Classification of Lipids

Classification of lipids

Lipids are broadly classified into simple,complex and derived which are further subdivided into different groups .The classification of lipids is shown in the chart below:

A) Simple lipids:

Simple lipids are the esters of fatty acids with various alcohols. They are further classified as ;

1) Triglycerides( fats and oils)

These are the esters of fatty acid with glycerol.The fats and oils that are widely distributed in both plants and animals are chemically triglycerides or triacylglycerides.They are insoluble in water and not- polar in nature and commonly known as neutral fat.

Note: R is same for simple oils and fats

R is different for mixed oils and fats

Oilsand fats are considered as lipids which on hydrolysis produce long chain monocarboxylic acid and glycerol.

Difference between fat and oil:

2) Waxes:

Waxes are the ester of long chain saturated and unsaturated fatty acids with long chain alcohols. waxes are considered as the lipid which on hydrolysis produce long chain fatty acids and long chain alcohols.

Waxes→hydrolysis→long chain fatty acids+ long chain alcohols

In the case of plankton (aquatic plants) waxes is the chief storage form of metabolic fuel . Biological waxes have a variety of applications in the pharmaceutical, cosmetics and other industries.

B) Complex Lipids:

On the basis of composition complex lipids are further classify as:

1) Phospholipids

2) Glycolipids

1) Phospholipids:

These are complex lipids containing phosphoric acid .They also contain nitrogen base and other substances and they are mainly deposited in the brain and nervous tissue.They are also present in cytoplasmic membrane due to which they are involved in cellular activities. They are classified into two groups:

1. Glycerophospholipids
2. Sphingophospholipids

1. Glycerophospholipids:

They are most abundant phospholipids . They contain 2 fatty acids esterified with first and second hydroxyl group of glycerol. the third hydroxyl group of glycerol forms an ester linkage with phosphoric acid.

Glycerophospholipids are further divided into following groups:

^{a) Phosphatidylcholine( Lecithins):}

It is widely distributed in the body mainly in the liver. In this phospholipids, collin is present as the base which prevents the deposition of abnormal fat in the liver. It is also the major constituents of acetyl choline.

^{b) Phosphatidylethanolamine (cephalin ) ;}

The base ethanolamine is present which occur together with lecithin and connected with the brain. It is also present in RBC.

^{c) Phosphatidylserine:}

The base amino acid serine is present. It has similar function with lecithin.

^{d) Phosphatidylinositol}:

It contains inositol as the base instead of nitrogen base. It is mainly found in plants and nervous tissue. Some hormones such as oxytocin,vasopressin are modified through phospholipids.

^{e) Phosphatidylglycerol:}

In this phospholipids, two molecules of phosphatic acids are held by the addition of glycerol to phosphate groups. It is found in the inner membrane of mitochondria.

^{f) Phosphatidylethanolamine (plasmalogens)}

In this phospholipids, fatty acids are linked with carbon and glycerol by an ester linkage. Generally, unsaturated fatty acid is linked at carbon.

Functions of phospholipids:

Phospholipids are constituents of an important group of compound lipids that perform a wide variety of functions:

• Phospholipids in the associations with proteins form the structural components of a membrane and regulate membrane permeability.
• Phospholipids in the mitochondria are responsible for maintaining the conformation of electron transport chain components and thus cellular respiration.
• Phospholipids participate in the absorption of fat in the intestine.
• Phospholipids are essential for the synthesis of different proteins ie. lipoproteins and thus participate in the transport of lipids.
• Accumulation of fat in the liver can be prevented by phospholipids ,hence they are regarded as the lipo protein factor.
• phospholipids participate in the reverse cholesterol transport and thus help in the removal of cholesterol from the body.
• Cephalins are the important group of phospholipids participate in the blood clotting.
• Phosphatidylinositol are involved in signal transmission across membrane.
• Phospholipids act as an agent for lowering surface tension.

