lipids - sacbiotech.files.wordpress.com · lipids are a large and diverse group of oils, fats, and...

Dr. Ruchika Yadu Department of Biotechnology SAC, Jabalpur

LIPIDS

(Notes)

Q1. What are lipids? How will you classify them?

Ans. The term lipid comes from the greek word “lipos” for fat. Lipids are a large and

diverse group of oils, fats, and fat like substances that occur in living organisms and that

characteristically are soluble in organic solvents but only sparingly soluble in aqueous

solvents. They are complex organic biomolecules which can be defined as “lipids are

esters of glycerol and fatty acids”, however this definition does not satisfy all lipids and its

derivatives.

• Lipids are not polymers, but mostly small molecules.

• They are Chief cellular storage form of energy

• They play a major role in cellular structure and biochemical functions

Classification of lipids:

Lipids are broadly classified into:

A. Simple lipids: (a) Fats & Oils

(b)Waxes

B. Complex Lipids: (a) Phospholipids

(b) Glycolipids

(c)Sphingophospholipids

C. Derived lipids: (a) Prostaglandins

(b) Steroids

Q2. Write down important properties/General characters of lipids

Ans. General Characters of lipids are:

Lipids are relatively insoluble in water.


They are soluble in non-polar solvents, like ether, chloroform, and methanol.

Lipids have high energy content and are metabolized to release calories.

Lipids also act as electrical insulators, they insulate nerve axons.

Fats contain saturated fatty acids, they are solid at room temperatures. Example,

animal fats.

Plant fats are unsaturated and are liquid at room temperatures.

Pure fats are colorless, they have extremely bland taste.

The fats are sparingly soluble in water and hence are described are hydrophobic

substances.

They are freely soluble in organic solvents like ether, acetone and benzene.

The melting point of fats depends on the length of the chain of the constituent fatty acid

and the degree of unsaturation.

Geometric isomerism, the presence of double bond in the unsaturated fatty acid of the

lipid molecule produces geometric or cis-trans isomerism.

Fats have insulating capacity, they are bad conductors of heat.

Emulsification is the process by which a lipid mass is converted to a number of small

lipid droplets. The process of emulsification happens before the fats can be absorbed

by the intestinal walls.

The fats are hydrolyzed by the enzyme lipases to yield fatty acids and glycerol.

The hydrolysis of fats by alkali is called saponification. This reaction results in the

formation of glycerol and salts of fatty acids called soaps.

Hydrolytic rancidity is caused by the growth of microorganisms which secrete enzymes

like lipases. These split fats into glycerol and free fatty acids.

Q3. Write a note on biological importance of lipids

Ans. Lipids perform several biological functions:

Lipids are storage compounds, triglycerides serve as reserve energy of the body.

Lipids are important component of cell membranes structure in eukaryotic cells.

Lipids regulate membrane permeability.

They serve as source for fat soluble vitamins like A, D, E, K.


They act electrical insulators to the nerve fibres, where the myelin sheath contains

lipids.

Lipids are components of some enzyme systems.

Some lipids like prostaglandins and steroid hormones act as cellular metabolic

regulators.

Cholesterol is found in cell membranes, blood, and bile of many organisms.

As lipids are small molecules and are insoluble in water, they act as signaling

molecules.

Layers of fat in the subcutaneous layer, provides insulation and protection from cold.

Body temperature maintenance is done by brown fat.

Polyunsaturated phospholipids are important constituents of phospholipids, they

provide fluidity and flexibility to the cell membranes.

Lipoproteins that are complexes of lipids and proteins, occur in blood as plasma

lipoprotein, they enable transport of lipids in aqueous environment, and their transport

throughout the body.

Cholesterol maintains fluidity of membranes by interacting with lipid complexes.

Cholesterol is the precursor of bile acids, Vitamin D and steroids.

Essential fatty acids like linoleic and linolenic acids are precursors of many different

types of ecosanoids including prostaglandins, thromboxanes. These play a important

role in pain, fever, inflammation and blood clotting.

Q4. What are fatty acids? Describe their structure and properties

Ans. They are the simplest form of lipids.

Fatty acids are carboxylic acids with hydrocarbon side chain ranging from 4 to 36

carbons long (C4 to C 36) having the general formula R-(CH2)n-COOH.

They occur mainly as esters in natural fats and oils but they also occur in the

unesterified form as free fatty acids, a transport form found in the plasma.

Usually contains an even number of carbon atoms (14C- 20C). The Chain may be

saturated (containing no double bonds) or unsaturated (containing one or more double

bonds).

A few contain three carbon rings, hydroxyl groups or methyl group branches.


The double bonds are almost never conjugated. They are always separated by methyl

groups.

Triple bonds are rare.

Fatty acid molecules also have two chemically distinct regions: 1) a long hydrophobic

hydrocarbon chain, which is not highly reactive; and 2) a carboxyl (-COOH) group, which

is hydrophilic and highly reactive.

The presence of double bonds results in the formation of bends or kinks in the

molecules, and impacts the capacity of the fatty acid chains to stack together.

