Lipid & it's metabolism [Part 1] (Viva)

Q.1 What are lipids?

Lipids are ester-like compounds of fatty acids that are insoluble in water but are soluble in fat solvents.

Q.2 Give the classification of lipids.

Lipids are classified into:

  1. Simple lipids.
  2. Compound lipids.
  3. Derived lipids.

Q.3 What is the biological importance of lipids in the body?

  1. Lipids are the most concentrated source of energy. Their caloric value is 9 KCal/gm.
  2. Lipids provide essential fatty acids.
  3. Supplies fat-soluble vitamins.
  4. As components of the cell wall.
  5. Lipids act as insulating material.

Q.4 What are fats?

Fats are the esters of fatty acids with glycerol.

Q.5 What is the difference between fat and oil?

Fats are solid at ordinary temperature whereas oil is liquid, i.e. (liquid) fat at ordinary temperature.

Q.6 What are saturated fatty acids? Give examples.

Fatty acids which do not have any double bond in their structure are called saturated fatty acids.
Examples: Acetic acid (CH3.COOH), propionic acid (C2H5COOH), butyric acid (C3H7COOH), etc.

Higher homologues like palmitic acid (C15H31COOH), stearic acid (C17H35COOH), etc.

Q.7 What are unsaturated fatty acids? What are the types?

Fatty acids which contain double bonds in their structure are called unsaturated fatty acids (UFA).

  • Types: Depending on the degree of unsaturation they are divided into two groups:

– Monounsaturated (monoethenoid): containing only one double bond.

– Polyunsaturated (polyethenoids): containing more than one double bond in their structure.

Q.8 Give an example of monosaturated FA (monoethenoid) found in our body fat.

Oleic acid C17H33COOH (Formula 18: 1; 9) is found in abundance in our body fat.

Q.9 Name the three polyunsaturated fatty acids (polyethenoids).

Three polyunsaturated fatty acids of biological importance are:

Linoleic acid:
Two double bonds between C9 and C10 and another between C12 and C13 (formula: 18: 2; 9, 12).
Linolenic acid:
contains three double bonds between carbons 9 and 10, 12 and 13, and 15 and 16. (Formula: 18: 3; 9, 12, 15).
Arachidonic acid:
It is a 20 C Fatty acid and contains four double bonds between 5 and 6, 8 and 9, 11 and 12, and 14 and 15. (Formula: 20: 4; 5, 8, 11, 14).


Q.10 What are the saturated fatty acids most abundant in fats?

  1. Palmitic acid.
  2. Stearic acid.

Q.11 What are essential fatty acids?

Essential fatty acids are those which cannot be synthesized by the body and hence are supplied in the diet. They are also called ‘polyunsaturated acids’.

They are:  No.of double bonds No.of carbon atoms
Linoleic acid   2 18
Linolenic acid   3 18
Arachidonic acid  4 20


Q.12 What are the functions of essential fatty acids?

  1. Essential fatty acids prevent the deposition of cholesterol in atherosclerosis.
  2. In the synthesis of prostaglandins

Q.13 Which EFA is important?

  • Linoleic acid is most important as arachidonic acid can be formed in the body from linoleic acid.
  • Biologically arachidonic acid is very important as prostaglandins (PGs) and leukotrienes (LTs) are formed in the body from it.

Q.14 What is saponification?

Hydrolysis of fat by alkali is called saponifications.

Q.15 What is the saponification number?

Saponification number is defined as the mg of KOH required to saponify 1 gm of fat.

Q.16 What does the saponification number of fat indicates?

  • Saponification number of fat gives an idea about the average chain length of the fatty acids present in the fat.
  • Higher the saponification number, the shorter (or smaller) will be the chain length of the fatty acid and vice versa.

Q.17 What is iodine number?
Iodine number is defined as the gms of iodine absorbed by 100 gm of the fat.

Q.18 What does the iodine number indicate?

 Iodine number indicates the degree of unsaturation of the fat.
Higher the iodine number, the more is the degree of unsaturation of the fat.

Q.19 What is an acid number?

