The document discusses the diverse structures and functions of lipids, including fats, oils, fatty acids, triglycerides, phospholipids, sphingolipids, steroids, and waxes, emphasizing their roles in biological membranes and energy storage. It explains the differences between saturated and unsaturated fatty acids, the significance of triglycerides in dietary fats, and the structural importance of phospholipids in forming lipid bilayers. Additionally, it covers the processes of hydrogenation, saponification, and the biological functions of membranes, including transport and signaling mechanisms within cells.

1. LIPIDS AND MEMBRANES
2016/17 ACADEMIC

2. Lipid
A naturally occurring molecule which is soluble in nonpolar organic
solvents.
Lipids are very diverse in structure
Lipid molecules include:
• Fats and oils
• Waxes
• Sterols
• Phospholipids
• Etc

3. Two commonest lipid molecules are:
1. Fats
2. Oils
Fats are solid at room temperature (25°C).
A mixture of triacylglycerols that is solid because it
contains a high proportion of saturated fatty acids.

4. •Oils are liquid at room temperature (25°C).
A mixture of triacylglycerols that is liquid because it
contains a high proportion of unsaturated fatty acids.
•Triacylglycerols in natural fats and oils are nonpolar,
hydrophobic molecules with no ionic charges.

5. Structure of Fat/Oil Molecule
There are two main components of
fats/oils:
Glycerol
Fatty acid

6. Glycerol
It is a trihydroxy alcohol

7. Fatty Acids
These are mostly long chain carboxylic acids (>10 C).
Naturally occurring fatty acids have even number of
carbon atoms.
Very often referred by their common names which
indicates their source.
Examples
Lauric acid-dodecanoic acid (12 carbons).
CH3(CH2)10COOH
Myristic acid-tetradecanoic acid (14 carbons).
CH3(CH2)12COOH

8. Palmitic acid-hexadecanoic acid (16 carbons).
CH3(CH2)14COOH
Stearic acid-octadecanoic acid (18 carbons).
CH3(CH2)16COOH
Arachidonic acid-eicosanoic acid (20 carbons).
CH3(CH2)18COOH

9. Saturated and Unsaturated fatty acids
The fatty acid molecule in the lipid can either be saturated
or unsaturated.
Saturated fatty acid:
A long-chain carboxylic acid containing only carbon–
carbon single bonds.
Saturated fatty acids of 12 - 20 C are common;
overall shape = straight

10. • Unsaturated fatty acid:
• A long-chain carboxylic acid containing one or more
carbon–carbon double bonds.
• Unsaturated fatty acids in nature are always cis
isomers.
• puts a “kink” in the chains & affects 3-D structure
(trans-fatty acids only formed synthetically)

13. • Polyunsaturated fatty acids have more than one C=C
double bond.
• As the number of double bonds increases
(polyunsaturated) melting points decrease

14. Linoleic and linolenic acids are essential in the human diet because
the body does not synthesize them.
They are needed for the synthesis of other lipids.

15. TRIGLYCERIDES
Triacylglycerols are carboxylic acid triesters of glycerol,
a three-carbon trialcohol and three fatty acids.
They make up the fats stored in our bodies and most
dietary fats and oils.
They are a major source of biochemical energy.

16. Animal fats and vegetable oils are the most plentiful
lipids in nature.
All fats and oils are composed of triesters of glycerol
(1,2,3-propanetriol, also known as glycerine) with three
fatty acids.
They are named chemically as triacylglycerols, but are
often called triglycerides.

19. Fatty acid on carbon 1 is typically saturated.
Fatty acid on carbon 2 is typically unsaturated.
Fatty acid on carbon 3 can be either saturated or
unsaturated.
The three fatty acids
esterified to glycerol
usually are not the same
type.

20. • Some key points about structure and properties of
triacylglycerols.
• 1) TAG that are solid at room temperature are
classified as fats (animal-based).
• 2) TAG that are liquid at room temp. are classified as
oils (vegetable-based).
• 3) In general, the more unsaturated the fatty acids
in a TAG, the less solid it is.

