglycerides are those in which the three glycerol-hydroxyls are esterit'ied with three molecules of the same fatty acid: examples are- tripalmitine. tristearin. triolein etc.
Mixed triglycerides contain two or three different fatty acids : examples are— stearo-oleo-palmitine. stearo-dipalmitine etc. Fats having higher melting points contain long chain saturated fatly acids whereas oils having low melting points contain smaller or unsaturated fatty acids mainly.
Another lipid classification is waxes. Waxes are fatty acid esters of alcohols other than glycerol. The fatty acids present in waxes are generally long chain acids and alcohols are monohydroxy alcohols. Examples are—lanolin, beeswax, carnauba wax, sperm oil etc. Sebum and cerumen also contain waxes.
Steroids are compounds containing 'cyclopentanoper-hydroplienanthrene' as the parent nucleus (or steroid nucleus). Many of these contain free hydroxyl groups and are called sterols. The sterols may remain in free form or as esters of fatty acids in natural lipids. Esterified sierols may also be considered as the lipid classification of waxes. However, the terms steroid and sterol are often used synonymously. Examples are— cholesterol and allied compounds e.g., hormones of adrenal cortex, testis and ovary. Un-esterified sterols are also included in lipids because they are soluble in typical lipid solvents.
These are another lipid classification, which are the fatty acid esters containing additional groups such as phosphate, sulphate, sugars, proteins etc., and accordingly they are classified into some subgroups; those are:
These are phosphoric acid containing lipids. The phosphoric acid remains as a diester i.e., esterified with the alcohol at one end and with a nitrogenous base or amino acid or sugar alcohol at the other end. Phospholipids are of two main types: —
(a) Glycerophosphatides: In these phospholipids, glycerol is the alcohol. They are also called phosphoglycerides. Example—lecithins, cephalins etc.
(b) Sphingomyelins: In this type, the nitrogenous alcohol sphingosine is present instead of glycerol. Example— phospholipids of myelin sheath.
Giycolipids or Cerebrosides
These lipid classification are fatty acid esters of sphingosine and contain in addition a hexose sugar. They differ from each other by containing different fatty acids. Examples are cerebron, nervon etc., that are found in brain and nerves.
Sulfolipids are cerebroside sulfuric acid esters, also called cerebroside sulfates. Example—cerebron sulfate.
These are combinations of lipids and proteins, the lipids being mainly phospholipids. Such lipids are abundant in cell membranes and blood.