 The amount of KOH in mgs required to neutralize free fatty acids presents in 1 gm of fat.

Q.20 Which will have a higher acid number, a pure fat or rancid fat?

A rancid fat will have a higher acid number.

Q.21 What is glycerol?

Glycerol is commonly called as glycerine. Chemically it is trihydric alcohol containing three -OH groups in the molecule.

Q.22 What are the sources of glycerol in the body?

Sources of glycerol in the body:

  • Exogenous: From dietary fats, approximately 22% of glycerol formed in the gut by lipolysis of dietary TG is absorbed directly to portal blood.
  • Endogenous: From lipolysis of fats (TG) in adipose tissue.

Q.23 What is acrolein test?

The presence of glycerol in a compound is detected by the acrolein test. Glycerol, when heated with KHSO4, two molecules of water are removed and it produces acryl aldehyde which has a characteristic pungent or acrid odor.

Q.24 Is glycerol produced in the body by lipolysis a waste product or useful?

Glycerol produced in the body is not a waste product. It is useful in that:

  • Re-esterified to form TG again
  • It has nutritive value. It can be converted to glucose/and glycogen by the process called gluconeogenesis.


Q.25 Can glycerol be used clinically in medicine?

Glycerol has been used orally or IV in cases of cerebrovascular diseases. It is nontoxic and it reduces cerebral edema with improvement in CS fluid. There is no rebound increase in intracranial pressure on discontinuation of therapy.


Q.26 What is rancidity?

Rancidity is a chemical resulting in the unpleasant odor and taste in the fat on storage when they are exposed to light, heat, and moisture.


Q.27 What are antioxidants?

Antioxidants are substances that prevent rancidity.


Q.28 Give the classification of antioxidants?

Primary antioxidants:
Prevent the formation of new free radicals,
e.g. Superoxide dismutase, glutathione peroxidase, ceruloplasmin.
Secondary antioxidants:
Remove preformed free radicals before they can initiate a chain reaction,
e.g. vitamin E and C, β carotene, uric acid, bilirubin.
Tertiary antioxidants:
Repair cell structures damaged by the free radical attack,
e.g. DNA repair enzymes.


Q.29 Which vitamin prevents rancidity?

Vitamin E.


Q.30 What are phospholipids?

Phospholipids are compound lipids that contain in addition to fatty acids, and glycerol/or other alcohol, a phosphoric acid residue, a nitrogen-containing base, and other constituents.


Q.31 State how phospholipids are classified?

As per Celmer and Cartar’s classification phospholipids are classified into 3 groups:

Containing glycerol as an alcohol group,
e.g. phosphatidyl choline (lecithin), phosphatidyl ethanolamine (cephalin), phosphatidyl serine, plasmalogens, etc.
Containing inositols as alcohol,
e.g. phosphatidyl inositol (lipositol).
Phospho sphingosides:
Containing an unsaturated 18 carbon amino alcohol called sphingosine (sphingol),
e.g. sphingomyelin.


 Q.32 What are phospholipases? What are the types? Mention their site of action.

Phospholipases are enzymes which hydrolyze phospholipid in a characteristic way. Five types described. Their specific site of action on lecithin are as follows:

Phospholipase A2:
Attacks β position and forms lysolecithin + one mole of FA.
Phospholipase A1:
Attacks ester bond in position 1 of lecithin.
Phospholipase B:
Substrate is lysolecithin. (lysophospholipase). It hydrolyzes ester bond in a position and forms glyceryl phosphoryl choline + one mole of FA.
Phospholipase C:
Hydrolyzes phosphate ester bond and forms α, β-diacyl glycerol + phosphoryl choline.
Phospholipase D:
Splits off choline and forms phosphatidic acid.


Q.33 What is sphingomyelin?

It is a phospholipid. It does not contain glycerol, instead contains an 18 carbon unsaturated amino alcohol called as sphingosine (sphingol).

On hydrolysis, sphingomyelin yields one molecule FA + phosphoric acid + nitrogenous base choline + sphingosine (alcohol).


Q.34 What are plasmalogens? How does it differ from lecithin/cephalin?

