Lipids - University of Winnipeg

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Transcript Lipids - University of Winnipeg

Welcome to class of
Lipids
Dr. Meera Kaur
Learning objectives
To understand
* various lipids by source, structure and use:
triglycerides, fats and oil and waxes
* the fluid mosaic model to describe the
movement of lipid molecule in membrane
* the source and functions of some sterols:
cholesterol. ergosterol etc.
Introduction
• Lipids (Greek: lipos, means fat or lard)
- are a heterogeneous class of naturally occurring organic
substances
- have a distinguished functional group or structural
features
- are insoluble in water and highly soluble in one or more
of the following solvents: ether, chloroform, benzene and
acetone.This property sets them apart from proteins,
carbohydrates,, nucleic acids and other biomolecules
- are widely distributed in the biological world
- play a wide variety of roles in plant and animal tissues
Functions of lipids
• Lipids are concentrated source of energy. One gram fat gives 9
K calories.
• It serves as a cushion for the vital organs and protects them
from external shocks or injuries.
• Lipids are the structural materials of cells and membranes
• Lipids serves as insulator for our body
• Lipids are the carrier / reservoir of fat soluble vitamins
• In food preparations lipids serves as a binding agent. It also
enhances the palatability of foods
Types of lipids
• Fats and oils:
Storage lipids
– are universally used as stored form of energy by living
organism. One gram of fat gives 9 K Calories
– are highly reduced compound, derivatives of fatty acids
– fatty acids are hydrocarbon derivatives, which are long
chain carboxylic acids containing up to 24 carbon atoms
– the most common fatty acids in plants and animals are the
even-numbered C16 and C18 species such as palmitate and
stearate
– fatty acid are classified as saturated (where all the carbon
are saturated with hydrogen) or unsaturated ( which
contain one or more double bonds)
Saturated Fatty acids
Stearic acid
The packing of fatty
acids depends on their
degree of saturation.
Stearic acid is shown
here in its usual
extended conformation.
Saturated fatty acids
are tightly packed and
stabilized by many
hydrophobic
interactions
Unsaturated Fatty acids
Oleic acid
The double bond
(shaded)introduces a
rigid bend in the
hydrocarbon tails. Fatty
acids with one or
several such bends
cannot pack together as
tightly as saturated fatty
acids
Triglycerides
• The fats and oils found in animals and plants
are triglycerides
• Triglycerides are:
– esters of fatty acids and glycerols
– simple lipids
– important as the storage form of fat in the human
body
Glycerol
Types of Triglycerides
• Simple triglycerides: They are triesters made from
glycerol and three molecules of one kind of fatty
acids. They are rare.
• Mixed triglycerides: They are triesters with different
fatty acid components. Animal fats and vegetable
oils are many different mixed triglycerides; e.g.,
Butterfat contains at least 14 different carboxylic
acids.
Triglycerides
Properties of Fats and Oils…
• Hydrogenation of fats and oils:
– The difference between fats and oils is:
• fats are solid in room temperature and oils are liquid in
room temperature.
– Hydrogenation is the process of converting
liquid oil to solid fat by adding hydrogen to some
of the double bond of the unsaturated carbon
chain in presence of nickel as catalyst; e.g.,
Margarine .
Properties of Fats and Oils…
• Rancidity:
– Triglycerides soon become rancid, developing an unpleasant
odor and flavor on exposure to moist air at room temperature.
Rancidity is caused by either hydrolysis of ester bond or
oxidation of double bond
– Hydrolytic rancidity is due to the hydrolysis of ester bond by the
enzyme lipase present in the airborne bacteria. It can be
prevented by storing food in refrigerator
– Oxidative rancidity is caused by the ruptured of the double
bond due to exposure to warmth air. As a result, aldehydes with
low molecular wt. are formed releasing off flavor; e.g., Off flavor
in cookies. Oxidative rancidity can be prevented by adding
antioxidant (Vitamin C, Vitamin E, -Tocopherol)
Properties of Fats and Oils
• Saponification of fats and oils:
– Hydrolysis of fats and oils by boiling with sodium
hydroxide is called saponification. This process is used to
make soap (Latin: sapon means soap). Soaps are the
alkali metal (Na, K or Li) salts of fatty acids.
– Glycerol is an important by-product of saponification
process. It is recovered by evaporating the water layer.
The crude soap is then purified, and coloring and
perfumes are added according to the market demands.
