Lipids in Biochemistry

1. Biochemistry I
MLT-301
Lipids
Fall, 2023

2. Lipids
A diverse group of compounds that are united by a common
feature of non polar in nature. These are hydrophobic and are
soluble in non polar solvent. This is because they are
hydrocarbons that include only nonpolar carbon-carbon or
carbon-hydrogen bonds.
Lipids perform many different functions in a cell.
 Cells store energy for long-term use in the form of lipids
called fats.
 Lipids also provide insulation from the environment for
plants and animals. For example, they help keep aquatic
birds and mammals dry because of their water-repelling
nature.
 Lipids are also the building blocks of many hormones and
are an important constituent of the plasma membrane.
Examples: Fats, oils, waxes, phospholipids, and steroids.

3. Lipids…
 A fat molecule, such as a triglyceride, consists of two main
components—glycerol and fatty acids.
 Glycerol is an organic compound with three carbon atoms, five
hydrogen atoms, and three hydroxyl (–OH) groups.
 Fatty acids have a long chain of hydrocarbons to which an
acidic carboxyl group is attached, hence the name “fatty acid.”
 The number of carbons in the fatty acid may range from 4 to 36;
most common are those containing 12–18 carbons.
 In a fat molecule, a fatty acid is attached to each of the three
oxygen atoms in the –OH groups of the glycerol molecule with a
covalent bond.
 During this covalent bond formation, three water molecules are
released.
 The three fatty acids in the fat may be similar or dissimilar.
 These fats are also called triglycerides because they have three
fatty acids.

5. Saturated and unsaturated Fatty acids
In a fatty acid chain, if there are only single bonds between
neighboring carbons in the hydrocarbon chain, the fatty acid is
saturated. Saturated fatty acids are saturated with hydrogen; in
other words, the number of hydrogen atoms attached to the
carbon skeleton is maximized.
When the hydrocarbon chain contains a double bond, the fatty
acid is an unsaturated fatty acid.
Most unsaturated fats are liquid at room temperature and are
called oils. If there is one double bond in the molecule, then it is
known as a monounsaturated fat (e.g., olive oil), and if there is
more than one double bond, then it is known as a
polyunsaturated fat (e.g., canola oil).

6. Saturated fats tend to get packed tightly and are solid at room
temperature. Animal fats with stearic acid and palmitic acid
contained in meat, and the fat with butyric acid contained in
butter, are examples of saturated fats. Mammals store fats in
specialized cells called adipocytes, where globules of fat
occupy most of the cell. In plants, fat or oil is stored in seeds
and is used as a source of energy during embryonic
development.
In the food industry, oils are artificially hydrogenated to make
them semi-solid, leading to less spoilage and increased shelf
life.
hydrogen gas is bubbled through oils to solidify them. During
this hydrogenation process, double bonds of the cis-
conformation in the hydrocarbon chain may be converted to
double bonds in the trans-conformation. This forms a trans-
fat from a cis-fat. The orientation of the double bonds affects
the chemical properties of the fat.

8. Essential fatty acids
Essential fatty acids are fatty acids that are required but not
synthesized by the human body. Consequently, they must be
supplemented through the diet. Omega-3 fatty acids fall into
this category and are one of only two known essential fatty
acids for humans (the other being omega-6 fatty acids).
Salmon, trout, and tuna are good sources of omega-3 fatty
acids. Omega-3 fatty acids are important in brain function and
normal growth and development. They may also prevent heart
disease and reduce the risk of cancer.

9. Phospholipids are the major constituent of the plasma
membrane. Like fats, they are composed of fatty acid chains
attached to a glycerol or similar backbone. Instead of three
fatty acids attached, however, there are two fatty acids and
the third carbon of the glycerol backbone is bound to a
phosphate group.
A phospholipid has both hydrophobic and hydrophilic
regions. The fatty acid chains are hydrophobic and exclude
themselves from water, whereas the phosphate is hydrophilic
and interacts with water.

10. Steroids and Waxes
steroids have a ring structure.
Although they do not resemble other lipids, they are grouped
with them because they are also hydrophobic.
Cholesterol is a steroid.
Cholesterol is mainly synthesized in the liver and is the
precursor of many steroid hormones.
It is also the precursor of vitamins E and K.
Cholesterol is the precursor of bile salts, which help in the
breakdown of fats and their subsequent absorption by cells.
Although cholesterol is often spoken of in negative terms, it is
necessary for the proper functioning of the body. It is a key
component of the plasma membranes of animal cells.

11. Waxes
 made up of a hydrocarbon chain with an alcohol (–OH)
group and a fatty acid.
 Examples of animal waxes include beeswax and
lanolin.
 Plants also have waxes, such as the coating on their
leaves, that helps prevent them from drying out.

