Lipids Introduction, Classification, and Chemical nature.

Lipids

Lipids are esters of fatty acids and alcohol or related substances. They are hydrophobic compounds that include fat, oil, wax, sterols, phospholipids, glycolopids, etc. Fats are esters of fatty acids and glycerol.

Classification of lipids

Lipids are classified into three classes:

• A. Simple lipid (Homolipid):

Simple lipids are esters of fatty acids with alcohol. They do not contain any additional groups. They contain only carbon, hydrogen, and oxygen. Ex. Fats, Oils, and Waxes

• 1. Fats: Fats are esters of three molecules of fatty acids and a molecule of trihydroxy alcohol: glycerol. They are solid at normal temperatures (around 20 degrees C). There are two types of fats.

  • i. Pure fats: A lipid containing all three similar fatty acids is called pure fat. Ex. Triesterine, Triolein

  • ii. Mixed fats: Lipids containing three dissimilar fatty acids or two similar and one dissimilar fatty acid are called mixed fats. Ex butter.

• 2. Oils: Oils are esters of three molecules of fatty acids and a molecule of trihydroxy alcohol: glycerol. They are liquid at normal temperature (around 20 degrees C).

• Fats and oils are also called triglycerides or triacyl glycerol.

• 3. Waxes: Waxes are esters of fatty acids with a long chain mono hydroxyl alcohol or high molecular weight monohydroxy alcohol. Ex. cerumen (ear wax), beeswax, carnuba wax, etc.

B. Compound lipid (Heterolipid or Conjugated lipid):

They are esters of fatty acids with alcohol that also have some additional groups. Ex. Phospholipid, Glycolipid, 

• 1. Phospholipid: Phospholipid is an ester of fatty acids with alcohol (glycerol) in which one fatty acid is replaced by phosphoric acid.

• 2. Glycolipid: A lipid with carbohydrates is called glycolipid.

C. Derived lipid:

Hydrolysis of simple lipid or compound lipid yields derived lipid. Ex. Sterols, fatty acids, terpenes,

PROPERTIES OF TRIGLYCERIDES (OILS AND FATS)

Fats and oils are triglycerides or triglycerols.

• 1. Esterification of three molecules of fatty acids with one molecule of glycerol (Trihydroxy alcohol) form triglycerides. Water is released during this esterification.

• 2. A lipid containing all three similar fatty acids is called pure fat. Pure fat is rarely found in nature. Ex. Triesterine, Triolein

• 3. Lipids containing three dissimilar fatty acids or two similar and one dissimilar fatty acid are called mixed fats. Most of the fats in nature are mixed fats. Ex butter.

• 4. Animal lipids are rich in saturated fatty acids and plant lipids are rich in unsaturated fatty acids.

• 5. Lipids with a large proportion of saturated fatty acids are solid at room temperature (about 200 C). They are considered as fats. Lipids with a large proportion of unsaturated fatty acids are liquid at room temperature (about 200 C). They are considered oils.

• 6. Hydrogenation of unsaturated fatty acids in oil yields saturated fatty acids and form fats. Hydrogenation of vegetable oil forms Vanaspati Ghee.

• 7. Oils containing fatty acids with more than one double bond are called polyunsaturated fatty acids (PUFA). Oils containing fatty acids with one double bond are called monounsaturated fatty acids (MUFA).

FATTY ACID CLASSIFICATION - BASED ON CHEMICAL & NUTRITIONAL REQUIREMENTS WITH EXAMPLES

Classification on the basis of chemical requirements: Fatty acids are organic compounds containing carbon, hydrogen, and oxygen in a linear carbon chain skeleton. It has one carboxyl group (-COOH) at one end with an even number of carbon atoms. Fatty acids with odd-number carbon atoms are very rare. The number of carbon atoms may be up to 30.

A. Classification of fatty acids on The basis of bonds in linear carbon chain skeleton

• 1. Saturated fatty acids (CnH2n+1COOH): Fatty acids with saturated hydrocarbons (no double bond) are called saturated fatty acids. Ex. Palmatic acid and Stearic acid.

