What is the role of Lipids in organisms?


Lipids are large and diverse group of compounds that are soluble in non__ polar organic solvents like alcohol. These compounds includes fats, oils, waxes and sterols. They are extensively found in cell membranes are extensively found in cell membranes and they act as energy storage and signalling molecules.

Fats and their derivatives are together  known as lipids. They contain the elements  carbon,hydrogen and oxygen. Lipids  are insoluble in water but soluble in non__ polar solvents  like chloroform, benzene, acetone, alcohol etc. Lipids in general  include fats, oils and waxes.

Many substances of daily use such as cooking oil, rubber, cholesterol  , waxes are either lipids  or rich in lipids. Lipids are present in plasma membrane,  carotene and lycopene ( plant pigments), vitamin A, E, K, menthol and  eucalyptus  oil. Carotene is common in carrot and lycopene is found in tomato.

Common Lipids :

Lipids ( G.lipos= fat) include highly varied substances, namely, cooking oil, butter, ghee,  waxes , cholesterol, essential oils, natural rubber, some plants pigments ( carotene of carrots, lycopene of tomatoes), menthol,¹ vitamins A, E and K and eucalyptus oil. The lipids are not  polymers , but they are assembled from smaller molecules by dehydration. They are large molecules, but not large enough to be called  macromolecules. 

Common Features: The lipids compounds differ in chemical structure and function,but have a few common features. They are nonpolar and, therefore, insoluble in water. They dissolve in nonpolar organic solvents, such as ether, chloroform, acetone and benzene. They disperse in water uniformly as minute droplets forming an emulsion. Generally the lipids do not polymerise to form macromolecules. However, they may combine with carbohydrates and proteins.


The lipids from about 3.5 per cent of the cell contents. 


The lipids are classified into subgroup as follows__ 

A.Simple Lipids: These are esters2 of fatty acids with various alcohols. They are further of two types__ 

1. Neutral or True Fats: These are esters of  fatty acids with glycerol ( glycerine). They are also   called glycerides.

                                                              [  O]
fatty acids linked by ester linkage [||  ]                                                                          [C_O]
formed between ( COOH) group of fatty acid and _ OH group of alcohol ( glycerol) .The neutral  lipids are known as fats, if present  in solid state at room temperature and oils, if present in liquid state at room temperature. Oils are rich in unsaturated    fatty acids e.g., group oil, rape seed and mustard oil, sesame seed oil and safflower oil. 

Notes: 1: Crystalline substance obtained from mint oil.

2: Organic acids and alcohols react to form eaters just as inorganic acids and bases react to form salts.

2: Waxes : These are esters of fatty acids with alcohol other than glycerol.waxes are another  class of lipids. They are formed by  the combination of long chain fatty acids with a long chain alcohol. Waxes have an important role to play in protection .They form water__ insoluble coating  on hair and skin  in animals  and stems, leaves,  fruits and also  reduce rate of transpiration in plants.

1): plant waxes They form a coating  on plant parts, i.e., leaf etc. to reduce the rate of transpiration  and prevent wetting. 

2): Bee was: This type of wax is secreted by abdominal  glands of worker honey_ bees. Its is palmitic  acid ester of myricyl alcohol. 

3): Lanolin: It is secreted by cutaneous  glands and closely  resembles  sebum. It mainly  consists of palmitic, oleic or stearic acid and cholesterol. 

B: Compound or Conjugarted Lipids: 

These are esters of fatty acids with alcohol, but contain some other substances also. They are of 4 types __ 

1): Phospholipids, which have a nitrogen __ containing base and a phosphate group in addition to fatty acids and glycerol.

These are esters of glycerol and a mixture  of  fatty acids   and phosphoric acid. These take part in the formation  of structural components of cells. Phosphate group is joined to one of  glycerol's outer _ OH groups. A nitrogen containing base as choline may be in bound form to phosphate group.Lacithin is a constituent of all cell membranes  and egg yolk.

