The following points highlight the ten important types of lipids. The types are: 1. Neutral or True Fats 2. Waxes 3. Cutin 4. Suberin 5. Phospholipids 6. Sphingolipids 7. Lipoproteins 8. Terpenes 9. Prostaglandins 10. Steroids.
Lipid: Type # 1. Neutral or True Fats:
They are triglycerides which are formed by esterification of three molecules of fatty acids with one molecule of trihydric alcohol, glycerol (glycerine or tri-hydroxy propane). Three molecules of water are eliminated.
The word triglyceride refers to the number of three molecules of fatty acids esterified to a molecule of glycerol. If the number of fatty acids attached to a glycerol happens to be two, the ester is called diglyceride or monoglyceride if there is only one molecule of fatty acid attached to a glycerol molecule.
In fats the three fatty acids are only rarely similar (e.g., tripalmitin, tristearin, triolein). They are called pure fats. Usually they are dissimilar or two of the three fatty acids are similar. They are known as mixed fats, e.g., Butter. Fats are named after the names of fatty acids, e.g., dipalmito-stearin, palmito-oleio-stearin, steario-oleio-palmitin.
Most neutral fats are mixtures of different triglycerides. For commercial use fats are differentiated into hard fats and oils. Oils are those fats which are liquid at room temperature of 20°C. This is because they have low melting point, e.g., groundnut (peanut) oil, cotton seed oil, rape seed oil, mustard oil, sesame oil, sunflower oil, safflower oil, etc.
The low melting property is actually the property of fatty acids. Oils are richer in either unsaturated fatty acids or fatty acids having small carbon chains. Unsaturated fatty acids can combine with oxygen and other chemicals.
Therefore, exposed oils have a tendency to solidify. Hence, they are called drying oils. Edible oils can be converted into hard fats through the process of hydrogenation. In hydrogenation unsaturated fatty acids are changed to saturated state. Vanaspati or vegetable ghee and margarine (emulsified animal or plant fat) are obtained from oils through hydrogenation.
Oils with polyunsaturated fatty acids (fatty acids with more than one double bond) are called polyunsaturated (PUFA). They are recommended by physicians to persons having hypertension, high blood cholesterol and other cardiovascular diseases.
It is because polyunsaturated fatty acids lower blood cholesterol. Edible oils having polyunsaturated fatty acids are safflower and sunflower oils. Hard fats are solid at room temperature of 20°C. They contain long chain saturated fatty acids, e.g., animal fat. Butter is soft because it contains good quantity of short chain fatty acids.
Lipid: Type # 2. Waxes:
They are fatty acid esters of long chain monohydric alcohols like cytyl, ceryl or mericyl. Other fat like substances also occur in waxes. Plant waxes occur in cuticle (along with cutin) and as greyish waxy coating or bloom around the plant organs. Wax found on the upper surface of floating leaves prevents wetting and submergence. Wax found on the surface of land plants is useful in reducing transpiration.
In animals cutaneous glands are known to secrete wax lanolin for forming a protective water insoluble coating on animal fur. Cerumen or ear wax is secreted by cutaneous glands for lubricating ear drum.
Bees build their hives from wax secreted by their abdominal glands. Beeswax is a complex of several waxes. The major component is ester of palmitic acid (C16 H32 O2) and mericyl alcohol (= triacontanol, C30H61OH). Second major component is ester of palmitic acid with hexacosonal (C26H53OH).
Tuberculosis and leprosy bacteria produce a wax called wax-D which is a major factor in their pathogenicity. Paraffin wax is obtained from petroleum. Candles are made of paraffin wax and stearic acid. Waxes are used in cosmetics and polishes. Waxed paper is used for wrapping.
Lipid: Type # 3. Cutin:
It is a complex lipid produced by cross-esterification and polymerisation of hydroxy fatty acids, as well as other fatty acids with or without esterification by alcohols other than glycerol. Cutin occurs in the aerial epidermal cell walls as well as a separate layer of cuticle on the outside of these epidermal cells. Cuticle has 50-90% cutin. Cutin reduces the rate of transpiration. It also binds epidermal cells.
Lipid: Type # 4. Suberin:
It is a mixture of fatty material having condensation products of glycerol and phellonic acid or its derivatives. Suberin makes the cell wall strong and impermeable. It occurs in the walls of cork cells and endodermal cells.
Lipid: Type # 5. Phospholipids (Common Membrane Lipids):
They are triglyceride lipids where one fatty acid is replaced by phosphoric acid which is often linked to additional nitrogenous groups like choline (in lecithin), ethanolamine (in cephalin), serine or inositol. Phospholipids are amphipathic carrying both hydrophilic polar and hydrophobic nonpolar groups.
The hydrocarbon chains of the two fatty acids function as hydrophobic non-polar tails of the phospholipid molecule. The phosphate and additional group (nitrogenous or non- nitrogenous) behave as hydrophilic polar head group of the molecule (Fig. 9.11).
In aqueous medium the phospholipid molecules arrange themselves to form a double layer or bilayer (Fig. 9.12). The polar or hydrophilic heads of molecules form the two surfaces which are in contact with water. The hydrophobic or nonpolar tails of the phospholipid molecules are towards the centre of the bilayer. Lipid bilayer is the basic component of all cell membranes.
Lipid: Type # 6. Sphingolipids:
They are lipids having amino alcohol sphingosine. Sphingomyelins contain an additional phosphate attached to choline like phospholipids. They occur in myelin sheath of nerves. Cerebrosides possess sugar residue galactose. They occur in nerve membranes. Ganglio- sides possess sugar residues glucose, galactose, sialic acid and acetyl glucosamine.
They influence ion transport through the membrane as well as function as receptors of viral particles. Gangliosides occur in grey matter. Excessive accumulation of gangliosides produces disorders like Tay-Sachs disease. Since cerebrosides and gangliosides contain sugar residues, they are also called glycolipids.
Lipid: Type # 7. Lipoproteins:
Lipoproteins are composed of lipids and proteins. They are present in blood, milk and egg yolk.
Lipid: Type # 8. Terpenes:
They are lipid like hydrocarbons formed of isoprene (C5Hg) units. Steroids like cholesterol are also derived from terpenes having 6 isoprene units. Essential oils of plant origin are terpenes with 2-4 isoprene units, e.g., camphor, menthol.
Gibberellins are a group of plant growth hormones with four isoprene units. An equal number of isoprene units occur in vitamin A, E and K. Phytol or tail of chlorophyll a molecule is a terpene with 4 isoprene units. Carotenoids (carotenes and xanthophyll’s) have 8 isoprene units. Natural and synthetic rubbers are terpenes with thousands of isoprene units arranged in linear fashion.
Lipid: Type # 9. Prostaglandins:
They are derivatives of arachidonic acid and other 20 Carbon fatty acids which have several functions like vasodilation, vasoconstriction, bronchoconstriction, acid production in stomach, cell communication and hormone modulation.
Lipid: Type # 10. Steroids:
They are a group of complex lipids that possess a hydrogenated cyclopentano-perhydrophenanthrene ring system (Fig. 9.13).
Position of carbon atoms are numbered. Attached to carbon atoms 10 and 13 are present methyl groups (having carbon atoms 19 and 18 respectively). In cholesterol a side chain is attached to carbon 17. Sterols have one or more hydroxyl groups. Others have carbonyl or carboxyl groups.