• 2. Unsaturated fatty acids: Fatty acids with unsaturated hydrocarbons (Containing double bonds) are called unsaturated fatty acids. Unsaturated fatty acids are further classified into The following subcategories depending on the number of double bonds present in the linear carbon chain structure.

  • i. Monounsaturated fatty acids (MUFA) (CnH2n-1COOH): These fatty acids have only one double bond in a linear carbon chain skeleton. They are also called mono ethnoid fatty acids Ex. Oleic acid.

  • ii. Polyunsaturated fatty acids (PUFA): These fatty acids contain two or more double bonds in a linear carbon chain skeleton. Polyunsaturated fatty acids are further sub-classified on the basis of the number of double bonds present in linear carbon chain skeleton.

    • a. Di unsaturated fatty acids (CnH2n-3COOH): They have two double bonds in a linear carbon chain skeleton. They are also called diethnoid acids.

    • b. Tri unsaturated fatty acids (CnH2n-5COOH): They contain three double bonds in a linear carbon chain skeleton. They are also known as triethenoid acids. Ex. Linolenic acid.

    • c. Tetra unsaturated fatty acids (CnH2n-7COOH): They contain four double bonds in a linear carbon chain skeleton. They are also known as tetraethenoid acids. Ex. Arachidonic acid

B. Fatty acids are classified into the following four classes depending upon the number of carbon in a linear carbon chain skeleton.

• 1. Short-chain fatty acids: Linear carbon chain skeleton of fatty acids with not more than 6 carbon atoms.

• 2. Medium chain fatty acids: Linear carbon chain skeleton of fatty acids with a number of carbon atoms in between 8 and 12.

• 3. Long-chain fatty acids: Linear carbon chain skeleton of fatty acids with number of carbon atoms in between 14 and 18.

• 4. Very long chain fatty acids: Linear carbon chain skeleton of fatty acids with number of carbon atoms not less than 20.

C. Other classification of fatty acids

• 1. Oxygenated fatty acids: Linear carbon chain skeleton of fatty acids with hydroxyl group, keto group, and epoxy group. Ex. ricinoleic acid, is a  major fatty acid in castor oil.

• 2. Cyclic fatty acids: Fatty acids containing cyclic rings are called cyclic fatty acids. This cyclic unit may have 3, 5 or 6 carbon atoms. Ex prostaglandins.

Classification of fatty acids depending upon nutritional requirements:

Fatty acids are classified into two classes depending on the ability of the human body to synthesize them.

• 1. Essential fatty acids: Fatty acids that are not synthesized inside the human body are called essential fatty acids. The human body depends upon external sources for these fatty acids. Ex. Linoleic acid, Linolenic aid and arachidonic acid.

• 2. Nonessential fatty acids: The human body can synthesize these fatty acids.

Alok Bains

STRUCTURE AND FUNCTIONS OF CHOLESTEROL

Structure of cholesterol: Hydrolysis of simple lipids and compound lipids yield derived lipids. The steroid is an example of derived lipids. Steroids consist of the fused saturated ring system. The fused saturated ring system has three saturated cyclohexane rings (A, B, and C) and one cyclopentane ring (D). This fused ring system is called a sterane nucleus (Phenanthrene nucleus). The sterane nucleus is a tetracyclic cyclopenta phenanthrene structure. Steroids containing hydroxyl group (-OH) are called hydroxyl steroids or Sterols. Cholesterol is an animal sterol.

Cholesterol (C27 H 45 OH) has a sterane nucleus. The cholesterol sterane nucleus has a hydroxyl group at carbon number three and a double bond between C5 and C6. C17 has octyl side chain. Two methyl groups are linked at C13 and C10 of the sterane nucleus. The carbon of these methyl groups is numbered as C18 and C19. The cholesterol molecule has four rigid ring nuclei and a flexible tail.

Functions of cholesterol in the human body:

Cholesterol plays a vital role in human life. Its functions can be enumerated as below:

• 1. Cholesterol is an essential structural and functional component of human cells.

• 2. It is a precursor for steroidal hormones, vitamin D, bile acid, and several metabolites.