A phospholipid molecule is bipolar.Its two long fatty acids ( tails) represent  the hydrophobic acid end while phosphate containing  end is hydrophilic. Hydrophilic( water loving ) represents polar groups and hydrophobic ( water heating) represent  non_ polar groups.

They are of following types: 

(a) Phosphatidic Acids: These are lipids which on hydrolysis give rise to one molecule each of glycerol and phosphoric  acid and two molecules of fatty acids of which one is saturated and other one is unsaturated. Phosphatidyl chine, phosphatidyl ethanolamine ( caphalin), phosphatidyl serine and phosphatidyl inositol are derivatives of phosphatidic acids

(b) Lecithins: These lipids on hydrolysis give rise to fatty acids, phosphoric acids,  glycerol and nitrogenous base choline.

                        CH2__ O__CO__R
                         |        Saturated fatty acids 
R___CO___O__CH       O
                          |          ||
Unsaturated  CH2_O_P_ O_CH2_CH2_N=(                                                                       CH3)3

Fatty acids                 |            +or - 
                                     OH      OH
(c) Cephalins: On hydrolysis  give rise  to fatty acids, glycerol, phosphoric acid and either the base ethanolamine ( choline) or the amino acid serine.

(d) Plasmalogens: These on  hydrolysis give rise to one molecule each of aliphatic aldehyde,  fatty acids,  glycerol,  phosphoric acid and nitrogen containing  base ( choline or ethanolamine).

Phospholipids (Lecithins) are constituent  of   cytoplasm and cell membrane of cell. Cephalin is found in brain and acts as insulation material of nerves. Phospholipid molecules arranged in a double layered membrane in the form of lipid bilayer in aqueous media. They play an important  biological role in cell activity and cell permeability. 

2): Glycolipids, which have fatty acids, an amino alcohol and a carbohydrate. These are compound of lipids containing  fatty acids, glycerol plasma ester and a sugar. They are found in plastid membranes. 

3): Lipoproteins, which have lipids ( mainly phospholipids) and proteins, and 
 Lipids is bound to   protein and found  mainly in mammalian plasma. These help in maintaining the permeability of cell membranes. 
4): Cutin and Suberin,  both are compounds lipids.

C: Derived Lipids: Derived lipids are the substances formed during the hydrolysis of simple and conjugated  lipids. This group consists mainly  steroids, which consists of three cyclohexane rings ( in a nonlinear arrangement) and a terminal cyclopentane ring. Some steroids are hormones ( e.g estrogen,  progesterone and costicosterone)  some are vitamins( e.e.Vit.D), other steroids are constituent of   plasma membranes of many animals cells, probably the best is cholesterol. 

It is particularly abundant in nervous system. It has the molecular formula C27 H45 OH.Dry white matter of brain contain 14% of cholesterol . They are composed of 4 fused  hydrocarbon  rings ( 3_ hexagonal rings and one pentagonal ring) and in most of sterols a side chain is attached  to the main basic ring.

These are lipid__ like substances,  such as sterols, or derivatives of lipids, e.g., steroids, terpenes and prostaglandins. 

Why do the birds Preen themselves?


Neutral or True Fats( Glycerides or Triglycerides):

● Composition:The neutral or true fats are composed of carbon, hydrogen and oxygen like the carbohydrates, but have far fewer oxygen atoms than carbon atoms unlike the carbohydrates. A fat molecule consists of two compounds: one molecule of alcohol called glycerol and one to three molecules of the same or different long __ chain fatty acids.

      ( i) Glycerol: A glycerol molecule has 3 carbons, each bearing a hydroxyl ( __ OH) group .


