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