12. [1] Lipids:
Biological Importance, Structures and Physical properties
 More soluble in organic solvents than water.
 Nonpolar lipids; (fats or oils) store energy (storage lipids).
(Oxidation of fats is highly exergonic.)
 Amphiphilic lipids (Polar); (phospholipids and sterols) help
make up biological membranes.
 Steroid Lipid; Steroids (found in membrane) are precursors
to many hormones
 Miscellaneous Lipids (other Lipids); Some are involved in
light-absorbing pigments, are enzyme cofactors (vitamin K),
hormones (estrogens, testosterone), signal molecules
(prostaglandins), electron carriers (ubiquinone) or help
solubilize other lipids during digestion (bile acids).

13. (1) Fatty acids: Carboxylic acids with hydrocarbon chains
(4-36 carbons)
R
C
O
OH
Saturated 18:0 Unsaturated 18:1(9)
[2] Storage Lipids: Fatty acids (FA) and derivatives

14.  Physiological Roles
-Precursors to glycerophospholipids and sphingolipids,
essential to biological membranes.
-Part of a lipoprotein (attached to a protein).
-Fuel, stored as triacylglcerols, metabolized for energy.
-Hormones and intracellular second messengers are
derivatives of FA.
-Na+ and K+ salts of FA are soaps – remove grease from
cloth by aggregating micelles.

15.  General Structure
pKa = 4.7
R-COOH R-COO‒
(At pH 7.0, in what form does it mostly exist?)
- Most FAs have an even number of C atoms in an
unbranched chain (usually 14-24, 16-18 is common)
- Saturated FAs: all carbon atoms “saturated” with H
C#  8: oily liquid (low m.p.)
C# ≥ 10: waxy solid (high m.p.)

17.  Unsaturated FA: are all oily liquids
- Monounsaturated FA: one double bond
- Polyunsaturated FA: two or more double bonds
- Fluidity of lipid bilayer alters as a function of the lipid
composition.
(the more unsaturated, lower m.p., the less rigid)
 16:0  16-carbon saturated palmitic acid
18:1  18-carbon oleic acid with one double bond
20:2(9,12)  a 20-carbon FA with one double bond
between C-9 and C-10 and another bet/ C-12 and C-13
cis
cis
C
OH
1
9
12
O

18. “Water off a duck’s back”
Tetramethyldecanoic acid is an example of
a branched-chain FA. (lower m.p.)
Produced in the uropygial gland of the duck
and used to preen its feathers
C
OH
1
O
Why 18:0 (solid at 25C) has higher melting point
than 18:1 (liquid at 25C)?
 Physical properties of FA depends on the chain length and
the number of double bonds.

19. [2] Storage Lipids: Fatty acids (FA) and derivatives
increase
decrease
increase
double
bonds
decrease
all cis

21. (2) Triacylglycerols (TAG, triglycerides, fats, or neutral fats):
Fatty acids joined by ester linkages that can be simple (all
same FA) or mixed (different FA)
The general formula is:
Nonpolar, essentially insoluble in water.
CH2O C
HC
CH2O
O
R1
C
O
R3
O C R2
O

22. Triacylglycerols serve as energy reservoirs in animals making
them the most abundant class of lipids and make up most fats
and oils in animals and plants.
 Stored in the adipocytes (fat cells) that can be almost entirely
filled with fat globules –thermal insulation.
(penguins, hibernating bears).
 Energy storage, fat or glycogen?
- Energy yield from (more reduced) lipids is two times that for
an equal weight of glycogen.
(Fat content of a normal human enables them to survive
starvation for 2-3 months, the glycogen supply would only offer a
day!)
- Unhydrated fats are lighter than hydrated carbohydrates.
 no extra weight to carry.
- But, readily soluble glycogen for instant access.

23. Oleic acid
a cis unsaturated fatty acid
that comprises 55–80%
of olive oil.
Elaidic acid
a trans unsaturated fatty acid
often found in partially
hydrogenated vegetable oils.
These fatty acids are geometric isomers
(chemically identical except for the arrangement of the double bond).

24.  Waxes: Esters of long-chain fatty acids (C14 ~ C36) with long
chain (C16 ~ C30) alcohols
- mp = 60 ~ 100C
- Energy storage form in plankton.
- Water repellent in birds, plants.
- Wide applications: ointments, polishes, etc.

25. [3] Structural Lipids in Membranes
All membrane lipids are amphipathic:
hydrophilic head + hydrophobic (bulky) tail
(1) Glycerophospholipids: belong to phospholipids
 glycerol + 2 FA (C1 & C2) + phosphate (C3)
 The major component of biological membranes

26. (2) Sphingolipids
 Major membrane components made from sphingosine
- the FA may differ.
 Sphingomyelin is prominent in myelin, a membranous sheath
that surrounds/insulates axons of some neurons.
 Prominent in membranes of neurons.
 Some serve as recognition sites on the cell surface
 Human Blood Type