2) Sphingophospholipids:

These are compounds formed by one molecule of long chain fatty acids and two molecules of long chain amino alcohol sphingosine or one of its derivative and their polar bond alcohols. They are found in the brain or nervous tissue. On the basis of composition Sphingolipids are two types:

a) Sphingomyelin:

It is the simplest and most abundant lipid which contains phosphocholine or phosphoethanolamine as the polar head region. It is present in the animal cell as myelin sheath which surrounds the nerve cell.

b) Ceramides: It is composed of sphingosine molecule and fatty acids:

2) Glycolipid:

It is the combination of lipid and carbohydrates which lack the phosphate group. It contains C 24 fatty acid molecule such as cerebric acid, lignoceric acid. It is mainly of two types:

a) Cerebrosides

b) Gangliosides

Sulfatides are also the class of glycolipids which are similar to cerebrosides except the 6th carbon of glucose and galactose is bonded with sulphuric acid. They are present in the myelin sheath.

a) Cerebrosides: It is the simplest form of glycolipids. Glucose and galactose are present as carbohydrate residue. Generally, carbohydrate chain of 2-20 units is present.

b)Gangliosides: It is a derivative of cerebroside and contains one or more molecule of N- acetyl neuraminic (NANA) . It is predominantly present on grey matter of the brain and neutralized tetanus toxins.

Function of glycolipids:

• glycolipids are essential components of all membranes in the body.
• They are located primarily in the outer layer of the plasma membrane where they interact with the extracellular environment.
• They play a role in the regulation of cellular interaction, growth, and development.
• They are antigenic and determine blood group of an individual.
• They also serve as cell surface receptors for cholera and diphtheria as well as for certain viruses.
• It forms the structural component of nervous tissues.

C) Derived Lipids:

These are the lipids containing cyclopentane rings, these are the complex fat soluble molecule with four fused rings. steroids are compounds containing a cyclic steroid nucleus namely Cyclopentanoperhydrophenanthrene (CPPP). It consists of a phenanthrene nucleus to which a cyclopentane ring is attached . these are several steroids in the biological system. These include cholesterol, bile acids, vitamin D,sex hormones etc. If the steroids contain one or more hydroxyl group it is commonly known as the sterol (means alcohol).

The most common sterol found in animal tissue is the cholesterol. The cholesterol consists of Cyclopentanoperhydrophenanthrene ring with OH group at carbon number three and the double bond between carbon number 5 and 6.

It is solid at room temperature, insoluble in water and occurs rarely in higher plants. the cell membrane of eukaryotes consists of cholesterol. It is the precursor of a large number of important steroids including bile acids, sex hormones, vitamin D etc. Cholesterol is synthesized by the liver and secreted in the blood. It is equally distributed in serum and plasma.

The normal blood cholesterol level is 130-250mg/dl. But cholesterol level is determined by metabolic functions which are influenced by heredity, nutrition,endocrine functions and integrity of vital organs like liver,kidney etc. If the cholesterol level falls below normal value the condition is called hypocholesterolemia. This condition is seen in hyperthyroidism acute infection,anemia,malnutrition etc. If the cholesterol level exceeds normal value the condition is called hypercholesterolemia. This condition is seen in hypothyroidism infection, poorly controlled diabetes mellitus,chronic hepatitis,nephrotic syndrome, obstructive Jaundice. Thus the determination of blood cholesterol level is the important diagnostic parameter from the clinical point of view.

Function of steroids/ cholesterols:

• It serves as a structural component of the cell membrane which determines the shape of a cell membrane .
• cholesterol functions as an insulating cover for the transmission of electrical impulses in the nervous tissue.
• It is the precursor of a large number of equally important steriods which include the bile acids,sex hormones and vitamin D.
• The low concentration of cholesterol in female may lead to sterility ,so cholesterol play important role in fertility .
• The excess cholesterol in the body leads to cardiovascular diseases.
• Steroids are the precursors of Vitmin D which maintains the serum calcium level.