Lipids are biomolecules known as fats. Lipids are part of the cell wall of plants. Apart from this there also are other biological functions of lipids which you will see here. You will also see their characteristics, classification, as well as the physical and chemical properties.
The term lipid is derived from lipos (a Greek word) which means fat. They are made of hydrocarbons and are mainly the reduced form of carbon. They exist as esters of fatty acids, glycerol, and triglycerides.
Lipids are a huge source of energy. This energy is released by their oxidation in the cells. They serve as building blocks of cell structure and other functional units of living cells. The common examples of lipids are fats, some vitamins, waves, oils, hormones, and some major parts of nonprotein membranes.
Characteristics of lipids
- Lipids are nonpolar and are soluble in nonpolar solvents like chloroform. It is insoluble in polar solvents like water.
- It can be extracted from animals and plants with the help of nonpolar solvents like ether, acetone, and chloroform. Fats, steroids, and phospholipids are examples of such lipids.
- Lipids with a functional ester group are soluble in water and can be hydrolysed. Eg., Neutral fats, waxes, glycolipids, phospholipids, etc.
- Lipids that lack a functional group cannot be hydrolysed and that includes steroids and other fat-soluble vitamins such as A, E, K, and D.
- The fats and oils are made of triglycerides and triglycerides. Their chemical composition is glycerol and three fatty acid molecules forming a triester. The complete hydrolysis of triglycerides gives these molecules.
Classification of Lipids
Lipids can be classified based on their structure into simple, complex, and derived.
A. Simple lipids
Simple lipids are fatty acid esters formed as a result of reacting with alcohols. Simple lipids can be heterogeneous, nonpolar, and insoluble in water. However, they are soluble in nonpolar
solvents like benzene and chloroform. Simple lipids include fats and oils, waxes, and ceramides.
- Fats and oils are the result of esterification of fatty acids and glycerol. They can be saturated or unsaturated, homogeneous since the hydroxyl groups of glycerol can be esterified, or heterogenous. They exist in liquid form.
- Butyric – Liquid form
- Palmitic – Melting point 63
- Stearic – Melting point 70
- Oleic – Liquid
- Linoleic – Liquid
- Linolenic – Liquid
- Waxes are the product of esterification of long-chain fatty acids and alcohol. The alcohol present here has a long chain with 12-32 carbon atoms. Waxes are used in cosmetics and ointments. Examples of wax include,
- Carnauba is found on the leaves of Brazilian palm trees
- Beeswax secreted by bees to make cells for honey and eggs
- Spermaceti found in the head cavities and blubber of the sperm whale
- Paraffin wax is used for candle making.
- Ear wax contains a mix of phospholipids and the esters of cholesterol.
- Ceramides are amides of fatty acids with long-chain di- or trihydroxy bases. These bases may have 12–22 carbon atoms in its chain. Eg. sphingosine.
- Cholesteryl esters are dietary lipids formed by the esterification of cholesterol and fatty acids. They have a lower solubility in water since they have hydrophobic molecules. They can be hydrolysed only by pancreatic enzymes such as cholesterol esterase. The hydrolysis of these molecules results in free fatty acids and cholesterol molecules. Eg. Cholesteryl Oleate
B. Complex lipids
Complex lipids are mainly found in the membranes of cells, in blood platelets, brain tissues etc. Here is the list of complex lipids and their source.
- Ceramide di- and poly hexoside are linked to disaccharide or tri- or oligosaccharide, respectively.
- Cerebroside sulfate is a ceramide mono hexoside esterified with a sulfate group.
- Cerebrosides are ceramide mono hexosides linked to a single sugar molecule at the terminal -OH group.
- Gangliosides are glycolipids containing 1-3 sialic acid residues and are structurally similar to ceramide polyhexoside. There might also be an additional amino sugar.
- Phosphatidic acid is an ester of diacylglycerol with phosphoric acid.
- Phosphatidyl acylglycerol has more than one ester of glycerol and phosphoric acid: e.g. cardiolipin and diphosphatidyl acylglycerol.
- Phosphatidylcholine is also called lecithin, which is a phosphatidic acid bonded to choline.
- Phosphatidylethanolamine.
- Phosphatidylinositol
- Phosphatidylserine
- Phospholipids are esters of fatty acids, glycerol, phosphoric acid and other groups containing nitrogen.
- Sphingolipids are derivatives of ceramides.
- Sphingomyelin is a ceramide phosphorylcholine.
C. Derived lipids
Derived lipids are formed from simple and compound lipids through hydrolysis. Examples of derived lipids include,
- Fatty acids and alcohols
- Fat-soluble vitamins A, D, E and K
- Sterols like cholesterol.
Cholesterol and Vit D
Cholesterol is the precursor to sex hormones and Vit D.
Our skin has plenty of cholesterol between the phospholipids which helps with the fluidity. Vitamin D is produced from the cholesterol molecules that rise to the surface layer of the skin. When these molecules are exposed to UV sunlight they undergo reactions to form Vit D.