21. 4) Most liquid TAG come from plant sources (olives,
corn, safflower).
5) Most solid or primarily saturated fats come from
animal sources.
6) 3-D structure of fatty acids affects packing which in
turn affects melting point.
Trans fatty acid is similar in shape to a saturated fatty
acid.
Unsaturated TAG do not pack as tightly due to shape

24. Fatty acids are a major component of:
•Lipoproteins especially LDL (low-density lipoproteins)
• Cell membranes.
•Oxidation degrades membranes and makes them less
fluid.

25. • Oxidation of fatty acids causes “rancidity” - oxidative cleavage of
unsaturated fatty acids leading to shorter chain aldehydes and acids.
• Antioxidants can protect the unsaturated fatty acids.
• These are compounds which react with free radicals (often by
forming a more stable free radical) and remove them from the site
before damage occurs.
• Packaging (air tight) also is used to reduce rancidity of fats

26. Hydrogenation of unsaturated fatty acids
Unsaturated fatty acids may be converted to saturated
fatty acids by the relatively simple hydrogenation
reaction.
Recall that the addition of hydrogen to an alkene
(unsaturated) results in an alkane (saturated).
A simple hydrogenation reaction is:
Ni/atm
H2C=CH2 + H2 ---> CH3CH3
alkene plus hydrogen yields an alkane

27. Vegetable oils are commonly referred to as
"polyunsaturated".
This simply means that there are several double bonds
present.
Vegetable oils may be converted from liquids to solids
by the hydrogenation reaction.
An important industrial process is the manufacture of
margarines and shortenings.
Margarines and shortenings are "hardened" in this way
to make them solid or semi-solids.

30. Saponification
Reaction of lipids with strong bases such as NaOH to
form a soap.
Hydrolysis
Breakdown of the triglyceride molecule to produce
glycerol and the fatty acids.
Transesterification?

31. Mono-and DI-glycerides
Diglycerides are formed when only two fatty acid
molecules are esterified to the glycerol molecule.
1,2-diglyceride
1,3-diglyceride

32. Monoglycerides are formed when only one fatty acid
molecule is esterified to the glycerol.
1-monoglyceride
2-monoglyceride

33. Phospholipids, sphingolipids and
the structure of cell membranes

34. • Phospholipids are a major component of all cell
membranes as they can form lipid bilayers.
• Most phospholipids contain a diglyceride, a phosphate
group, and a simple organic molecule such as choline.
• The first phospholipid identified as such in biological
tissues was lecithin, or phosphatidylcholine, in the
egg.
• The structure of the phospholipid molecule generally
consists of hydrophobic tails and a hydrophilic head.

36. Phosphatidyl choline (PC)

37. Phosphatidylserine (PS)

38. • In general, phospholipids are composed of a phosphate group, two alcohols, and
one or two fatty acids.
• On one end of the molecule are the phosphate group and one alcohol; this end
is polar, i.e., has an electric charge, and is attracted to water(hydrophilic).
• The other end, which consists of the fatty acids, is neutral; it is hydrophobic and
water-insoluble but is fat-soluble.
• This amphipathic nature (containing both hydrophobic and hydrophilic groups)
makes phospholipids important in membranes.
• They form a two-layer structure, called the lipid bilayer, with the polar head
facing out on each surface to interact with water, and with the neutral “tails”
driven inward and pointing toward one another.

40. • The lipid bilayer is the structural basis of all cell membranes
and is nearly impermeable to ions and most polar molecules.
• The major role is as a barrier between cells and their
environment.
• Thus separating the cytoplasm and cellular structures from the
extracellular fluid and each other.
• Proteins embedded in the phospholipid matrix transport many
substances through the membrane.

42. Sphingolipids

43. Sphingosine is the alcohol instead of glycerol
NOTE that an amino group replaces an OH
group in sphingosine

44. • The sphingolipids comprise a complex range of lipids in
which fatty acids are linked via amide bonds to a long-
chain base or sphingoid.
• sphingolipids are extremely versatile molecules.
• Found in nerve tissue.
• They are also found in a few bacterial genera (but
especially Sphingomonas and Sphingobacterium).