Waxes
• Waxes: Waxes are part of the lipid family. Waxes are esters of
long-chain saturated and unsaturated fatty acids (having 14-36
carbon atoms) with long-chain alcohol(having 16-30 carbon
atoms).
• Waxes are low- melting, stable solids which appear in nature in
both plants and animals. A wax coat protects surface of many
plant leaves from water loss and attack by microorganisms.
• Carnauba wax, a major ingredients of car wax and floor polish,
comes from the leaves of a South American palm tree. Beeswax
is largely myricyl palmitate , the ester of myricyl alcohol and
palmitic acid
• Waxes also coat skin, hairs and feathers, and help keep them
pliable and water-proof
Beeswax
The term ‘wax’ originates in the Old English word weax,
meaning the “material of the honeycomb”.
Structural lipids
• Chemical analysis of the isolated materials shows
that lipids are the major components of most
membranes. This lipids are not triglycerides but
another group of compound called complex
lipids.
• There are two types of complex lipids:
– Phospho lipids
– glycolipids
Complex Lipids…
* Phospholipids: They are esters of phosphoric
acids.There are two main types of phospholipids in cellular
membranes:
Phosphoglecerides: They are also known as Phosphaitdyl choline
(lecithin). They are built from long chain fatty acid, glycerol and
phosphoric acids.
sphingomyelins: They do not contain glycerol. Instead, they contain
sphingosine, a long-chain unsaturated amino alcohol. Only one fatty
acid is attached to the sphingisine. Sphingomylins are found in brain
and nervous tissue and in the myelin sheath, the protective coat of
nerves.
3-phosphoglycerol (building block for phosphoglycerides)
General structure of a glycerophospholipid. Note the
glycerol-3-phosphate backbone
Glycerophospholipids
Electron microscopy (EM) of myelinated nerve fibres.
Note spirally wrapped membranes around each nerve
axon. The myeline sheath may be 10-15 layers thick. Its
high lipid content makes it a good electrical insulator
Complex Lipids
Glycolipids: A lipid molecule that contains
carbohydrates, which is usually a simple sugar like
glucose or galactose
They are also called cerebrosides because of
their abundance in brain.
Membrane lipids
• Phospholipids: naturally aggregate in form of
bilayers (which fuse to form spherical liposomes)
– glycerophospholipids
– sphingophospholipids
• Sterols:
– e.g. cholesterol (animal sterol) ergosterol( plant
sterol).
Lipid bilayers
The fundamental component of a biological
membrane is lipid bilayer.
In a vigorously shaked mixture of phosphatidyl choline
and water, the lipid molecules form microscopic sphere.
These lipid sphere or liposome are packages of
solvent surrounded by a lipid bilayer- a two layer thick
wall of phosphatidyl choline
Electron micrograph of a liposome
Diagram indicating spherical micelle (b), and effect of crowding in too
many lipid molecules. The micelle could develop a water-filled centre
(c), or could flatten out to collapse the hollow centre but would
develop water-filled spaces (d)
Two-tailed amphipathic molecules are more
cylindrical, and tend to naturally form a bilayer
structure (as opposed to a micelle)
Model (snapshot) of a lipid bilayer surrounded by water
Fluid mosaic model
• The fluid mosaic model of membrane structure
proposes that lipids of the bilayer are in constant
motion, gliding from one part of their bilayer to
another at high speed
The fluid mosaic model of membrane structure
Steroids
• Steroids are a family of lipids found in plants and
animal
– A steroid contains four fused carbon ring: 17 carbon atoms
make the structural unit of steroids known as steroid
nucleus.
– Steroid nucleus is found in a number of extremely important
biological molecules: cholesterol, adrenocorticoid hormones,
the sex hormone and bile acids.
Cholesterol
• Cholesterol is found only in animal cells. A typical
animal cell membrane contains about 60%
phospholipids and 25% cholesterol.
• Cholesterol imparts rigidity to cell membrane. The
higher the % of chlesterol, the more rigid the
membrane.
• Bile acids contain cholesterol.
Cholestorol (membrane sterol)
Scanning EM of adipocytes: each contains a fat globule
that occupies nearly the entire cell
Sperm Whales
Triacylglycerols are
biologically useful.
Spermaceti organ
contains spermaceti oil,
a mixture of
triacylglycerol and
waxes. This oil is liquid
at 31degree C, and
becomes solid when
the temperature drops
several more degrees
O
C
O
(Z)-7-dodecenyl acetate
pheromone of
cabbage looper moth
and
asian elephant
CH3
Lipids in biomolecules