Lipids

1. LIPIDS
•Classification, Structure & important
biological functions
Submitted to,
Dr. Elsam Joseph
Asst.Professor
Dept. of Botany
St Teresa’s
college,EKM.
Submitted by,
Ancy Varghese
1 MSc Botany
No.6
St.Teresa’s
college,EKM

2. LIPIDS : INTRODUCTION (GREEK :LIPOS, FAT)
• Lipids are one of the major organic biomolecules present in our
body, others include nucleic acids, carbohydrates, and proteins.
• But unlike the other macro biomolecules, lipids are not polymers –
they aren’t composed of monomers.
• They have a chemical composition of mainly Hydrogen, Oxygen
and Carbon.
• There are several different families or classess of lipids, but all
derive their distinctive properties from the hydrocarbon nature of
a major portion of their structure.
• Structurally, they are esters or amides of fatty acids.
2/53

3. • This class includes a range of molecules, such as fats, oils,
hormones, and certain components of membranes.
• These molecules are either non-soluble or are poorly soluble in
water.
• They are hydrophobic in nature because of the predominance of
hydrocarbon chains (–CH2–CH2–CH2–CH2–) in their structures.
• Although Lipids are a distinct class of biomolecules, we often see
them occur combined, either covalently or through weak bonds,
with members of other classes of biomolecules to yield hybrid
molecules such as glycolipids, lipoproteins…etc having the
properties blended to accomplish specialised biological functions.
3/53

4. • They are soluble in non-polar solvents, such as ether, alcohol,
chloroform, acetone, and benzene and are insoluble in water.
• Lipid molecules have no ionic charges.
• Pure fats and oils are colorless, odorless, and tasteless.
• Lipids are considered hydrophobic or amphiphilic small molecules.
GENERAL PHYSICAL PROPERTIES OF LIPIDS
4/53

5. • Lipids are greasy in texture and stored in adipose tissues( Also
called fat tissue.Adipocytes- cells that compose adipose tissue –
are cells specialised in storing energy as fat, mainly triglycerides,
in organelles called lipid droplets
• Lipids are either liquid or non-crystalline solid at room
temperature.
• Lipids can either be present in saturated (having only single bonds)
or unsaturated (having one or more double bonds) structural form.
5/53

6. CHEMICAL PROPERTIES OF LIPIDS
• Hydrolysis of triglycerides: Triglycerides (neutral lipids) on reacting with water
form carboxylic acid and alcohol.
• Saponification: Triglycerides on hydrolysis with alkali (NaOH or KOH) or
lipase enzymes (termed alkaline hydrolysis) lead to the formation of two
products: soap or fatty acid salts of sodium or potassium, and glycerol.
• Hydrogenation: The breakage of double bonds occurs after the reaction of
unsaturated fatty acids with hydrogen. This turns the molecules into saturated
fatty acids.
• Halogenation: Free or combined fatty acids in the reaction with halogens gain
double bonds and cause decolorization of halogen solutions.
• Rancidity: Oxidation and hydrolysis of fats and oil to generate a
disagreeable odor – this is known as rancidity.
6/53

7. CLASSIFICATION OF LIPIDS
• Lipids have been classified in several different ways.
• Most satisfactory classification is based on their backbone
structures.
7/53

8. 1.BASED ON BACKBONE
STRUCTURE
• Complex lipids characteristically
contain fatty acids as components
(Acylglycerols, Phosphoglycerides,
sphingolipids, waxes) – differ by to
which fatty acids they are covalently
joined.
• Also called saponifiable Lipids (they
yield soaps on alkaline hydrolysis-
salts of fatty acids)
• Simple lipids do not contain fatty
acids- hence nonsaponifiable
• Lipid type Backbone
• Complex (saponifiable)
1. Acylglycerols Glycerol
2. Phosphoglycerides Glycerol 3- phosphate
3. Sphingolipids Sphingosine
4. Waxes Nonpolar alcohols of high
molecular weight.
Simple (nonsaponifiable)
1. Terpenes
2. Steroids
3. Prostaglandins
8/53

9. CLASSIFICATION OF LIPIDS
• Lipids can be classified also in four different ways, depending on:
• chemical composition,
• fatty acids,
• requirements,
• and sources.
9/53