48. • A long-chain base, such as sphingosine, is the simplest
possible functional sphingolipid
• Ceramides which contain a fatty acid linked by an amide
bond, are also the precursors of phospholipids and glycolipids
• Sphingomyelin has structural similarities to
phosphatidylcholine, but has very different physical and
biological properties
• Oligoglycosylceramides and gangliosides.
These are Complex sphingolipids located mainly in the
plasma membrane of mammalian cells where they have a
structural function, and also serve as adhesion sites for
proteins from the extracellular tissue.
• Sphingolipids and their metabolites have important roles in
signal transduction.

49. STEROIDS
ISOPRENOIDS

50. Steroids form an important group of compounds based
on the fused four saturated carbon rings
1,2-cyclopentanoperhydrophenanthrene (sterane).

51. • Steroids comprise a group of cyclic organic compounds.
• The most common characteristic is an arrangement of
seventeen carbon atoms in a four-ring structure.
• There are three rings composed of 6-carbons (rings A,
B, and C) followed by one with 5-carbons (ring D).
• They have an 8-carbon side chain attached to a carbon
on ring D, and two or more methyl groups at the points
where adjacent rings are "fused".

52. Cholesterol

53. Progesterone
Lanosterol
Bile acid
Testosterone
Estrogen

54. KEY POINTS
• Steroids are lipids because they are hydrophobic and
insoluble in water, but they do not resemble lipids since
they have a structure composed of four fused rings.
• Cholesterol is the most common steroid and is the
precursor to vitamin D, testosterone, estrogen,
progesterone, aldosterone, cortisol, and bile salts.
• Cholesterol is a component of the phospholipid bilayer and
plays a role in the structure and function of membranes.

55. Steroids are found in the brain and alter electrical
activity in the brain.
Because they can tone down receptors that
communicate messages from neurotransmitters, steroids
are often used in anesthetic medicines.

56. WAXES
• Waxes are a class ofchemical compounds that are
plastic (malleable) near ambient temperatures.
• They melt above 45 °C (113 °F) to give a low viscosity liquid.
• Waxes are insoluble in water but soluble in organic,
nonpolar solvents.
• All waxes are organic compounds, both synthetic and
natural

57. Plant and Animal Waxes
• Waxes are synthesized by many plants and animals.
• Those of animal origin consist of wax esters derived
from a variety of carboxylic acids and fatty alcohols.
• Waxes of plant origin are mainly unesterified
hydrocarbons. The composition depends not only on
species, but also on geographic location of the
organism.
• Because they are mixtures, naturally produced waxes
are softer and melt at lower temperatures than the
pure components.

58. Animal waxes
• The most commonly known animal wax is beeswax,
• A major component of the beeswax used in
constructing honeycombs is the ester myricyl palmitate
which is an ester of triacontanol and palmitic acid.
• Its melting point is 62-65 °C.
• Lanolin is a wax obtained from wool, consisting of
esters of sterols.

59. Table 3. Relative composition (wt % of the total) of the non-polar lipids from the skin
surface of various species.
Squal
ene
Sterols
Sterol
esters
Wax
esters
Diesters
Glyceryl
ethers
Triacyl-
glycerol
s
Free
acids
Free
alcohols
Human 12 1 3 25 41 16
Sheep 12 46 10 21 11
Rat 1 6 27 17 21 8 1
Mouse 13 10 5 65 6
Adapted from Downing, D.T. Mammalian waxes. In: Chemistry and Biochemistry of
Natural Waxes. (Ed. P.E. Kolattukudy, Elsevier, Amsterdam) (1976).

60. Plant waxes
• Plants secrete waxes into and on the surface of
their cuticles as a way to control evaporation,
wettability and hydration.
• The epicuticular waxes of plants are mixtures of
substituted long-chain aliphatic hydrocarbons,
containing alkanes, alkyl esters, fatty acids, primary
and secondary alcohols diols ketones aldehydes.
•

61. Commercially the most important plant wax is
Carnauba wax, a hard wax obtained from the Brazilian
palm Copernicia prunifera.
It contains the ester myricyl cerotate and has many
applications, such as confectionery and other food
coatings, car and furniture polish, floss coating and
other uses.

