What are Lipids and Fatty Acids ? Structure, Classification, Functions, Sources and Examples of Lipids ~ Study Notes, Exams Guide, and Carrier Paths

A complete and comprehensive note on one of the most important topics of Biochemistry i.e. Lipids. This article explains What are lipids? What is the chemical composition of lipids? What are fatty acids and how we can classify them? Here you will also find the classification of Lipids into groups and sub-groups.

Introduction to Lipids

Definition of Lipids

Organic compounds that are made up of fatty acids, and alcohols and may contain other compounds called Lipids.

General Characteristics of Lipids

Lipids are non-soluble in water or any other polar solvent. So they are Hydrophobic.
They are soluble in non-polar solvents only e.g. in ether, chloroform, benzene, acetone, etc.
Being lighter they have a low density than water.
They leave greasy marks on paper.
Some of the examples of lipids are Triglycerides, Phospholipids, Cholesterol, and Cholesterol derivatives, such as Steroid hormones, Bile salts, Fats-soluble vitamins, Prostaglandins (PG), etc.

Fatty Acids - The Major component of Lipids

Important points to know about Fatty Acids

Fatty acids are organic acids that occur in nature and are Aliphatic monocarboxylic acids made up of hydrocarbon chains.
They are made up of a minimum of two carbon atoms.
The chain length of lipid-forming fatty acids ranges from 4 to 24 carbon atoms.
Fatty Acids, which occur in neutral fats, usually contain an even number of Carbon atoms.

Classification of Fatty acids

Fatty acids can broadly be divided into saturated fatty acids and unsaturated Fatty Acids.

1. Saturated Fatty Acids

The fatty acids that do not contain double bonds, between their carbon atoms, are known as saturated fatty acids.

Examples of Saturated Fatty Acids

Butyric acid
Caproic acid
Palmitic acid
Stearic acid, etc.

The saturated Fatty acids, having less than eight carbon atoms, are liquid at room temperature and are volatile.

2. Unsaturated Fatty Acids

The Fatty acids that contain at least one double bond between their Carbon atoms are called Unsaturated Fatty acids.

According to their degree of unsaturation, Fatty Acids are further classified into mono-unsaturated, and poly-unsaturated Fatty acids.

a. Mono-Unsaturated Fatty Acids

The type of unsaturated fatty acids that contain one double bond. e.g. Oleic Acid (Found in nearly all fats).

b. Poly-Unsaturated Fatty Acids

The type of unsaturated fatty acids that contain more than one double bond.

Following are the three biologically important Polyunsaturated Fatty Acids, i.e.

Linoleic acid (di-poly unsaturated, having two double bonds).
Linolenic acid (tri-poly unsaturated, having three double bonds).
Arachidonic acid (tetra-poly unsaturated, having two double bonds).

Polyunsaturated Fatty acids, that are not synthesized in our body, are termed essential Fatty acids. So, should be taken from oils such as oil from corn, wheat germ, peanut, soya bean, etc.

A lack of essential Fatty acids can produce growth retardation and other deficiency symptoms.

Physical Properties of Fatty Acids

For saturated Fatty acids, the melting point increases with the increase in the number of carbon atoms, while for unsaturated fatty acids the melting point increase with the decrease in the number of double bonds.
Solubility of Fatty acids increases with the increase of the number of double bonds and vice versa.

Chemical properties of Fatty Acids

1. Formation of salts

Upon boiling with alkali metals and alkaline are metals, Fatty acids form salts i.e. Na+, and K+ (soluble salts) while Ca++, and Mg++ (insoluble salts).

The salts of sodium, potassium, calcium, and magnesium are soaps, The process is called saponification.

Classification of Lipids

Lipids are broadly classified into two classes i.e. Simple Lipids and Compound Lipids

1. Simple Lipids

Esters of Fatty acids with various alcohols form simple lipids. Simple lipids are further classified into neural fats and waxes.

A. Neutral fats or Tri-acyl-glycerols (TAG)

TAGs are esters of fatty acids with glycerol.
The fatty acid moiety in lipid esters is called the “acyl” group.
Depending upon the numbers of ‘OH’ of glycerol, in an acylglycerol, they may be mono, di and triglycerols.
TAGs with different fatty acid compositions are known as mixed TAGs.

Functions of TAGs

Mono and di-acyl-glycerols play an important role in digestion, as well as act as metabolic intermediates.
TAGs are the major storage and transport form of lipids.

Physical properties of Neutral Fats (TAGs)

TAGs may either be liquid at room temperature and are called Oils, while some are non-crystalline solids.
Pure fats and oils are colorless, odorless, and tasteless.