10. 2.BASED ON THE CHEMICAL COMPOSITION
• Lipids based on chemical composition are divided into three categories:
simple lipids, compound lipids, and derived lipids.
• 1. Simple Lipids
• includes esters of fatty acids and glycerol that are also termed neutral fats
or triglycerides.
• They make up 98-99% of food and body fats and oil.
• Its three classes are fatty acids, triglycerides, and waxes.
• A. Fatty Acids
• Fatty acids are the simplest form of lipids. They are a long chain of
hydrocarbons (4 to 36 carbons long) with one carboxyl group.
10/53

11. • B. Triacylglycerols
• Triacylglycerols (or triglycerides) are tri-esters of fatty acids and glycerol.
• The two types of triacylglycerols include simple and mixed types.
• The triglycerides containing only a single type of fatty acids are called
simple triglycerides,& those with two or more different types of fatty acids
are called mixed triglycerides.
• C. Waxes
• Waxes are esters of long-chain fatty acids and long-chain alcohol.
11/53

12. • 2. Compound Lipids
• The complex or compound lipids contain some other organic molecules in
addition to fatty acids and glycerols.
• They include phospholipids, glycolipids, and lipoproteins.
• A. Phospholipids
• Phospholipids consist of four components: fatty acids, glycerol or sphingosine,
phosphate, and alcohol attached to phosphate.
• It includes phosphoglycerides, ether glycerophospholipids, and
sphingophospholipids.
• B. Glycolipids
• Glycolipids contain a carbohydrate group (attached through a glycosidic
bond) in combination with glycerol and fatty acids. It’s the third major class of
membrane lipids.
12/53

13. • C. Lipoproteins
• Lipoproteins are lipid-protein complexes. They help lipids derived from food
or synthesized in one organ, such as triglycerides, phospholipids, cholesterol,
and cholesterol esters, to be transported throughout the body.
• Lipoproteins soluble in the blood are categorized into four groups based on
their densities:
1. Chylomicrons
2. Very Low-Density Lipoproteins (VLDL)
3. Low-Density Lipoproteins (LDL)
4. High-Density Lipoproteins (HDL).
• The lipoproteins have a core containing neutral lipids, triacylglycerols and
cholesterol esters. The core is coated with a single layer of phospholipids,
embedded with apolipoproteins and cholesterol.
13/53

14. • 3. Derived Lipids
• Derived lipids are released during hydrolysis of simple and compound lipids.
• They include steroids and some fatty acids.
• A. Steroids
• Steroids consist of four fused rings called steroid nucleus. They are complex
derivatives of triterpenes.eg. cholesterol which is an essential component in
animal cell membranes.
• B. Eicosanoids
• They are a family of biological signaling molecules that act as short-range
messengers.
• They originated from 20 carbon arachidonic acids, and include prostanoids
and leukotrienes.
14/53

15. 3.BASED ON FATTY ACIDS
• Lipids are divided into two categories based on the type of fatty acids
present in them: saturated and unsaturated fatty acids.
• 1. Saturated Fatty Acids
• Saturated fatty acids have no double or triple bonds.
• They are a simple, unbranched, and linear chain of CH2 groups connected
with a carbon-carbon single bond and one carboxylic acid at its end.
• general formula is CH3 – (CH2)n – COOH., where n represents the number of
methylene groups.
• Some examples of saturated fatty acids include lauric, myristic, palmitic,
stearic, behenic, and lignoceric acids.
15/53

16. • 2. Unsaturated Fatty Acids
• Unsaturated fatty acids have one or more double or triple bonds.
• So, they can either be monounsaturated or polyunsaturated.
• The naturally occurring fatty acids are generally in the cis
configuration, rather than in trans configuration.
• There are only a few naturally occurring fatty acids with triple
bonds and they’re often of plant origin.eg. stearolic acid.
16/53

17. CAN LIPIDS BE POLAR ?
• According to their polarity, the complex lipids can also be classified as non-polar
lipids (also named neutral lipids) or polar lipids, including phospholipids and
glycolipids .
• Lipids, i.e., fatty molecules, on the other hand, are non-polar,
• meaning that the charge distribution is evenly distributed, and the molecules do not
have positive and negatively charged ends..
• Non-polar molecules do not dissolve well in polar solutions like water; in fact, polar
and non-polar molecules tend to repel each other in the same way that oil and
water don’t mix and will separate from each other even if they are shaken
vigorously in an attempt to mix them.
• This distinction between polar and non-polar molecules has important consequences
for living things, which are composed of both polar molecules and non-polar
molecules.
17/53