   (ii) Fatty Acids : Fatty acids are so named because some of them occur in fats. A fatty acid molecule is an unbranched chain of carbon atoms with each carbon atom  (C) forming four bonds to other atoms. It has a carboxyl group ___ COOH at one end and hydrogen atom ( H) bounded to all or most carbon atoms, forming  a hydrocarbon chain. The carbon__ hydrogen bonds are nonpolar. Therefore, the hydrocarbon chain does not dissolve in water. The carboxyl group gives the molecule its acidic properties as it can dissociate  into hydrogen ion ( H+) and a negatively charged ____ COO__ group. Because the carboxyl group contains the polar C= O and OH groups, it tends to dissolve in water even though the rest of fatty acid molecule will not. Dissociation cannot occur when the fatty acid is a part of a fat molecule,  hence the name " neutral fat." Water insolubility of lipids is due to the repulsion present between the nonpolar lipid molecule and the polar water molecule.

    Most fatty acids have an even number of carbon atoms between 14 and 22, mostly 16 or 18.

    Fatty acids are of two types: saturated and Unsaturated 

(a) Saturated Fatty Acids:

In saturated fatty acids, all carbon atoms are interlinked by single bonds and each carbon atom carries a hydrogen atom. They cannot take up any more hydrogen, hence saturated. The saturated fatty acids are straight chains. They have higher melting points and are solid at normal temperature. They have a general formula of CnH2nO2. They include palmitic acid, C16H32O2, and stearic acid, C18H36O2.Their melting points are 63.1C° and 69.6C° respectively. 

(b) Unsaturated Fatty Acids:

In unsaturated fatty acids, two adjacent carbon atoms at one or more places lack hydrogen atoms and are interlinked by double bonds( C= C) . They can take up additional hydrogen, hence Unsaturated. These include 18 __ carbon oleic, linoleic and linolenic acids and 2p__ carbon arachidonic acid. They have 1,2, 3 and 4 double bonds respectively. Their general formula is CnH2n_2xO2. Formula of oleic acid is C18H34O2, of linoleic acid is C18H30O2, and of arachidonic acid is C20H32O2. The Unsaturated fatty acids have lower melting points ( 13.4C° for oleic acid), and are liquid at normal temperature. They are more abundant in the living organisms. Unsaturated fatty acids have a bend or kink at each double bond. The kinds make the fatty acid chains more disordered and hence more fluid at ordinary temperatures. 

      The fatty acids having more than one double  bonds are called polyunsaturated fatty acids. The oils having such fatty acids are termed Polyunsaturated. The latter are recommended by physicians for persons who have high blood cholesterol or cardiovascular diseases. Their use lowers the  blood cholesterol level. Hydrogenation changes unsaturated  fatty acids to saturated form and oil to a solid fat, e.g., vanaspati ghee, peanut butter and margarine¹.

●Formation Of Fats:

The three hydroxyl group of a glycerol molecule join with the carboxyle groups of three fatty acid molecules to form a fat or glyceride molecule.

The chemical linkage between the glycerol and fatty acids is called the ester bond. It results from the elimination of three molecules of water ( condensation or dehydration). The fatty acids present as components of most of the animal fats are 16__ carbon palmitic acid and 18__ carbon stearic acid. The neutral fats formed with these fatty acids are named tripalmitin and tristearin respectively. Both are pure fats. A fat having two molecules of palmitic acid and one molecule of stearic acid is termed dipalmito__ stearin, and a fat having a molecule each of palmitic, oleic and stearic acid is called palmito__ oleio__ stearin. These are mixed fats. Butter is also a mixed fat.

Types of Fats:

A true fat having three molecules of fatty acid or acids is called triglyceride. Triglycerides are the most common lipids in cells. A fat having only 1 or 2 fatty acid molecules is termed monoglyceride or diglyceride.Mono__  and diglycerides are not so common as triglycerides are. They occur as intermediates in certain biosynthetic reactions.