Plasmalogens are phospholipids. They are predominantly found in the brain and nervous tissue

On hydrolysis, they yield one molecule of FA + glycerol + phosphoric acid + a nitrogenous base usually ethanolamine (sometimes choline) + one molecule of long-chain aliphatic aldehyde. Thus it differs from lecithin cephalin in having a long-chain aliphatic aldehyde.


Q.35 What is cardiolipin?

 It is a phospholipid found in the mitochondrial inner membrane. Chemically, it is diphosphatidyl glycerol and is formed from phosphatidyl choline.


 Q.36 What is dipalmitoyl lecithin (DPL)? What is its clinical importance?

  • Dipalmityl lecithin (DPL), is secreted in lung alveoli by type II granular pneumocytes lining the alveolar wall. It acts as a surfactant and lowers the surface tension in lung alveoli and prevents the collapse of lung alveoli.
  • Absence of DPL, in premature infants, may produce the collapse of lung alveoli producing difficulties in respiration and death. It is called as respiratory distress syndrome (RDS) or hyaline membrane disease.


Q.37 What are glycolipids? Give examples.

Glycolipids are lipids that contain carbohydrate moiety in their molecule.

They are mainly of 2 types:
– Cerebrosides, and
– Gangliosides.


Q.38 What are cerebrosides?

Cerebrosides are glycolipids. A cerebroside is built as follows:

Sphingosine (alcohol)
  - FA of high molecular weight
  - Usually galactose (may contain glucose sometimes)


Q.39 What are the different types of cerebrosides? How do they differ from each other?

Cerebrosides are mainly 4 types. They differ from each other by the fatty acid content.

They are:

  • Kerasin: Contains FA lignoceric acid
  • Cerebron: Contains hydroxy lignoceric acid called “cerebronic (phrenosin) acid”.
  • Nervon: Contains an unsaturated homologue of lignoceric acid called “nervonic acid”.
  • Oxynervon: Contains a hydroxy derivative of nervonic acid.


Q.40 What is ceramide?

Ceramide is formed by esterification of sphingosine with FA of high molecular weight.

Principally found in the white matter of the brain, in myelin sheaths, and medullated nerves.


Q.41 What is the composition of lecithins?

Lecithins contain glycerol, fatty acids, Phosphoric acid, and choline.


Q.42 What is the composition of cephalins?

Cephalins contain glycerol, fatty acids, Phosphoric acid, and ethanolamine.


Q.43 What is the sugar component present in cerebrosides?



Q.44 What are sphingolipidoses? Describe some important sphingolipidoses.

Sphingolipidoses are a class of heterogeneous group of inherited disorders relating to sphingolipids and they primarily affect the central nervous system 


Q.45 What are steroids?

Substances possessing cyloperhydrophenanthrene nucleus are called steroids.


Q.46 Give the structure of cholesterol. 


Q.47 State the characteristic features of cholesterol structure.

Characteristic features of cholesterol structure are:

  • Possesses “cyclopentanoperhydrophenanthrene” nucleus.
  • α -OH group at C3
  • an unsaturated double bond between C5 and C6.
  • has two-CH3 groups at C10 and C13
  • and has an eight carbon side chain attached to C17.


Q.48 What are the sources of cholesterol in the body?

Two sources:

  • Exogenous: Dietary cholesterol approximately 0.3 gm/day. Diet rich in cholesterol are butter, egg yolk, milk, cream, meat, etc.
  • Endogenous: Synthesized in the body from two carbon units, acetyl CoA

(CH3.C~S – CoA). Approximately 1.0 gm/day.


Q.49 What are the forms in which cholesterol exists in blood and tissues?

Cholesterol occurs both in free form in which -OH group on C3 is free and in ester form in which -OH group is esterified with fatty acids.

Q.50 How does cholesterol esterified?

 Two ways:

Some cholesterol esters are formed in tissues by the transfer of “acyl” groups from acyl-CoA to cholesterol by the enzyme acyl transferase.

