ZO503 ANIMAL PHYSIOLOGY AND
PHYSIOLOGICAL CHEMISTRY

BLOCK – III: PHYSIOLOGICAL CHEMISTRY

Dr.Shyam S.Kunjwal

Department of Zoology
Uttarakhand Open University

Haldwani

INTRODUCTION TO LIPIDS

Lipids are a very important heterogonous group of organic substances which are widely distributed

throughout the plant and animal kingdom. The plant they are present in seeds, nuts and fruits, while in

animals they are stored in adipose tissues, bone marrow and nervous tissues. Chemically they are

various types of esters of fatty acids and alcohol. The addition to fatty acids and alcohols, some of the

lipids may contain phosphoric acid, nitrogenous group and carbohydrate. Lipids are relatively

insoluble in water and readily insoluble in water and readily soluble in organic solvents such either,

chloroform, carbon disulphide benzene, hot alcohol etc. Bloor [1947] defined lipids as “naturally

occurring compound which are insoluble in water, and soluble in one or more organic solvent such as

benzene, chloroform, either acetone the so called fat solvent on hydrolysis field fatty acids which are

utilized by the living organisms’’

10.10 CLASSIFICATION OF LIPIDS

Lipids are generally classified into three main classes

1. Simple Lipids

2. Compound Lipids

3. Derived Lipids

Fig.10.10. Classification of Lipids

10.10.1 SIMPLE LIPIDS

Simple lipids are the esters of fatty acids with various alcohols. These can be further categorized into

fats and waxes. Fats are triesters of glycerol and fatty acids. A fat in the liquid state is called oil.

Waxes are the esters of fatty acids with long chain (higher mol.wt.) monohydric alcohols.

Fats

Fats are solids at room temperature. Chemically, fats are triglycerides since, three molecules of fatty

acids condense with one molecule of glycerol, e.g. three molecules of stearic acid are linked to

glycerol to yield glyceryl tristearate, a fat.

If all the three molecules of fatty acids are similar, the product is a simple glyceride. If fatty acids

molecules are different, it is called a mixed glyceride. Natural fats are largely composed of mixed

glycerides. Since these glycerides have no acid or basic groups, they are often called natural fats. The

melting point of fats depends upon the chain length and degree of saturation of fatty acids. The

melting points of fats are always higher than the solidification point, e.g. tristearin melts at 720C but

solidifies on cooling at 520C.

Fats develop unpleasant odor on aging, this is due to auto oxidation of fat. This is called

rancidification.

The chemical changes that occur during rancidification are called rancidity. Fats which are liquid at

room temperature are called oils. Oils are also esters of fatty acids and glycerol, but the fatty acids

found in oils are unsaturated fatty acids. The unsaturated fatty acids have one or more double bonds.

They have low melting point and are insoluble in water. Hydrolysis of fats with alkali or enzymes

lipase yields fatty acids and glycerol. When the fats are hydrolyzed with alkali, the few fatty acids

react with alkali to form salts. These salts are soaps and this process is called saponification.

Waxes

Waxes are another class of simple lipids. These are the esters of fatty acids with high molecular weight

alcohol. Waxes contain one molecule of fatty acids and one molecule of alcohol. The bees wax, the

fatty acid constituent is a smaller chain acid, palmitic acid and [160C] and alcohol is myristic

palmitate. Ambretolide found in the seeds of abelmoschus esculentus is a hay hydro nil acid and is

responsible for the characteristic smell of the seed. Being highly insoluble in water and having no

double bonds in their hydrocarbon chains, waxes are chemically insert. And very resistant to

atmospheric condition also not digested by the fat splitting enzymes. They can be split slowly with hot

alcoholic KOH, however. They also have higher melting point. They serve as protective coating on

fruits and leaves. They play on important role in provide water barrier for insects, birds and animals.

They are used in furniture polishing.