 ● Physical Nature:The physical nature of a neutral fat depends upon the kind of fatty acid present.Some have saturated fatty acids, and are solid at ordinary temperature ( 20°C), e.g., butter, lard¹, suet², coconut oil.They are often called hard fats. Others have Unsaturated fatty acids and are liquid at ordinary temperature. They are called oils, e.g., cod liver oil, shark liver oil, groundnut oil, grape rape seed and mustard oil, sesame or til oil, linseed oil, sunflower oil, safflower³ oil,corn oil, olive oil and cotton seed oil. Most plant and fish lipids have Unsaturated fatty acids and most animal fats have saturated fatty acids. Animals inhabiting the cold regions, such as arctic and antarctic fishes, are exception to this rule. They form abundant Unsaturated fatty acids that keep their bodies flexible in cold waters.They also keep the lipids functional, for example as a constituent of membranes. There is evidence that the saturated fats ( animal fats) cause increased production of cholesterol that leads to atherosclerosis ( narrowing of arteries due to deposits, called plaques ,on their lining) and arteriosclerosis ( hardening of arteries) in humans. Therefore, use of saturated fats should be minimised or avoided. 


Animals mainly get neutral fats from food.These may also be formed from excess carbohydrates ⁴and proteins. Many animals needs 3 fatty acids in their diet.These are called essential fatty acids because they cannot be synthesized in  the animal cells but are necessary for growth and good health. These are linolenic, linoleic and arachidonic acids. Bacteria and plants can synthesize these fatty acid. Animals depends on these sources for  them.

Most of the animals  usually  get  the suppley of neutral  fats from food.Three essential  fatty  acids required by most  of the animals are linolenic,  linoleic and arachidonic acid. They are essential  in the sense that they cannot be made by animal cells and are must for normal health. Animals  get the supply of these acids from plants and bacteria . Fats many also be formed from excess of carbohydrates and proteins. 

Vegetable oils_ " rich in polyunsaturate": It signifies fatty  acids in the said vegetables oil contain more than one double bond. These types of oils, are recommended to persons having blood cholesterol and other cardiovascular diseases instead of hydrogenated vanaspathi ghee and margarine. Because  vanaspathi ghee becomes  solid fat on hydrogenation of Unsaturated fatty acids. These unsaturated fatty acids turn into saturated  fatty acids on hydrogenation. By using vegetable oil,  polyunsaturated fatty acids diet reduces the high blood cholesterol level raising  the fat.

● Properties:

The true fats are insoluble in water, but dissolve in organic solvents. They disperse in water as minute droplets, forming an emulsion. In cells, they are dispersed throughout the cytoplasm. They have well defined melting points and solidifying points, the latter temperatures being only a few degrees lower. They have low specific gravity and float on water. Oils spread on water to form thin monomolecular layers. All fats are greasy.The neutral fats are hydrolyzed in the cells by the action of enzymes called lipases.
          A rather unusual property of fats is that their glycerol end is hydrophilic ( water loving) and the fatty acid end is lipophilic ( fat loving) .In other words, the glycerol end is polar, attracts water molecules, forms hydrogen bonds with them and thus dissolves in water; and the fatty acid end is nonpolar, insoluble in water, and dissolve in fats.

Properties of Cholesterol 

1):  It is white crystalline substance. 

2): Insoluble in water, acids and alkalies, so they are deposited in the arteries  and  veins,if blood cholesterol rises. This leads to high blood pressure  and heart  diseases. 

3): It acts as precursors for bile salts, steroid  hormones  and vitamin D2.

4): As the liver cells esterify  cholesterol,  infectious hepatitis, cirrhosis and most types of liver damages are accompanied by a decreased percentage  of cholesterol esters.

Phytosterols ( sterols of higher plants)

(a) Stogmasterol: It is found in  calabar and soyabean oils.It can be converted into hormones of corpus luteum and progesterone. 

(b) Sitosterols: It is present in wheat germ oil.

(c) Ergosterols: It is found and yeast. It is precursors for vitamin D2.It has chemical formula as C28H43OH.

Other _ sterol

(a) Sex hormones: The principal hormones of ovary is estradiol and of testis is testosterone. 

(b) Adrenal conticosteroid: Adrenal  cortex forms costicosterone steroid, which is of great metabolic  importance. 

(c) Diosgenin: A steroid prepared from yam plant Dioscorea, is used for preparing antifertility pills.