18. HOW POLARITY IS DEVELOPED IN A LIPID
• Phospholipids constitute another important class of lipids.
• These are similar to triglycerides in that they have a glycerol backbone, but there are
only two fatty acids connected to glycerol.
• The third carbon of the glycerol backbone is attached to a phosphate group (an atom
of phosphorus bonded to four atoms of oxygen),
• and the phosphate group is attached to a base molecule of choline, serine, or
ethanolamine.
• The part of the phospholipid with phosphate and the base is actually very polar, and it
tends to rotate away from the two fatty acids.
18/53

19. • This makes phospholipid molecules have a hairpin shape.
• The head of the hairpin is very polar and therefore likes to associate with water (it is
hydrophilic), while the two fatty acid chains (the “tails”) are very non-polar and tend
to avoid water (hydrophobic) and associate with other hydrocarbon chains.
• Phospholipids can be described
as amphipathic,because they have
this dual nature (part polar and
part non-polar). This characteristic
causes phospholipids to self-associate
into large macromolecular complexes
in an aqueous (watery) environment.
19/53

20. 4.BASED ON POLARITY
• On the basis of polarity, lipids are divided into two classes:
• (1) Polar and (2) Non-Polar
• 1. Polar (Glycerophospholipids & Sphingolipids):
• It forms bilayer impulsively in water. Eg. Phosphatidyl choline (PC, a purified
polar lipid from membranes)
• There are three sub-classes of polar lipids: anionic, uncharged and zwitter-
ionic. No cationic lipids are reported in nature.
• Anionic Lipids
• Examples of the anionic lipids are Cardiolipin (CL), Phosphotidyl glycerol
(PG), Phosphotidyl inositol (PI), Phosphotidyl serine (PS) and many others.
20/53

21. • Uncharged Lipids
• Eg.Cerebrosides, Gangliosides and Glycolipids.
• Zwitter-Ionic Lipids
• Eg. Phosphotidyl choline (PC), Phosphatidyl ethanolamine (PE), Sphingomyelin
(SM)
• 2. Non-polar (Triglycerides)
• A lot of lipids, on the other hand, are non-polar, implicating that the charge
allocation is uniformly distributed.
21/53

22. IMPORTANT BIOLOGICAL FUNCTIONS
1. Energy Storage
• Triacylglycerol or triglycerides, residing in adipose tissues,
are a major source of energy in both plants and animals.
• The complete breakdown of fatty acids releases about 38
kJ/g (9 kcal/g) caloric content.
• Unlike other body cells that can store fat in limited supplies,
fat cells are specialized for fat storage and also to expand
almost indefinitely in size.
• An overabundance of adipose tissue can result in undue stress
on the body and can be detrimental to health.
22/53

23. • The excess energy from the food we eat is digested and
incorporated into adipose tissue, or fatty tissue.
• Most of the energy required by the human body is provided by
carbohydrates and lipids.
• Glucose is stored in the body as glycogen. While glycogen
provides a ready source of energy, lipids primarily function
as an energy reserve.
• Glycogen is quite bulky with heavy water content, thus the body
cannot store too much for long.
• Alternatively, fats are packed together tightly without water and
store far greater amounts of energy in a reduced space.
• A gram fat is densely concentrated with energy—it contains more
than double the amount of energy than a gram of carbohydrate.
2
3
/
5
3

24. • 2.Regulating and signalling
• Triacylglycerols control the body’s internal climate, maintaining a constant temperature.
• Those who don’t have enough fat in their bodies tend to feel cold sooner, are often
fatigued, and have pressure sores on their skin from fatty acid deficiency.
• Triacylglycerols also help the body produce and regulate hormones. Eg. adipose tissue
secretes the hormone leptin, which regulates appetite.
• In the reproductive system, fatty acids are required for proper reproductive health;
women who lack proper amounts may stop menstruating and become infertile.
• Omega-3 and omega-6 essential fatty acids help regulate cholesterol and blood clotting
and control inflammation in the joints, tissues, and bloodstream.
24/53

25. • Fats also play important functional roles in sustaining nerve
impulse transmission, memory storage, and tissue structure.
• More specifically in the brain, lipids are focal to brain activity
in structure and in function. They help form nerve cell
membranes, insulate neurons, and facilitate the signaling of
electrical impulses throughout the brain.
• They activate different signaling pathways either by binding
with G-coupled receptors or nuclear receptors. Some of the
lipid molecules involved in signaling functions include:
• Sphingosine-1-phosphate: It’s a potent messenger molecule,
involved in calcium mobilizing regulations, cell growth, and
apoptosis.
25/53

