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NURS 1012: Basic Biochemistry

Lecture #4: Lipid Structure and Function

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

Properties of fatty acidsSaturated fatty acids Unsaturated fatty acids

Solids at room temperature

Liquid at room temperature

They are found in animals

They are found in plants

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 (b) sphingomyelin

most common

The bipolar nature of phospholipids allows for the formation of

bimolecular sheets (bilayers)

Diagram of the phospholipid bilayer

http://www.bioteach.ubc.ca/Bio-industry/Inex/graphics/lipidbilayer.gif

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) (b)

They are produced by the cells of the body 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

Diagram showing plaque formation in the arteries

http://www.abc.net.au/health/library/img/cholesterol_arteries.jpg

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