Fatty Acids

Fatty acids are obtained from the hydrolysis of fats. Since all the fats contain glycerol, their properties

differ according to the nature of fatty acids present in them. A fatty acid can be defined as an organic

acid that occurs in a natural triglyceride and is a monocarboxylic acid ranging in chain length from C4

to about 24 carbon atoms. A few have branched chain or contain hydromel group or have a cyclic

chain at the end. Fatty acids that occur in natural fats usually contain an even number of carbon atoms,

one carboxylic group and are straight chain derivatives. On the basis of presence or absence of double

bonds, fatty acids may be classified into two main classes-

i. Saturated fatty acids (saturated with hydrogen)

ii. Unsaturated fatty acids

1. Saturated fatty acids

The fatty acids which do not contain any double bond are called saturated fatty acids. The general

formula is Cn H2n+1 COOH e.g. Butyric acid C3 (CH2)2 COOH. The most abundant saturated fatty

acids in nature are palmitic acid (C18) and stearic acid (C16). The saturated fatty acids are straight chain

acids. In addition to these straight chain acids, there are some branched chain acids, with odd or even

number of carbon atoms.eg. Isopalmitic acid, anti-isopalmitic acid and tuberculostearic acid.

2. Unsaturated fatty acids

The fatty acids which contain one or more double bonds are called unsaturated fatty acids. On the

basis of number of double bonds, the unsaturated fatty acids may be divided into two groups-

Monosaturated fatty acids: – having one double bond e.g. crotonic acid, oleic acid, palmitoleic acid,

nervonic acid etc.

Polyunsaturated fatty acids:-having more than one double bonds e.g. linoliec acid, eleostearic acid etc.

In most of the monosaturated fatty acids there is a single bond lying between carbon atoms 9 and 10.

This is designated as ∆9. The symbol ∆ with the superscript number nine (9) indicates the positions of

the double bond. When there are more than one double bond (polyunsaturated fatty acids), the

additional bonds occur between the ∆9 double bond and the methyl terminal end of the chain. The

symbol 18:3 signifies that there are three double bonds and symbol ∆9,12,15 signifies that the position of

double bonds are between carbon atom 9 and 10, 12 and 13 and 15 and 16. Presences of double bonds

in the fatty acids lower their melting point considerably. Most plant fats contain unsaturated fatty acids

as oleic acid and linoleic acid and hence they are liquid at room temperature. Contrary to this animal

fats have more of saturated fatty acids and hence solid at room temperature.

Essential and Non Essential Fatty Acids

Essential fatty acids: The fatty acids which cannot be synthesized by human body but are essential for

the normal maintenance of the body are called essential fatty acids .These fatty acids must be included

in our diet. Three polyunsaturated fatty acids, linoleic acid, linolenic acid and arachidonic acid are the

essential fatty acids.

Non essential fatty acid: These are the fatty acids which can be synthesized by our body. Thus they

need not be included in our diet. They are unsaturated fatty acids and are synthesized from their

corresponding saturated fatty acids by introducing a single bond .e.g. palmitoleic acid and oleic acid.

10.10.2 COMPOUND LIPIDS

Compound lipids contain some additional groups or elements besides fatty acids are alcohol. The

addition group may contain phosphorus, nitrogen, sulpher or it may be a protein. Compound lipids can

be categorized into the following: Phospholipids, Glycolipids, Other compound lipids.

Phospholipids

Phospholipids are those compound lipids which contain a phosphorus atom. Phospholipids are wide

spread in bacteria, animal and plant tissues and their general structures are quiet similar. These have

been termed as amphipathic and compound since they process both polar and non polar function.

In addition to phosphorus, phospholipids may also contain nitrogen as a key component. There are

various types of phospholipids including- lecithin, cephalins, plasmalogens, phosphoinositides and

phosphosphingosides.

Lecithin

Lecithin is widely distributed in nature. In animals it is found in liver, brain, nerve tissues, sperm and

egg yolk. In plant it is abundant in seeds and sprouts. On hydrolysis, lecithin yields glycerol, fatty

acids, phosphoric acid and nitrogenous base – choline. It is also called phosphatidyl choline. The fatty

acids commonly found in lecithin are palmitic, stearic, oleic, linolenic and arachidonic acids. Lecithin

is yellowish grey solid which is soluble in with soluble in ether and alcohol but insoluble in acetone.