26. • Diacylglycerol (DAG)and phosphatidylinositol phosphate: They are involved in
calcium-mediated activation of protein kinase C.
• Prostaglandins: It’s an eicosanoid, involved in inflammation and immunity.
• Estrogen, testosterone, and cortisol: These are hormones, modulating several
functions including metabolism, reproduction, and blood pressure.
• Oxysterol: It’s involved in regulating biological responses by binding to liver X
receptors which is a nuclear receptor essential for cholesterol, fatty acid, and
glucose homeostasis.
• Phosphatidylserine: It’s involved in signaling phagocytosis of apoptotic cells by
exposing themselves to the outer leaflet of the bilayer cell membrane.
26/53

27. • 3.The Structural Component of the Cell Membrane
• The plasma membrane of cells is made of a lipid bilayer with proteins
embedded in it. The lipid bilayer is composed of amphipathic
glycerophospholipid molecules.[3] All the glycolipids and phospholipids
present in the cell membrane act as structural components of the membrane.
• The cellular membrane also contains some non-glyceride lipids, which include
sphingomyelin and sterols that are involved in membrane flexibility.
27/53

28. • 4.Insulating and Protectin
• up to 30 percent of body weight is comprised of fat tissue.
• Some of this is made up of visceral fat or adipose tissue surrounding delicate organs.
• Vital organs such as the heart, kidneys, and liver are protected by visceral fat.
• The composition of the brain is outstandingly 60 percent fat, demonstrating the major
structural role that fat serves within the body.
• subcutaneous fat, or fat underneath the skin - blanket layer of tissue insulates the body
from extreme temperatures and helps keep the internal climate under control.
• It pads our hands and buttocks and prevents friction, as these areas frequently come in
contact with hard surfaces.
28/53

29. • 5.Aiding Digestion and Increasing Bioavailability
• By carrying fat-soluble nutrients through the digestive process, intestinal absorption is
improved.
• This improved absorption is also known as increased bioavailability.
• Fat-soluble nutrients are especially important for good health and exhibit a variety of
functions. Vitamins A, D, E, and K—the fat-soluble vitamins—are mainly found in foods
containing fat.
• Some fat-soluble vitamins (such as vitamin A) are also found in naturally fat-free foods
such as green leafy vegetables, carrots, and broccoli.
• These vitamins are best absorbed when combined with foods containing fat.
29/53

30. • Fats also increase the bioavailability of compounds known as phytochemicals, which are
plant constituents such as lycopene (found in tomatoes) and beta-carotene (found in
carrots).
• Phytochemicals are believed to promote health and well-being.
• As a result, eating tomatoes with olive oil or salad dressing will facilitate lycopene
absorption.
• Other essential nutrients, such as essential fatty acids, are constituents of the fats
themselves and serve as building blocks of a cell.
• Removing the lipid elements from food also takes away the food’s fat-soluble vitamin
content. When products such as grain and dairy are processed, these essential nutrients
are lost. Manufacturers replace these nutrients through a process called enrichment.
30/53

31. 31/53

32. • 6. Lipids as hormones
• Most lipid hormonesare derived from cholesterol and thus are structurally similar to it.
• The primary class of lipid hormonesin humans is the steroid hormones.
• Chemically, these hormones are usually ketones or alcohols; their chemical names will end in
“-ol” for alcohols or “-one” for ketones.
• Examplesof steroid hormonesinclude estradiol, which is an estrogen, or female sex hormone,
and testosterone, which is an androgen, or male sex hormone.
• Other steroid hormones include aldosterone and cortisol, which are released by the adrenal
glands along with some other types of androgens.
• Steroid hormonesare insoluble in water, and they are transported by transport proteins in
blood.
• As a result, they remain in circulation longer than peptide hormones. For example, cortisol has
a half-life of 60 to 90 minutes, while epinephrine, an amino acid derived-hormone, has a
half-life of approximately oneminute.
32/53

