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Lipids

Lipids are important constituent of of the diet because they are a source of high energy value. Lipids are also important because of the fat-soluble vitamins,  and essential fatty acids found in the fat of the natural food stuffs. Body fat serves as a very good source of energy, it is stored in adipose tissues. They also act as insulating material in the subcutaneous tissues and are also seen around certain organs. Lipids combined with proteins are important constituents of the cell membranes and mitochondria of the cell. Lipids are not generally macromolecules. 

Lipids are naturally occurring organic compounds, commonly known as oils and fats. Lipids occur through out the living world in  microorganisms, higher plants and animals and also in all cell types. Lipids contribute to cell structure, provide stored fuel and also take part in many biological processes.

Lipid Bilayer Structure

 

Lipids Definition

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Lipids are naturally occurring hydrophobic molecules. They are heterogenous group of compounds related to fatty acids. They include fats, oils, waxes, phospholipids, etc. They make up about 70% of the dry weight of the nervous system. Lipids are crucial for the healthy functioning of the nerve cells. Lipids are greasy or oily organic substances; lipids are sparingly soluble in water and are soluble in organic solvents like chloroform, ether and benzene.

Characteristics of Lipids

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General characters of lipids are 
  • Lipids are relatively insoluble in water.
  • They are soluble in non-polar solvents, like ether, chloroform, methanol.
  • Lipids have high energy content and are metabolized to release calories.
  • Lipids also act as electrical insulators, they insulate nerve axons.
  • Fats contain saturated fatty acids, they are solid at room temperatures. Example, animal fats.
  • Plant fats are unsaturated and are liquid at room temperatures. 
  • Pure fats are colorless, they have extremely bland taste. 
  • The fats are sparingly soluble in water and hence are described are hydrophobic substances. 
  • They are freely soluble in organic solvents like ether, acetone and benzene. 
  • The melting point of fats depends on the length of the chain of the constituent fatty acid and the degree of unsaturation. 
  • Geometric isomerism, the presence of double bond in the unsaturated fatty acid of the lipid molecule produces geometric or cis-trans isomerism. 
  • Fats have insulating capacity, they are bad conductors of heat. 
  • Emulsification is the process by which a lipid mass is converted to a number of small lipid droplets. The process of emulsification happens before the fats can be absorbed by the intestinal walls. 
  • The fats are hydrolyzed by the enzyme lipases to yield fatty acids and glycerol. 
  • The hydrolysis of fats by alkali is called saponification. This reaction results in the formation of glycerol and salts of fatty acids called soaps. 
  • Hydrolytic rancidity is caused by the growth of microorganisms which secrete enzymes like lipases. These split fats into glycerol and free fatty acids. 

Types of Lipids

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In the year 1943 Bloor proposed the following classification of lipids based on their chemical composition. 

Illustration of Lipids Classification


Simple Lipids or Homolipids


Simple lipids are the esters of fatty acids with various alcohols. 

Fats and Oils (triglycerides and triacylglycerols) - These are esters of fatty acids with a trihydroxy alcohol, glycerol. A fat is solid at ordinary room temperature, an oil is liquid. 

Simple Triglycerides - Simple triglycerides are one in which three fatty acids radicles are similar or are of the same type. Example: Tristearin, Triolein.

Mixed Triglycerides are one in which the three fatty acids radicles are different from each other. Example: distearo-olein, dioleo-palmitin. 

Triglycerides from Glycerol

Waxes  are the esters of fatty acids with high molecular weight monohydroxy alcohols. Example: Beeswax, Carnauba wax. 

Compound Lipids or Heterolipids


Heterolipids are esters of fatty acids with alcohol and possess additional groups also. 

Phospholipids or Phosphatids are compound containing fatty acids and glycerol in addition to a phosphoric acid, nitrogen bases and other substituents. They usually possess one hydrophilic head and tow non-polar tails. They are called polar lipids and are amphipathic in nautre. 
Phospholipids can be phosphoglycerides, phosphoinositides and phosphosphingosides.

Phosphoglycerides are major phospholipids, they are found in membranes. It contains fatty acid molecules which are esterified to hydroxyl groups of glycerol. The glycerol group also forms an ester linkage with phosphoric acid. Example: Lecithin, Cephalins.

Phosphoinositides are said to occur in phospholipids of brain tissue and soybeans. The ply important role in transport processes in cells.

Phospholipid

Phosphosphingosides are commonly found in nerve tissue. Example: sphingomyelins. 

Glycolipids are the compounds of fatty acids with carbohydrates and contain nitrogen but no phosphoric acid. The glycolipids also include certain structurally related compounds comprising the groups gangliosides, sulpholipids and sulfatids. 