• 3. It plays a very important role in cell signaling, nerve conduction, gene transcription, and transport processes inside the human body

• 4. It is present in the myelin sheath of nerve fibers. Thus it is vital for the nervous system, especially the brain.

• 5. It maintains the integrity and fluidity of the cell membrane.

LIPOPROTEIN- TYPES, COMPOSITION AND FUNCTIONS

• Types of lipoproteins: There are six types of lipoproteins depending upon their density.

• 1. Chylomicron: The density of chylomicron is less than 0.93gm/mL. It is produced in the intestine. It is rich in triglycerides.

• 2. Chylomicron Remnant: Chylomicron becomes the chylomicron Remnant after release of triglyceride into tissue. It is smaller in size than chylomicron

• 3. Very low-density lipoproteins (VLDL): It is rich in triglyceride. It is synthesized in the liver. Its size depends upon the amount of triglyceride in VLDL. But its size shall be always smaller than to of chylomicron.

• 4. Intermediate density lipoproteins (IDL): It is also called VLDL Remnant. Triglyceride from VLDL enters into adipose tissue and muscle tissue. It becomes a VLDL Remnant.

• 5. Low-density lipoproteins (LDL): LDL is derived from VLDL and IDL. They are rich in cholesterol. Most of the cholesterol remains with LDL. There is variation in the size and density of LDL present inside the human body. LDL is harmful to the human body. It is considered bad cholesterol.

• 6. High-density lipoproteins (HDL): HDL is a heterogeneous compound. It is rich in cholesterol and phospholipids. It is beneficial lipoprotein. It carries cholesterol from peripheral tissues to the liver. It is considered good cholesterol.

• Composition of lipoproteins: Lipoproteins are made of lipid and protein. The center of lipoprotein protein particles has nonpolar lipids as the hydrophobic core. It has mainly cholesterol ester and triglycerides. The hydrophilic membrane surrounds the hydrophobic core. The hydrophilic portion is made of phospholipid, free cholesterol, and apolipoprotein. Lipoprotein may be soluble lipoprotein or insoluble lipoprotein depending upon its composition. Soluble lipoproteins are found in blood plasma and egg yolk. Insoluble lipoproteins are found in cell membranes.

Lipoproteins

1 Chylomicron: Major constituents

• 84% Triglycerides

• 7% Phospholipid

• 2% Protein

• Density: Less than 0.93gm/mL

2 Very low-density lipoproteins (VLDL):  Major constituents

• 50% Triglycerides

• 18% Phospholipid

• 10% Protein

• Density: 0.93 to 1.006 gm/mL

3 Intermediate density lipoproteins (IDL):  Major constituents

• 31% Triglycerides

• 22% Phospholipid

• 18% Protein

• Density: 0.93 to 1.006 gm/mL

4 Low-density lipoproteins (LDL)

• 25% Triglycerides

• 21% Phospholipid

• 4% Protein

• Density: 1.019 to 1.063 gm/mL

5 High-density lipoproteins (HDL)

• 33% Triglycerides

• 29% Phospholipid

• 8% Protein

• Density: 1.06 to 1.121 gm/mL

Functions of Lipoproteins:

Cholesterol is insoluble in blood plasma. Lipoproteins play an important role in the transportation of cholesterol from one part of the human body to another part of the human body through blood plasma. This function is performed by the two types of lipoproteins. These are low-density lipoprotein (LDL) and high-density lipoproteins (HDL).

• 1. Role of LDL: Cholesterol is synthesized in the liver. LDL carries this cholesterol from the liver and transports it to several cells of the human body.

• 2. Role of HDL: HDL carries unused cholesterol present in the different cells of the human body to the liver. The liver breaks down this cholesterol into bile acids. These bile acids are excreted from the human body.

• 70% of blood plasma cholesterol binds with LDL to be transported. This cholesterol develops fatty deposits inside blood vessels. This develops the disease of atherosclerosis. The remaining 30% of blood plasma cholesterol binds with HDL. HDL reduces or inhibits cholesterol deposits inside blood vessels. HDL carries cholesterol to the liver. Thus HDL cholesterol is considered as good cholesterol and LDL cholesterol is considered as bad cholesterol.

Alok Bains