Lecture #4: Lipid Structure and Function
NURS 1012: Basic Biochemistry
Lipids
Lipids are related to fatty acids They include fats, oils and waxes
Fatty acids have the general formula R-COOH
where R = H or a hydrocarbon
The R group is therefore non polar and is hydrophobic
Fatty acids
The hydrocarbon chain of fatty acids sometimes contain
double bond(s) and are called unsaturated fatty acids
If the hydrocarbon chain contains no double bonds then the
fatty acid is saturated
Saturated Fatty Acids
http://academic.brooklyn.cuny.edu/biology/bio4fv/page/3fatty.gif
Unsaturated Fatty Acids
http://academic.brooklyn.cuny.edu/biology/bio4fv/page/3fatty.gif
Saturated fatty acids Unsaturated fatty acids
Solids at room temperature Liquid at room temperature
They are found in animals They are found in plants
Properties of fatty acids
Fatty acids
Unsaturated fatty acids from plants can have their double
bonds removed by adding hydrogen.
This is called hydrogenation and forms margarine a plant fat
solid at room temperature
Functions of Lipids
Lipids are a storage form of energy
They are insoluble and can act as food storage
They are involved in membrane structure involvement
They have high insulating capacity
They play a role in the absorption and transport of fatty acids
(phospholipids)
They play a role in hormone synthesis (cholesterol)
They are carriers of fat soluble vitamins (Vit. A, D, E)
Classification of Lipids
Lipids can be classified as –
(a) Simple lipids/homolipids – e.g. triglycerides (fats), oils
and waxes
(b) Compound lipids/ heterolipids – e.g. phospholipids
and glycolipids
(c) Derived Lipids - they are derived from simple and
compound lipids e.g. steroids
Simple Lipids: Triglycerides/Fats
Triglycerides are the commonest lipids in nature
They are non polar and are therefore insoluble in water
They are less dense than water and as a result are able to
float
They are formed via esterification i.e. a reaction between an
alcohol (glycerol) and a tricarboxylic acid. Therefore they
can undergo hydrolysis to form free glycerol and fatty acids
Diagram showing the condensation reaction between glycerol and a fatty acid (tricarboxylic acid)
Structure of triglyceride
Function of Triglycerides/Fats
Triglycerides store long term energy
They provide more energy per gram than proteins or
carbohydrates (4.9 kcal/gram)
They act as insulators for the body
They act as shock absorbers for organs
Compound Lipids: Phospholipids
Phospholipids are the most abundant membrane lipids
They are the structural components of the membrane
They differ from triglycerides having one polar head and two
hydrophobic tails
There are two types of phospholipids
(a) phosphoglycerides – most common
(b) sphingomyelin
The bipolar nature of phospholipids allows for the formation
of bimolecular sheets (bilayers)
http://www.bioteach.ubc.ca/Bio-industry/Inex/graphics/lipidbilayer.gif
Diagram of the phospholipid bilayer
http://courses.cm.utexas.edu/jrobertus/ch339k/overheads-2/ch12_lipid-bilayer.jpg
Compound Lipids: Glycolipids
Glycolipids are compounds comprised of lipids linked to
carbohydrates. The carbohydrate portion forms a polar head
Glycolipids are an important constituent of cell membranes,
particularly in the myelin sheath and outer surface of the
nerve cells and the chloroplast membrane
Both phospholipids and glycolipids form self-sealing lipid
bilayers that are the basis of all cellular membranes
http://www.ucl.ac.uk/~sjjgsca/CellMembraneComplex.gif
Derived Lipids: Cholesterol
Cholesterol is in the blood of all animals
Blood plasma levels range from 15 – 250mg/100mls
Cholesterol is acquired by the body in two ways -:
(a) They are produced by the cells of the body
(b) They are found in dietary sources e.g. meat and
dairy products
Sites of production for cholesterol include the liver,
gonads, adrenal glands, nervous tissues and intestines
Structure of Cholesterol
Derived Lipids: Cholesterol
Cholesterol is absent from plant cells, as a result vegetable
oil is considered as cholesterol free
It is an important component of the cell membrane
It is an important precursor of many biological compounds
e.g. bile acids and steroid hormones
The end products of cholesterol metabolism are steroids (bile
salts)
Bile salts are formed in the liver and secreted into the small
intestines to absorb lipids
When the bile salts are attached to the lipid the complex is
called a chylomicron
Derived Lipids: Cholesterol
Bile salts are largely reabsorbed during lipid absorption
Many steroid hormones are formed from cholesterol. These
include :-
(a) Aldosterone
(b) Progesterone
(c) Cortisol
(d) Testosterone
(e) Estradiol
Derived Lipids: Cholesterol
Cholesterol is insoluble in blood
It travels in the blood bound to lipoproteins
There are two types of lipoprotein
(a) low density lipoprotein (LDL)
(b) high density lipoprotein (HDL)
LDL is often referred to as bad cholesterol and HDL is often
referred to as good cholesterol
HDLs carry LDLs away from the walls of the artery
LDL sticks to the artery walls and can lead to plaque build up
(atherosclerosis)
Derived Lipids: Cholesterol
Higher proportion of LDL compared to HDL is associated
with cardiovascular diseases such as heart attack and stroke
http://www.abc.net.au/health/library/img/cholesterol_arteries.jpg
Diagram showing plaque formation in the arteries
Derived Lipids: Cholesterol
Diet and exercise help to maintain balance in the LDL and
HDL concentrations
There exist two important cholesterol diseases
(a) Familial hypercholesterolemia – This is an over production
of cholesterol
(b) Zanthomatosis – This is a build up of cholesterol in the
skin