Derived Lipids


Derived lipids are the substances derived from simple and compound lipids by hydrolysis. These includes fatty acids, alcohols, monoglycerides and diglycerides, steroids, terpenes, carotenoids. 

The most common derived lipids are steroids, terpenes and carotenoids.

Steroids do not contain fatty acids, they are nonsaponifiable, and are not hydrolyzed on heating. They are widely distributed in animals, where they are associated with physiological processes. Example: Estranes, androstranes, etc.

Illustration of Steroids Structure

Terpenes in majority are found in plants. Example: Natural rubber. gernoil, etc.

Carotenoids are tetraterpenes. They are widely distributed in both plants and animals. They are exclusively of plant origin. Due to the presence of many conjugated double bonds, they are colored red or yellow. Example: Lycopreene, carotenes, Xanthophylls. 

Essential fatty acids are those that cannot be constructed through any chemical pathways, known to happen in humans. They must be obtained from the diet. Linoleic acid and linolenic acid are the essential fatty acids.

Non-essential fatty acids are those which are not necessary to be taken through diet, they are synthesized through chemical pathways. 

Unsaturated fatty acids have one or more double bonds between carbon atoms. The tow carbon atoms are bound to each other through double bonds and can occur in cis or trans configuration. 

Saturated fatty acids are long chain carboxylic acids and do not have double bonds. Example: Arachidic acid, Palmitic acid, etc.

Structure of Lipids

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Lipids has no single common structure. The most commonly occurring lipids are triglycerides and phospholipids. 
Triglycerides are fats and oils. Triglycerides have a glycerol backbone bonded to three fatty acids. If the three fatty are similar then the triglyceride is known as simple triglyceride. If the fatty acids are not similar then the fatty acids are known as mixed triglyceride. 

The second most common class of lipids are phospholipids. They are found in membranes of animal and plants. Phospholipids contains glycerol and fatty acids, they also contain phosphoric acids and a low-molecular weight alcohol. Common phospholipids are lecithins and cephalins. 

Function of Lipids

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Lipids perform several biological functions: 
  • Lipids are storage compounds, triglycerides serve as reserve energy of the body. 
  • Lipids are important component of cell membranes structure in eukaryotic cells. 
  • Lipids regulate membrane permeability. 
  • They serve as source for fat soluble vitamins like A, D, E, K. 
  • They act electrical  insulators to the nerve fibres, where the myelin sheath contains lipids. 
  • Lipids are components of some enzyme systems. 
  • Some lipids like prostaglandins and steroid hormones act as cellular metabolic regulators. 
  • Cholesterol is found in cell membranes, blood, and bile of many organisms. 
  • As lipids are small molecules and are insoluble in water, they act as signalling molecules. 
  • Layers of fat in the subcutaneous layer, provides insulation and protection from cold. Body temperature maintenance is done by brown fat. 
  • Polyunsaturated phospholipids are important constituents of phospholipids, they provide fluidity and flexibility to the cell membranes. 
  • Lipoproteins that are complexes of lipids and proteins, occur in blood as plasma lipoprotein, they enable transport of lipids in aqueous environment, and their transport throughout the body. 
  • Cholesterol maintains fluidity of membranes by interacting with lipid complexes. 
  • Cholesterol is the precursor of bile acids, Vitamin D and steroids. 
  • Essential fatty acids like linoleic and linolenic acids are precursors of many different types of ecosanoids including prostaglandins, thromboxanes. These play a important role in pain, fever, inflammation and blood clotting. 

List of Lipids

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Lipids are a diverse group of naturally occurring organic compounds. Below are the list of lipids:

List of Lipids

Examples of Lipids

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Few well known examples of lipids are as follows:

Fatty acids - Oleic acid, Linoleic acid, Palmitoleic acid, Arachidonic acid. 

Fats and Oils - Animal fats -  Butter, Lard, Human fat, Herring oil. Plant oils - Coconut oil, Corn, Palm, Peanut, Sunflower oil.

Waxes - Spermacti, Beeswax, Carnauba wax. 

Phospholipids - Lecithins, Cephalins, Plasmoalogens, Phosphatidyl inositols, Sphingomyelins.

Glycolipids - Kerasin, Phrenosin, Nervon, Oxynervon. 

Steroids - C 29, C 28, C 27, C 24, C 21 steroids. 

Terpenes - Monoterpenes, Sesquiterpenes, Diterpenes, Triterpenes.

Carotenoids - Lycopene, Carotenes, Xanthophylls.
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