33. STRUCTURES OF MAJOR LIPID GROUPS
33/53

34. FATTY ACIDS
• They are carboxylic acids with long- chain hydrocarbon side groups.
• In higher plants & animals, the predominant fatty acid residues are those of
the C16 and C 18 : Palmitic, oleic, linoleic and stearic acids
• Over half of the fatty acid residues of plant & animal lipids are unsaturated
(contain double bonds) and are often polyunsaturated (two or more double
bonds).
• Bacterial fatty acids are rarely polyunsaturated, but are commonly branched,
hydroxylated, or contain cyclopropane rings.
• 1st double bond of an unsaturated fatty acid commonly occur between it’s C9
& C10 atoms counting from carboxyl C atom. This bond is called a ∆9 or 9-
double bond.
34/53

35. 35/53

36. • Hydrocarbon chain is almost unbranched in animal fatty acids.
• The configuration of the double bonds in most unsaturated fatty acids is cis.
• Double bonds in polyunsaturated fatty acids are separated by at least one
methylene group.
• Properties of fatty acids and lipids derived from them depend on chain length
and degree of saturation.
• Unsaturated fatty acids have lower melting point than saturated fatty acids of
same length.
• M.p of Stearic acid = 69.6°C
• Oleic acid (contain one cis double bond) = 13.4°C
• Melting point of polyunsaturated fatty acids of C 18 series are even longer.
• Chain length also affects m.p (Palmitic- C16 is 6.5 °C less than Stearic acid –
C 18.
36/53

37. • Unsaturated fatty acids predominate over the saturated
ones, particularly in higher plants and animals living at
low temperatures.
• Fatty acids that cannot be produced or synthesized in
our bodies are called essential fatty acids.
• These fatty acids need to be taken through a diet to
fulfill the body’s requirement for different metabolic
functions. It includes linoleic acid, linolenic acid, and
arachidonic acid.
• Non-essential fatty acids include those lipids that are
synthesized by our body. They are not needed to be
taken through any outside food source. It includes
palmitic acid, oleic acid, and butyric acid.
37/53

38. TRIGLYCERIDES
• Fatty acid esters of the alcohol glycerol are
called acylglycerols or glycerides.
• When all three hydroxyl groups of glycerol
are esterified with fatty acids, the structure
is called Triacylglycerol ( earlier called
Triglycerides)
• Most abundant family of lipids & major
storage lipid.
• Triacylglycerol – solid at Room temp.= fat.
_ liquid at room temp. = oil
38/53

39. WAXES
• Waxes are esters of long-chain fatty acids and long-chain alcohol.
• They are solid at room temperature and completely water-insoluble.
• They are formed by the esterification of long-chain fatty acids and monohydroxy alcohol of
higher molecular weight.
• The popularly known beeswax contains triacontanyl palmitate as a major molecule.
• The hydrophobic nature of waxes allows them to function as water repellents on leaves of some
plants, feathers, and cuticles of insects. They also serve as energy storage for planktons and
higher aquatic animals
39/53

40. PHOSPHOGLYCERIDES
• Also called glycerol phosphatides.
• Major components of cell membranes.
• In Phosphoglycerides one of the primary hydroxyl groups of Glycerol is
esterified to phosphoric acid.; the other hydroxyl groups are esterified to
fatty acids.
• Because Phosphoglycerides posses a polar head in addition to their Nonpolar
hydrocarbon tails, they are called amphipathic or polar lipids.
• The different types of Phosphoglycerides differ in the size, shape, and electric
charge of their polar head groups.
• The simplest form of phosphoglyceride is phosphatidic acid.
40/53

41. 41/53

42. STEROIDS
• Unlike phospholipids and fats, steroids have a fused
ring structure.
• Although they do not resemble the other lipids, they
are grouped with them because they are also
hydrophobic and insoluble in water.
• All steroids have four linked carbon rings, and many
of them, like cholesterol, have a short tail.
• Many steroids also have the –OH functional group,
and these steroids are classified as alcohols called
sterols.
42/53

43. STEROLS
• Steroids are derivatives of the saturated tetracyclic hydrocarbon
perhydrocyclopentanophenanthrene.
• All steroids originate from the linear triterpene squalene which cyclizes readily.
• The first important steroid product of this cyclization is lanosterol, - the precursor of
cholesterol in animal tissues – the most abundant steroid in animal tissues.
• Cholestrol & lanosterol are members of a large subgroup of steroids called the sterols.
• Cholesterol is an important component of the cell membrane and is required for the
synthesis of sex hormones, vitamin D, and bile salts.
43/53