62. Table 1. The major constituents of plant leaf waxes.
n-Alkanes CH3(CH2)xCH3 21 to 35C - odd numbered
Alkyl esters CH3(CH2)xCOO(CH2)yCH3 34 to 62C - even numbered
Fatty acids CH3(CH2)xCOOH 16 to 32C - even numbered
Fatty alcohols (primary) CH3(CH2)yCH2OH 22 to 32C - even numbered
Fatty aldehydes CH3(CH2)yCHO 22 to 32C - even numbered
Ketones CH3(CH2)xCO(CH2)yCH3 23 to 33C - odd numbered
Fatty alcohols
(secondary)
CH3(CH2)xCHOH (CH2)yCH3 23 to 33C - odd numbered
β-Diketones CH3(CH2)xCOCH2CO(CH2)yCH3 27 to 33C - odd numbered
Triterpenols Sterols, α-amyrin, β-amyrin, uvaol, lupeol, erythrodiol
Triterpenoid acids Ursolic acid, oleanolic acid, etc

63. Table 2. Relative proportions (wt %) of the common wax constituents in some plant
species.
Arabidopsis
leaf
Rape leaf Apple fruit
Rose
flower
Pea leaf
Sugar cane
stem
Hydrocarbons 73 33 20 58 40-50 2-8
Wax esters - 16 18 11 5-10 6
Aldehydes 14 3 2 - 5 50
Ketones 4 20 3 - -
Secondary
alcohols
1 8 20 9 7 -
Primary
alcohols 8 12 6 4 20 5-25
Acids 1 8 20 5 6 3-8
Other components present include various diol types and triterpenoid acids

64. BIOLOGICAL MEMBRANES
• Biological membranes or biomembranes are
enclosing or separating membranes that act as
selectively permeable barriers within living cells.
• They consist of a phospholipid bilayer with
embedded integral and peripheral protein.
• The proteins are used in communication and
transportation of chemicals and ions.

65. Functions of Biological Membranes
• Active transport of K+
,Na+
,Ca2+
, maintaining the
osmotic equilibrium in all cells (cytoplasmic)
• Binding of hormones and switching on of mechanisms
of intracellular signaling in majority of cells.
• Generation of action potential by nerve and muscle
cells.

66. Transfer of electrons on to oxygen and synthesis of ATP
(oxidative phosphorylation) by majority of cells in
Inner membrane of mitochondria.
Transfer of Ca2+
from cell solutions into vesicles in
majority of cells.
Absorption of light quanta and generation of
intracellular signal by eye epithelium cells.

67. MEMBRANE STRUCTURE
• The phospholipids within the membrane are
amphipathic in nature
– Polar heads favour contact with water
– Hydrocarbon tails interact with one another in the middle
• The preferred structure for most phospholipids and
glycolipids in aqueous media is a bimolecular sheet
(lipid bilayer)

69. Lipid Bilayer

70. Cell Membrane Asymmetry
• The cell membranes tend to have different composition on one side of the membrane
than on the other.
• The differences can be caused by the different ratios or types of amphipathic lipid-
based molecules, the different positioning of the proteins (facing in or facing out), or
the fixed orientations of proteins spanning the membrane.
• In addition, there are different enzymatic activities in the outer and inner membrane
surfaces.
• The reason the cell membrane is asymmetric is because when the proteins are
synthesized by the preexisting membranes, they are inserted into the membrane in an
asymmetric manner.
• The cell membrane's phospholipids are distributed asymmetrically across the lipid
bilayer, in a phenomenon called membrane phospholipid asymmetry.

72. Signal for destruction membrane
asymmetry

73. Protein Targeting
• Protein targeting or protein sorting is the biological
mechanism by which proteins are transported to the
appropriate destinations in the cell or outside of it.
• Proteins can be targeted to the inner space of an
organelle, different intracellular membranes
plasma membrane, or to exterior of the cell via
secretion.
• This delivery process is carried out based on
information contained in the protein itself. Correct
sorting is crucial for the cell; errors can lead to diseases.