On exposure to air they rapidly darken colour and absorb water, forming dark grassy mass. Lecithins

are broken down by the enzyme lecithinase to lysolesithin which is present in venoms of bee and

cobra. When injected into the blood, lysolecithins cause rapid haemolysis of the red blood cells.

Cephalins

Cephalins are found in animal tissues in close association with lecithin. They are also found in soya

bean oil. The basic difference between cephalins and lecithin is the nature of nitrogenous base.

Cephalins contain ethanolamine in place of choline. The fatty acid components of cephalins are

stearic, oleic, linoleic and arachidonic acid. They are less soluble in alcohol then lecithin.

PLASMALOGENS

Plasmalogens are abundant in brain and muscles they are also found in the seeds of higher plants.

Structurally, they resemble lecithin and cephalins except in having an aldehyde group attached to ex –

carbon atoms of glycerol. They are soluble in all lipid solvents.

Plasmalogen

Phosphoinositides

They are present in brain tissues and nervous tissues. They can be either mono or diphosphoinositides.

Monophosphoinositides contain hexahydric alcohol inositol. The name lipoinosital was also proposed

for them.

PHOSPHOSPHINGOSIDES

These lipids are abundant in lacking in plant and microorganisms. In these lipids glycerol is replaced

by an 18 carbon unsaturated amino alcohol called ‘sphingosine’. On hydrolysis they yield fatty acids,

phosphoric acid, choline and sphingosine.

Glycolipids

Glycolipids are compounds containing carbohydrates and high molecular weight fatty acids like

sphingosine but no phosphoric acid. These are of two types: Cerebrosides and Gangliosides.

Cerebrosides: They occur in large amount in brain and myelin sheath of nerves. The structure of

cerebrosides is somewhat similar to sphingomyelin. Here the fatty acid ceramide is linked either to

galactose or glucose.

Gangliosides: These are found in ganglion cells of nervous tissues and also in parenchymatous tissues

like spleen and RBCS, They are the most complex glycosphingolipids. They are ceramides with

attached oligosaccharides that include at least one sialic acid residue.

Other compound lipids

Sulfolipids: Lipids containing sulfer, widely distributed in plant (localized in chloroplast) and bacteria.

Lipoprotein: They are the component of membranes found in the membranes of mitochondria, ERL

The lipid component consists of triacylglycerol, phospholipids and cholesterol. The protein

components of lipoprotein have a relatively high portion of non polar acid residues.

10.10.3 DERIVED LIPIDS

Derived lipids are the product of hydrolysis of simple lipids and compound lipids and in addition other

lipid like compounds such as steroids, terpenes, fatty acids, alcohols, carotenoids, essential oils etc.

Steroids

Steroids are the derivatives of cyclopentanoperhydrophenanthrene, a compound consisting of four

fused non planer rings. They are named as A, B, C and D. The rings A, B and C are hexagons and are

called cyclohexane rings while D is a pentagon and is called cyclopentane.

Sterols

The steroids may have one or more alcoholic groups. The steroids having alcoholic groups are called

sterols. They are crystalline compounds and differ from alcohols in being solids that is the reason they

are called so. Steroids are widely distributed in plant, animals and microorganism. They are found in

cell membranes and other cellular component containing lipids. Unlike other lipids, sterols cannot be

saponified and by this process they can be separated from other lipids. Ergosterol is present in food in

small amount. It has been isolated from parasitic fungus Claviceps pupurea (Ergon) growing on rye

plants. Other plant sterols are spinasterol obtained from spinach and cabbage, stigmasterol from

coconut and soyabeans and sitosterol from cereal seeds.

Cholesterol

The best known animal sterol is cholesterol, which is a major component of animal plasma membrane.

It is classified as sterol because of its C30H group It is present in relatively high concentration in

nervous tissues and in bile. Cholesterol is a crystalline solid with rhombic crystals and its solution is

levorotatory. It has a high melting point.

In mammals, cholesterol is the metabolic precursor of steroid hormones – adrenocorticoids and sex-

steroids.

Adrenocorticoids

They are secreted by the adrenal cortex hormone contains – glucocorticoids, aldosterone, cortisterone,

desoxycortisterone adrenosterone and other mineral corticoids influence a wide variety of vital

functions.