Other differences between fatty acids include the length of the hydrocarbon chains, as

well as the number and position of the double bonds.

The presence of the double bond will also influence the melting point, as unsaturated

fatty acids have a lower melting point than saturated fatty acids.

The melting point is also influenced by whether there is an even or odd number of

carbon atoms; an odd number of carbons is associated with a higher melting point.

Furthermore, saturated fatty acids are highly stable, while unsaturated fatty acids

are more susceptible to oxidation.

Unsaturated fatty acids are present as Polyunsaturated fatty acids (PUFA) or

Monounsaturated fatty acids (MUFA). Monounsaturated fatty acids contain one

carbon-carbon double bond, which can be found at different positions throughout the

fatty acid chain. Polyunsaturated fatty acids are produced only by plants and

phytoplankton, and are essential to all higher organisms

Unsaturated fatty acids can be present in cis (meaning that the hydrogen atoms are

oriented in the same direction), or trans configuration( meaning that the hydrogen atoms

are oriented in the opp. direction) .Moreover, the cis configuration is associated with


thermodynamic instability and, thus, a lower melting point compared to trans and

saturated fatty acids.

Fatty acids (FA) differ from one another in:

(1) Length of the hydrocarbon tails

(2) Degree of unsaturation (double bond)

(3) Position of the double bonds in the chain

Fats of animal origin are simpler than that of plant origin.

Q6. Differentiate between saturated and unsaturated fatty acids

S.No. Saturated fatty acids Unsaturated fatty acids

1. They do not possess any double

bonds in their fatty acids.

They contain one or more double

bonds in their fatty acids.

2. All carbon atoms are fully saturated. Carbon atoms are unsaturated in the

region of double bonds.

3. They have straight chains. The chain bends at the double bond.

4. They are solid at ordinary

temperature.

They are liquid at ordinary temperature.

5. They have higher melting point. They have lower melting point.

6. Animal fats are mostly saturated fats. Plant fats are generally unsaturated

fats.

7. They increase blood cholesterol. Unsaturated fats lower blood

cholesterol.


8. Hydrogenation has no effect on

saturated fats.

Hydrogenation converts unsaturated

fats into saturated ones

9. Essential fatty acids are absent. Essential fatty acids are present.

10. They are not much affected by

exposure to air.

On exposure to air, they tend to solidify.

Q6. Describe nomenclature of fatty acids

Ans. Standard nomenclature assigns the number 1 to carboxyl carbon (C1) and α to the

carbon next to it.

Each line segment of the zigzag represents a single bond between adjacent

carbons.

A simplified nomenclature for unbranched fatty acid specifies the chain length and

number of double bonds separated by a colon.

Eg. 16:0 means 16 carbon fatty acid chain with 0 double bond (saturated) –palmitic acid

18:1 means 18 carbon unsaturated fatty acid chain with 1 double bond- Oleic acid

The position of double bond(s) is indicated by Δ followed by a superscript number

Δn indicating the lower numbered carbon in the double bond

• An alternative convention is also followed (generally for polyunsaturated fatty acids-

PUFAs). The numbering is done from opposite direction, assigning number 1 to methyl

carbon at the other end of the chain; this carbon is also designated as ω. The positions

of the double bonds are indicated relative to the ω carbon.

Q7. What are Simple Lipids?

OR

Write short notes on

I. Fats and Oils

II. Waxes


Ans. Triacylglycerols or simple lipids are the products of a reaction in which three OH

groups of glycerol are esterfied with fatty acids.

Simple lipids are the esters of fatty acids with various alcohols. They are divided into:

I. Fats and Oils (triglycerides and triacylglycerols) –

These are esters of fatty acids with a trihydroxy alcohol, glycerol.

Fat is the name given to a class of triglycerides that appear as solid or semisolid at

room temperature, fats are mainly present in animals

Oils the name given to class triglycerides that appear as a liquid at room temperature,

oils are mainly present in plants and sometimes in fish

The fact that saturated fatty acid tails can bunch up closely together, its allows the

triacylglycerols relatively high melting points, which in turn allows them to appear

as solids at room temperature.

The opposite goes for unsaturated fatty acids, their tails cannot pack as closely

together so in turn they have relatively low melting points which causes them to

appear as liquids at room temperature.

Fats usually consist of saturated fatty acids while oils usually consist of unsaturated

fatty acids.

One important characteristic of a triacylglycerol is its state at room temperature. The

degree of saturation and the length of their chains attached to the glycerol backbone

both determine their state at room temperature.

Short-chain unsaturated triglycerides are liquid at room temperature.

Long-chain saturated triglycerides are solid at room temperature.

Animal fats (lard) contain a high amount of saturated triglycerides while plant oils

(vegetable oil) contain a high amount of unsaturated triglycerides. While neither is

healthy when consumed in excess, vegetable oils are far healthier than lard.


Waxes are the esters of fatty acids with high molecular weight monohydroxy alcohols.

Example: Beeswax, Carnauba wax.