44. • It’s stored in cells as fatty acid esters and act as precursors of many other
steroids in animal tissues, including the bile acids, detergent like components
that aid in emulsification and absorption of lipids in the intestine;
• The steroid hormones ..eg. Estrogens – female sex hormones
• Androgens- male sex hormones – testosterone, dihydrotestosterone
• Progestational hormone – progesterone
• Steroid hormones of adrenal cortex – cortisol, aldosterone, corticosterone.
44/53

45. 45/53

46. BIOLOGICAL SPECIFIC ACTIVITIES OF STEROIDS
• The adrenal cortex produces the adrenocortical hormones, which consist
of the glucocorticoids and the mineralocorticoids.
• Glucocorticoids like cortisol also help to maintain normal blood pressure,
and their anti-inflammatory and immunosuppressive actions have
rendered them useful in treating rheumatoid arthritis and preventing the
rejection of transplanted organs.
• Mineralocorticoids such as aldosterone help maintain the balance
between water and salts in the body, predominantly exerting their
effects within the kidney.
46/53

47. • Androgens - responsible for development and maintenance of
reproductive function and stimulation of the secondary sex
characteristics in the male.
• Androgens also have an anabolic function in stimulating the
production of skeletal muscles and bone as well as red blood cells.
• Estrogens promote the development of the primary and secondary
female sex characteristics; they also stimulate linear growth and
skeletal maturation.
• In other mammals these hormones have been shown to precipitate
estrus (heat). The ovarian production of estrogen plummets during
menopause.
47/53

48. • Progestins, the most important of which is progesterone, are the other type of
female sex hormone and are named for their role in maintaining pregnancy
(pro-gestation).
• Estrogens and progestins are secreted cyclically during menstruation.
• During the menstrual cycle, the ruptured ovarian follicle (the corpus luteum) of
the ovary produces progesterone, which renders the uterine lining receptive to
the implantation of a fertilized ovum.
• To accomplish this, Placenta becomes the main source of progesterone, without
which the pregnancy would terminate.
• As pregnancy progresses, placental production of progesterone increases,
and these high doses suppress ovulation, preventing a second conception.
48/53

49. ISOPRENOIDS
• The isoprenoids are branched lipids, also referred to as
terpenoids, that are formed by chemical modifications of the
isoprene molecule .
• Depending upon no. of isoprene units containing there are
monoterpenes, sesquiterpenes, di terpenes…etc.
• Terpenes may be either linear or cyclic molecules.
49/53

50. • Of very large no.of terpenes identified in plants, have characteristic odors or
flavors – major component of essential oils.
• Eg. Monoterpenes geraniol, limonene, menthol, camphor,and carvone are
major components of oil of geranium, lemon oil, mint oil, camphor oil and
caraway oil.
• Triterpenes include squalene, an imp. Precursor in the biosynthesis of
cholesterol.
• Carotenoids – tetraterpenes. An important carotenoid is B- carotene, the
hydrocarbon precursor of Vit. A.
• Fat soluble vitamins A, E and K – are important Terpenes needed in trace
amounts in diet of mammals.
50/53

51. • Polyprenols in the form of their phosphate esters have a co- enzyme like function in the
enzymatic transfer of sugar groups from the cytoplasm to the outer surface of the cell during
the synthesis of cell surface & cell wall lipopolysaccharides, peptidoglycans, teichoic acids
and glycoproteins.
• Some terpenoids function as co- enzymes – Ubiquinone or co- enzyme Q family of
compounds which function as hydrogen carriers for biological oxidations in the mitochondria.
• Plastiquinones –( analogous compounds) are found in chloroplasts, where they function in
photosynthesis.
• Long-chain isoprenoids are also found in hydrophobic oils and waxes.
• In humans, the main wax production occurs within the sebaceous glands of hair follicles in the
skin, resulting in a secreted material called sebum, which consists mainly of triacylglycerol,
wax esters, and the hydrocarbon squalene.
51/53

52. REFERENCES
• Lehninger,A.(1975). Biochemistry (2 nd edition).Kalyani publishers,Ludhiyana,New
Delhi.
• www.biology online.com – Lipid definition and examples.
• Study.com – A review of lipids function and importance.
• Conduct science.com – Classification and biological functions of lipids. 21 July 2021.
• www.nature.com – Adipocytes .latest research.
• Med.libretexts.org – What are lipids?
52/53