By interaction of lecithin and cholesterol:
Plasma cholesterol esters are produced in plasma by transfer of an acyl group, usually unsaturated, from β-position of lecithin to cholesterol by the enzyme lecithin-cholesterol acyl transferase (LCAT).

Lecithin + Cholesterol ?LCAT?→ Cholesterol ester + Lysolecithin


Q.51 What are the bad effects of cholesterol?

Excessive cholesterol is harmful to the body in that it gets deposited in arterial walls producing atherosclerosis. This can narrow the lumen of the blood vessel impeding blood flow which can cause thrombosis.


Q.52 Is cholesterol good for the body? What are the good effects?

In normal quantities, cholesterol has a number of good effects.

They are:

  • Bile acids are produced from cholesterol by its oxidation in the liver.
  • Cholesterol is converted to steroid hormones in the adrenal cortex and to sex hormones in the gonads.
  • Cholesterol forms 7-dehydrocholesterol and in the skin, it is converted to Vit. D3 by UV rays.
  • Cholesterol is a poor conductor of heat and hence acts as an insulator.
  • It is also a poor conductor of electricity and has a high dielectric constant.
  • Cholesterol is in abundance in the brain and nervous tissues where it functions as an insulating covering of structures that generate and transmit electrical impulses.


Q.53 What are the tests by which cholesterol is detected?

  1. Libermann-Burchard reaction.
  2. Salkowaki test.

Q.54 What is Liebermann-Burchard reaction?

A chloroform solution of cholesterol when treated with acetic anhydride and conc. H2SO4 gives a grass green color (cholestapolyene sulphonic acid is formed). This reaction is utilized in the colorimetric estimation of cholesterol in the blood by Sackett’s method.


Q.55 What is Zak’s reaction?

When the solution of cholesterol is treated with glacial acetic acid, ferric chloride, and conc H2SO4, it gives purple red color (cholestapolyene carbonium ion). This reaction forms the basis for the colorimetric estimation of cholesterol by Zak’s method.


Q.56 What is 7-dehydrocholesterol? Why it is called pro-vitamin D3?

7-dehydrocholesterol is produced in the body from cholesterol and it is present in the skin epidermis. Ultraviolet ray of sunlight changes 7-dehydrocholesterol to vitamin D3 (cholecalciferol). Hence it is called as provitamin D3.


Q.57 What is pro-vitamin D2?

Ergosterol is a plant sterol.

When it is irradiated with UV rays [long wave 265 μm (millimicron)] it changes to vitamin D2. Over irradiation may produce toxic substances viz. Toxisterols and suprasterols.


Q.58 What are chylomicrons?

Chylomicrons are the central core of triglycerides, phospholipids, and cholesterol combined with a small amount of proteins.


Q.59 What are lipoproteins?

The combination of lipid with proteins are called lipoproteins.

Q.60 What are the various functions of plasma lipoproteins?

  1. High-density lipoproteins, i.e. α-lipoproteins.
  2. Low-density lipoproteins, i.e. β-lipoproteins.
  3. Very low-density lipoproteins, i.e. pre-αlipoproteins.
  4. Chylomicrons.


Q.61 Which fraction of the lipoprotein contains maximum cholesterol content?

Low-density lipoprotein fraction (LDL).

Q.62 LCAT is present on which lipoprotein?

LCAT is present on the HDL molecule. It converts cholesterol and lecithin to cholesteryl ester and lysolecithin respectively.


Q.63 What is the other name for VLDL remnant?

Intermediate density lipoprotein (IDL).


Q.64 LDL receptor can be expressed by another which method?

APO B-100, E receptor.


Q.65 What are apolipoproteins?

The protein component of plasma lipoproteins are called apolipoproteins.


Q.66 What are bile salts?

They are sodium and potassium salts of glycocholates and taurocholates.


Q.67 What is the role of bile salts?

  1. As powerful emulsification agents.
  2. Bile salt lowers the surface tension of the media and thus aid in the absorption of fats.

Q.68 At which step bile acid synthesis is regulated?

7-α hydroxylase step.


Q.69 What do you understand by respiratory distress syndrome?

Deficiency of lung surfactant cause this disease.