II. Waxes

Waxes consist of a long-chain fatty acid linked through an ester oxygen to a long-

chain alcohol.

These molecules are completely water-insoluble and generally solid at biological

temperatures.

Their strongly hydrophobic nature allows them to function as water repellents on the

leaves of some plants, on feathers, and on the cuticles of certain insects.

Waxes also serve as energy-storage substances in plankton (microscopic aquatic

plants and animals) and in higher members of the aquatic food chain.

Plankton apparently use the biosynthesis of waxes to adjust their buoyant density and

thus their depth in the ocean.

It has been suggested that a major source of petroleum found in deep-sea sediments

originates from the deposition of wax-rich dead plankton over vast periods of time.

Whales and many fishes also store large quantities of waxes.

Waxy coating protects the plant from becoming infected with fungi and bacteria which

causes disease.

Q8. Write notes on

I. Phospholipids

• They are abundant in all biological membranes.

• A phospholipid made up of three subunits

Glycerol: Forms the backbone of the phospholipid molecule

Fatty acids: Two fatty acids are attached

Phosphate group: A charged phosphate group is attached to one end of the glycerol

molecule, usually with a charged organic molecule attached to it


• The fatty acid components provide a hydrophobic barrier, whereas the remainder of the

molecule has hydrophilic properties to enable interaction with the environment.

• In phospholipids, two of the OH groups in glycerol are linked to fatty acids while the third

OH group is linked to phosphate group.

Phospholipids are amphipathic, as they have a charged/ polar hydrophilic phosphate

head with a non-charged/ non-polar hydrophobic fatty acids tails

• Phospholipids are further divided into :

Phosphoglycerides: Glycerophospholipids, also called phosphoglycerides, are

membrane lipids in which two fatty acids are attached in ester linkage to the first and

second carbons of glycerol, and a highly polar or charged group is attached through a

phosphodiester linkage to the third carbon.

Phosphoinositiol: Phosphoinositides have been found to occur in phospholipids of brain

tissue and of soybeans and are of considerable importance because of their role in

transport processes in cells.

Phosphotidylcholines: Also known as lecithins.

Phosphotidylethanolamines: Also known as cephalins


II. Sphingophospholipids

These compounds are commonly found in nerve tissue esp., in the myelin sheath of the

nerve (hence their name, sphingomyelins) and apparently lack in plants and the

microorganisms.

These differ from other phospholipids in their lack of glycerol and the presence of

another nitrogenous base sphingosine or a closely related dihydrosphingosine, besides

choline, in place of glycerol.


Sphingomyelins are electrically charged molecules and contain phosphocholine as their

polar head groups.

III. Glycolipids

Glycolipids are present in all tissues on the outer surface of the plasma membrane.

Glycolipids are components of cellular membranes comprised of a hydrophobic lipid tail

and one or more hydrophilic sugar groups linked by a glycosidic bond.

Generally, glycolipids are found on the outer leaflet of cellular membranes where it plays

not only a structural role to maintain membrane stability but also facilitates cell-cell

communication acting as receptors, anchors for proteins and regulators of signal

transduction.

The basic structure of a glycolipid consists of a mono- or oligosaccharide group attached

to a sphingolipid or a glycerol group (can be acetylated or alkylated) with one or two

fatty acids.

The phosphate residue typical of phospholipids is absent.

They basically include two major groups: Cerebrosides and Gangliosides

Found abundantly in myelin sheath of nerves and white matter of brain

Cerebrosides Cerebrosides have a single sugar linked to ceramide; those with

galactose are characteristically found in the plasma membranes of cells in neural

tissue, and those with glucose in the plasma membranes of cells in nonneural tissues.

Galactocerebrosides are typically found in neural tissue, while glucocerebrosides are

found in other tissues.


Ganglioside: A ganglioside is a molecule composed of a glycosphingolipid (ceramide

and oligosaccharide) with one or more sialic acids (e.g. n-acetylneuraminic acid,

NANA) linked on the sugar chain.

They are found predominantly in the nervous system where they constitute 6% of all

phospholipids

Q9. Describe Cholesterol

Cholesterol has a molecular formula, C27H45OH.

In addition to an OH group at C3, there is a double bond at C5.

The hydroxyl group constitutes its polar head, the rest of the molecule is

hydrophobic.

It is a white crystalline solid and is optically active, [α]D 39°.

The crystals are rhombic plates with one of the angles broken.

It has a melting point of 149°C.

Cholesterol is generally believed to be notorious as a major cause of heart

disease.

There are 2 types of cholesterol, the low density lipoprotein cholesterol (LDL-C)

and the high-density lipoprotein cholesterol (HDL-C).

It is the principal sterol of higher animals and is especially abundant in nerve

tissues and in gallstones.

It occurs either free or as fatty esters in all animal cells.

It was first isolated in 1784, from human gallstones which consist almost entirely

of cholesterol and hence so named (cholesterol literally means ‘solid alcohol

from bile’).

Its main sources are fish liver oils and the brain and spinal cord of cattle.

Cholesterol is, however, not found in plant fats.

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