Functions the True Fats:

The true fats have a number of functions __

(i) Reserve Food Materials The neutral fats serve as food reserves in both plants and animals. They are stored in special regions__ connective tissue around the kidneys,under the dermis in mammals, in the fat bodies in frogs and lizards, and in the liver in fish. The adipose cells, adipocytes, which make up the fatty tissues of the body, specialize in storing fat droplets and contain little else. The fat deposits are not long__ term reserves of stored food used only in starvation, but are constantly being used up and reformed.

         In plants, oil is stored in the seeds to provide nourishment to the germinating embryo. The oil extracted from the seeds of groundnut, mustard, soyabean, sunflower, coconut and castor plants is used for cooking and other purposes. 

    The fats from a better storage product than the carbohydrates. Fats, being nonpolar,  can be stored in cells without the addition of water. Carbohydrates, being polar, must be stored surrounded by water.Plants, being immobile, can have bulky energy storage, i.e., starch. Animals have to carry the energy store with them, so it is beneficial to have a more compact fuel reservoir, namely, fat.

(ii) Fuel: The neutral fats form a concentrated fuel, producing more than twice as much energy per gram as do the carbohydrates. This is because the fats have a higher proportion of hydrogen and very little oxygen compared with carbohydrates. The fats, thus, represent an economical food reserve in the body.This is why the lipids have become increasingly important food reserves for animals and plants during the course of evolution. Many migratory birds increase their fat reserves before starting journey for warmer areas. Frogs do so before hibernation. 

(iii) Insulation  Coats:The neutral fats form an insulating layer under the skin. Women tend to have a thicker layer of adipose tissue than men have, and are more tolerant to cold. The polar bears and whales have very thick layer of adipose tissue under the skin for insulation. Subcutaneous adipose tissue, called blubber in whales and seals also contributes to buoyancy. 

(iv) Shock__ absorbing Cushions:The neutral fats serve as a shock__ absorbing cushion around the eyeballs, kidneys and gonads.

(v) Improvement of Physique:The subcutaneous fat in humans keeps the skin firm and helps in rounding off the contour of the body.

  (vi) Protection of Feathers and Hair: Birds spread oil from a gland on their tail into the feathers.  This makes features less likely to become brittle and get wet. It also increases buoyancy and insulation of body in aquatic birds. The only material,called sebum, secreted by the sebaceous glands of mammalian skin lubricates the hair to keep them soft and flexible and to prevent their felting together. It prevents wetting of hair and drying up of the skin also.

(vii) Source of Waterz: Fats yield water on oxidation. This metabolic water is very useful to some desert animals,  such as the kangaroo rat, which stores fat for this purpose. Camel gets metabolic water from the fat stored in the hump.

Important point : 

1): Fat from pig's abdomen clarified for use in cooking and pharmacy. 

2): Hard fat round kidneys and Lions od cattle and sheep used in cooking. 

3): Oil extracted from the seeds of Carthamus tinctorius.

4): This shows that starch and sugar are fattening. 

5): Edible fat formed from animal and vegetable oils. 

Differences Between saturated and Unsaturated Fatty Acids 

Saturated Fatty Acids 

1): All carbon atoms are linked up by single bonds and bear hydrogen atoms. 

2): Cannot take up more hydrogen. 

3): Are straight chains.

4): Have higher melting point than Unsaturated fatty acids.

5): Are solid at ordinary temperature.

6): Are less abundant in living organisms. 

7): Occur in most animals fats.

8): Increase blood cholesterol and cause arteriosclerosis. 

Unsaturated Fatty Acids 

1): At one or more places, carbon atoms are joined by double bonds and lack hydrogen atoms. 

2): Can take up more hydrogen. 

3): Have a bend at the double bond. 

4): Have lower melting point than saturated fatty acids.

5): Are liquid at ordinary temperature. 

6): Are more abundant in living organisms. 

7): Occur in most plant fats.

8): Lower blood  cholesterol and donot cause arteriosclerosis. 


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