Sex steroids

The testes secrete testosterone, andosterone and adrenosterone, while estrogens and progesterone are

secreted by the ovary. They affect sexual development and functions.

10.11 SOURCES OF LIPIDS

Lipids are widely distributed in plants and animals. Common sources of fatty acids are butter, coconut

oil, animal fats and some bacteria. Lecithin, a type of phospholipids is found in liver, brain, nerve

tissue, sperm and yolk sac in animals while in plants it is abundant in seeds and sprouts. Similarly

other phospholipids and glycolipids are also found in animal tissues like brain, muscles and nervous

tissues. Terpenes and carotinoids are types of derived lipids which are exclusively of plant origin.

10.12 BIOLOGICAL SIGNIFICANCE OF LIPIDS

1. Rich source of energy: fats provide food of high calorific value (1g fat produces about 9.3 kilo

calories of heat).

2. As food reserve: Fats are stored in body as reserve food material, because these could be

readily stored in the body on account of insoluble character sticks. Triglycerides stored in

adipocytes (fat cells) of adipose tissue are the principal fat reserve.

3. As heat insulators: Fats deposited in the subcutaneous tissues act as insulators conserving body

heat.

4. Solvent: Lipids act as a solvent for fat soluble vitamins like vitamin A, D and E.

5. Structural constituents: Phospholipids, glycolipids and sterols are structural components of all

the membrane system of cell (i.e. cell membrane, nuclear membrane, membranes of the

endoplasmic reticulum etc.)

6. Fat transport: Phospholipids play an important role in the absorption and transportation of fatty

acids.

7. Hormone synthesis: Adrenocorticoids, sex hormones, vitamin D and cholic acids are

synthesized from cholesterol.

8. As shock absorber: The fat deposited around the visceral organs and underneath the skin acts

as cushion and absorbs mechanical shocks.

9. As electric insulators: Myelin sheath around medullated nerve fibres forms an electric

insulation.

10. Prostaglandins: They control local activities in the body.

11. Protective layers: Lipids form a protective waxy covering on the aerial parts of plants to check

loss of water by evaporation.

12. Thrombokinase: helps blood clotting.

13. Leukotrienes: a group of eicosanoid helps in respiration.

14. Some isoprenoids form insect hormones.

15. Some insoprenoids form volatile oil and pigments. Natural rubber is also an isoprenoid.

16. Glycolipids help in cell recognition.

17. Complex lipids form phospholipid bilayer of plasma membrane.

18. Steroid act as hormones and neurotransmitters in mammals.

10.13 DEFICIENCY DISEASES OF LIPIDS

The deficiency of lipids in human body causes dry, scaly skin, hair loss, loss of mensuration, cold

intolerance, power resistance to infection and bruising, poor growth, poor wound healing and low

body weight.

10.14 SUMMARY

 Carbohydrates are the compounds of carbon, hydrogen and oxygen are the major source of

energy in body.

 Carbohydrates are classified into three major groups- monosaccharides, oligosaccharides

and polysaccharides.

 Monosaccharides are the simplest carbohydrates and are either aldoses or ketoses.

 Oligosaccharides consist of few molecules of monosaccharide, most common are

disaccharides consist of two molecules of monosaccharides.

 In polysaccharides a number of monosaccharides are linked by glycosidic bonds.

 Polysaccharides maybe either homopolysaccharide or heteropolysaccharide.

 Monosaccharides are colourless and crystalline compounds which are optically active.

 Lipids are a heterogeneous group of molecules with are soluble in organic solvents.

 Chemically lipids are the esters of fatty acids with alcohols.

 Lipids are good source of energy and important constituent of biological membranes.

 Lipids are generally classified into simple, compound and derived lipids. Fats and wax are

simple lipids.

 Fatty acids are obtained from the hydrolysis of fats and can be saturated or unsaturated.

 Phospholipids are the compounds lipids which contains a phosphorus group.

 Glycolipids contain carbohydrate and high molecular weight fatty acids.

 Derived lipids are the products of hydrolysis of simple and compound lipids and include

steroids, terpenes, carotenoids etc.