When the level of cholesterol exceeds a particular level in the bile, it leads to the formation of gallstones. Gallstones are cholesterol and some amount of minerals such as calcium.
Physical Properties of Fatty Acids
- The length of the fatty acid chain and the unsaturation of the hydrocarbon chain determine its physical properties.
- The longer chain and fewer double bonds in fatty acids cause lower solubility in water and a higher melting point.
- More double bonds and an increase in chain length reduces the melting point of fatty acids.
- Trivial names of fatty acids show their derivatives from their natural sources.
- Lauric acid isolated from seed fat of Lauraceae
- Myristic acid from seed fat in Myristicaceae
- Palmitic acid from the seed fat of Palmae
- Oleic acid is taken from the seed fat of olive oil.
Chemical Properties of Lipids
- Acid number is the amount of KOH required to neutralize free fatty acids in 1g of fat or oil. This number provides the number of fatty acids and also indicates the state of fat or oil that is stored under unfavourable conditions. The acid number is important as it helps calculate the hydrolytic rancidity of fats and oils.
- Esterification combines free fatty acids with glycerol to produce triglycerides. The result will be mono or diglycerides. Monoglycerides are used as emulsifying agents in the food industry.
- Interesterification happens when the fatty acids are moved from one triglyceride molecule to another. Interesterification is also called randomization, modification, or rearrangement. This is used in the food industry to produce confectionery, margarine oils, processed edible oils, etc.
- Hydrolysis of fatty acids yields mono and diglycerides and free fatty acids. Hydrolysis can be accelerated by pressure, high temperature, and excess amount of water in the medium.
- Hydrogenation of fatty acids is done to make hydrogenated fats. This process is used to make omega fatty acids hard at room temperature. This process is dependent on the nature of the target substance, temperature, pressure, concentration of hydrogen, and that of the catalyst.
- Halogenation of fatty acids is done with chlorine, iodine, and bromine. The addition of these elements adds double bonds for the unsaturated fatty acids.
- Isomerization is the process by which one molecule of a compound is transformed into
- another molecule having the exact same atoms but with a different atomic arrangement. The two important types of isomerism of fatty acids are Geometric and positional isomerism.
- Geometrical isomerism will have two configurations of double bonds in cis and trans forms. When the hydrogen atoms are on the same side, the arrangement is Cis and when hydrogen atoms are on opposite sides, it is trans. All natural fats and oils have a cis form. Fats from remnants have a trans -form of fatty acids.
- Positional Isomerism has the unsaturated fatty acid isomerised in alkaline or acidic conditions, or at high temperatures when the double bond moves from one position to another. At times, hydrogenation processes cause a shift in double bonds in fatty acid chains leading to cis-trans isomerisation.
- Oxidation of fats and oils happens when they come in contact with oxygen in the air. Here oxidation happens at the double bonds, affecting the flavour of the fat. The oxidation rate increases directly with an increase in temperature, oxygen exposure, lights, and metals such as copper. The oxidation that takes place at room temperature is called autoxidation. Peroxide value is used to measure the degree of oxidation or rancidity of unsaturated fats and oils. It is the quantity of peroxide formed per kg of fat.
- Polymerization of fatty acids happens when there is excessive oxidation such as deep frying of food at a higher temperature. These conditions cause heat stress. The rate of polymerization is directly proportional to the amount of unsaturation and viscosity.
- Riechert Messel Number of lipids refers to the amount of water-soluble volatile fatty acids. It is the amount of alkalis required to neutralize these fatty acids in 5 g of fat.
- Saponification is the breaking down or degradation of neutral fat into its components of glycerol and fatty acids when treated with alkali. The saponification number is the amount of KOH needed to to saponify 1g of fat.
Biological Functions of Lipids
Lipids perform several biological functions:
- Lipids such as triglycerides serve as storage compounds that reserve energy for the body.
- They are integral components of cell membranes in eukaryotic cells.
- It regulates membrane permeability.
- Serve as a source of fat-soluble vitamins.
- Act as electrical insulators for nerve fibers such as the case of myelin sheath.
- Are components of certain enzyme systems.
- Lipids such as prostaglandins and steroid hormones serve as metabolic regulators.
- Cholesterol is part of cell membranes, bile, and blood.
- Cholesterol is also a precursor for steroids, bile acids, and vitamin D. It also maintains membrane fluidity.
- They act as signaling molecules.
- Wax over the green tissues of plants forms a thin hydrophobic layer that serves as a chemical as well as a physical barrier.
- Fat in the subcutaneous layer serves as insulation and protection from cold and helps maintain body temperature.
- Polyunsaturated phospholipids are part of phospholipids that provide flexibility and fluidity to plasma membranes.
- Plasma lipoproteins transport lipids in an aqueous environment, and throughout the body.
- Essential fatty acids like linoleic and linolenic acids are precursors of prostaglandins, thromboxanes, etc. They are also important in pain, inflammation, fever, and blood clotting.