For your information:

Due to dissolved impurities, fats and oils may change their color, taste, or smell. For example, Butter is yellow in color due to the presence of a Pigment molecule i.e. Carotene. Similarly, the taste of butter is due to diacetyl and 3-hydroxy-2-butanone (produced by Bacteria).

Chemical properties of Neutral fats (TAGs)

a. Hydrolysis

Upon hydrolysis, Neutral fats yield glycerol and fatty acids.
In the human body, lipases (pancreatic enzymes) are responsible for the hydrolysis of dietary fats.

b. Saponification

The process of formation of soaps, from boiling of TAGs with alkalis (NaOH, and KOH), is called saponification.

c. Hydrogenation

The process by which hydrogen is added to unsaturated double bonds present in oils.

Upon hydrogenation, the unsaturated Fatty acids (in oils) become saturated. Thus by this process Liquid fats i.e. oils are converted to solid fats i.e. ghee.

d. Halogenation

The addition of Halogen (that takes place at double bond) in which unsaturated fatty acids, in free or combined form, react with Halogens is called Halogenation.

e. Rancidity

A slow process of developing an unpleasant odor and taste to any fat or oil is termed rancidity.

2. Compound Lipids

The lipids that contain other substances in addition to fatty acids and alcohols are known as compound lipids.

Compound lipids include Phospholipids, Glycolipids (Cerebrosides), Gangliosides, Sulfo-lipids, Lipoproteins, and Lipopolysaccharides. Their detail is given as

A. Phospholipids

As their name indicates, Phospholipids are phosphate-containing lipids.
They are biologically very important being present in membranes, serum lipoproteins, bile, pulmonary surfactant, and egg yolk.
They also play an important role in biochemical processes, such as Electron Transport Chain, Oxidative Phosphorylation, and energy-linked transport of ions across the membranes.

Two main types of Phospholipids

i. Glycerol-phospholipids (GPL) or Phospho-glycerides or Glycerol-phosphatides

GPLs are Chemically composed of Fatty acids, Glycerols, Phosphoric Acid (H₃PO₄), and in many cases Nitrogenous base as well.

Functions of GPL include

They act as good emulsifying agents.
GPLs are the very important constituents of cellular structures, i.e. cell membrane.
They help in signal transmission across the cell membrane.
They are the component of lung surfactants.

Classification of GPL

GPL is of many types, such as Phosphatidic Acid, Lecithins, Cephalins, and Plasmalogens. They are described as

a. Phosphatidic Acid

The type of GPL formed when one of the Fatty acids of Triglyceride is replaced by H₃PO₄ is known as Phoshatidic acid.

Being the simplest, among all GPLs, it acts as the parent compound of all GPLs.
It is present in cells in small amounts.

b. Lecithins

These GPLs are derivatives of alpha-phosphatidic acid, in which choline (a nitrogenous base) is joined with H₃PO₄.

Depending upon the types of fatty acids, present in Lecithins, they are of many types.
These are the most abundant of the phsopho-lipids in serum and bile.
They are good emulsifying agents for fats.
They are important constituents of lung surfactants.

c. Cephalins

Except that, in these GPLs the choline is replaced by ethanol-amine, serine, or inositol, otherwise structurally identical to Lecithins.

Being present in high concentrations in our brain tissue (Diencephalon, and Telencephalon parts), they are named Cephalins.
They also take part in blood clotting.

d. Plasmalogens

In most Plasmalogens, the base is ethanol-amine (though choline and serine are also present).

They are most widely present in skeletal muscles, heart, brain, liver, and Platelets.
They were originally known as “Platelet activation factors” as they cause Platelet aggregation and degranulation.
They may cause pulmonary edema and hypersensitivity reactions.
They mediate acute inflammatory reactions and may cause anaphylactic shock (a life-threading condition).
They cause neutrophils and macrophages to generate superoxide radicals.

ii. Sphingo-phospholipids (SPL)

Sphingo-phospholipids (SPLs) are the sub-types of Phospholipids that contain Sphingosine. Sphingosine is an unsaturated Nitrogen containing 18-Carbon alcohol.

The chief SPL in this class is Sphingo-myelin, which is present in the myelin sheath.
Sphingomyelin contains Fatty acids, Sphingosine, and Phosphoryl-choline or Phosphoryl-ethanol-amine.
The sphingosine + Fatty acid combination is called Ceramide.
Sphingomyelin is present in large amounts in brain and nerve tissues, and in smaller amounts in the blood.

B. Glycolipids (Cerebrosides)

Glycolipids are Ceramide sugars i.e. Ceramide + Hexose (Glucose + Galactose).

They are present mostly in the white matter of the brain and also in the myelin sheath of the nerves.
Except for Cerebronic acid (a 24-Carbon Fatty acid) which is common in all types of Glycolipids, other Glycolipids differ from each other by the type of Fatty acids present in them.

C. Gangliosides

Gangliosides are composed of Ceramide + Galactose or Glucose + N-Acetyl Neuraminic acid or N-Acetyl Hexosamine.

They are present in the brain, spleen, RBCs, and Nerve cells.
They are the constituents of cell membranes.
They play role in tissue immunity and transmission of nerve impulses.

D. Sulfo-lipids (Sulfatides)

Sulfo-lipids are composed of Ceramide + Galactose + Sulfate.

They are mostly present in the white matter of the brain and also in other tissues.

E. Lipo-proteins

Lipo-proteins are composed of a neutral lipid core (containing Tri-acyl-Glycerol or Cholesteryl esters or both) surrounded by a shell of apo-lipo-proteins (also known as apo-proteins), phospho-lipids, and non-esterified cholesterol.

Lipo-proteins transport lipids in the blood.
Apoproteins, present in Lipo-proteins, are purified proteins that are designated as A-H.

Types of Lipo-proteins

There are five types of Lipo-proteins i.e. Chylomicrons, Very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Their detail is given as under

I. Chylomicrons

Due to the highest concentration of Tri-acyl-Glycerol and fewer proteins, Chylomicrons are the lowest in density and largest in size of all the blood lipo-proteins.

Chylomicrons are synthesized in our intestinal cells from dietary lipids.
Major lipoproteins are apoB-48, C-II, and E.
Chylomicrons (and VLDL) are the major carrier of Tri-acyl-Glycerols.
By the action of Lipoprotein lipase, Tri-acyl-Glycerols are hydrolyzed into Fatty acid and glycerol.
Fatty acids synthesis, by Chylomirons and VLDL, can either be oxidized to produce energy or converted back to Triacylglycerols to be stored for later use.
While Glycerol of Chylomicrons is used for the synthesis of Triacylglycerol or converted to Dihydroxy acetone Phosphate (DHAP).
DHAP is then oxidized to produce energy either through Glycolysis or after conversion to glucose in the liver.
Remnants of Chylomirons are taken up by the liver, degraded by liver Lysosomal enzymes, and finally, products of degradation (amino acids, fatty acids, glycerol, and cholesterol) are released in the cytosol to be reused.

II. Very-low Density Lipoproteins (VLDL)

VLDLs are denser, i.e. low proteins high lipids ratio, than Chylomicrons but still have a high content of Triacylglycerols.

VLDLs are synthesized by our liver by combining Triacylglycerol, cholesterol, apoproteins A-1, B-100, C-II, and E, and Phospholipids and then release into the blood.
VLDLs and Chylomirons are the major carriers of Triacylglycerol.
In the Peripheral tissues i.e. in adipose and muscles, Triacylglycerol of VLDL is hydrolyzed by lipoprotein lipase of Fatty acid, glycerol, and IDL (Intermediate-density lipoprotein).
Fatty acid and glycerol can either get oxidized to produce energy or can be converted back to Triacylglycerol or DHAP respectively.
In blood, IDL is further degraded to LDL.

III. Intermediate density Lipoprotein (IDL)

They are produced from VLDL in the peripheral tissues.

These are denser than chylomicrons but still have a high content of Triacylglycerol.

IV. Low-density Lipoprotein (VLDL)

These are produced in plasma by the degradation of IDL.

They are denser than IDL, as they have less Triacylglycerol and more protein.
They have the highest concentration of cholesterol and esters.
An increased level of LDL has a strong association with an increased risk of Cardiovascular diseases.
Obese and inactive individuals tend to have higher levels of LDL.
They contain an apoprotein, known as B-100.

V. High-Density Lipoprotein (HDL)

After their synthesis in the Liver, HDL is released into the blood.
The major proteins present in HDL are Apo-A, AI, CII, and E.
It is the densest lipoprotein. It has the lowest Triacylglycerol and highest protein content.
They carry cholesterol from peripheral tissues to the liver, where they are hydrolyzed to free cholesterol, which is then used for the synthesis of VLDL or converted to salt.
Increased levels of HDL cholesterol have been associated with a decreased risk of heart disease i.e. HDL is cardio-protective.
HDL is protective against hyperlipidemia, hypercholesterolemia, and Coronary heart diseases.

VI. Lipopolysaccharides

These conjugated molecules are formed by the combination of Lipids + Polysaccharides.
They occur in the cell wall of some bacteria.