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Lipids Notes
 Lipids are formed by condensation reactions between fatty acids and an alcohol
Fatty Acids
 Contain the acidic carboxyl –COOH group.
 Have the general formula R.COOH where R is hydrogen or a group such as CH3, C2H5, or
C3H7 and so on.
 Usually many carbons in the fatty acids used to make lipids.
 Most naturally occurring fatty acids have an even number of carbons ranging from 14 to 22.
 Fatty acids have a characteristically long chain of carbon and hydrogen atoms forming a
hydrocarbon tail.
 The tails determine many of their properties including insolubility in water as they are
hydrophobic.
 Sometimes contain one or more double bonds and so are unsaturated e.g. oleic acid.
 Unsaturated fatty acids (e.g. oleic acid) melt at much lower temperatures than saturated
fatty acids (e.g. stearic acid).
Alcohols
 Most lipids are triglycerides and so are made from the alcohol, glycerol.
 Glycerol has three hydroxyl groups, all of which can condense with a fatty acid.
 Triglycerides are the most common lipids in nature and are classified as fats or oils
depending on whether they are solid or liquid at room temperature.
 Triglycerides are non-polar so they do not hydrogen bond with water and are therefore
insoluble in it – hydrophobic.
 Less dense than water and therefore float.
 Tails vary in length according to the fatty acid used.
 Major function of lipids is to act as energy stores. They have a higher calorific content than
carbohydrates (i.e lipids will yield more energy on oxidation) as they contain more hydrogen.
 Animals store extra fat for hibernating and fat is also found beneath the dermis of
vertebrates for insulation. This is extensive in mammals such as whales etc. It also
contributes to buoyancy.
 Plants usually store oils rather than fats. Seeds, fruits and chloroplasts are often rich in oils
and some seeds are commercial sources of oils.
 When fats are oxidised, water is a product. This metabolic water is very useful to desert
animals such as the camal and kangaroo rat.
Phospholipids
 Lipids containing a phosphate group.
 Commonest type is where one of the –OH groups of the glycerol combines with phosphoric
acid instead of a fatty acid. The molecule consists of a phosphate head with 2 hydrocarbon
tails from the two fatty acids. The phosphate head carries an electric charge and is therefore
hydrophilic so is soluble in water. The tails are hydrophobic and insoluble in water.
 Important in the structure and function of cell membranes.
Glycolipids
 Lipid and carbohydrate association
 Carbohydrate forms a polar head to the molecule.
 Found in cell membranes.
Cholesterol
 Made of 4 carbon rings
 Found in all biological membranes
 Small and hydrophobic so can sit between phospholipid hydrocarbon tails and regulate
fluidity and strength of the membrane. .
 Steroid hormones oestrogen and testosterone are made from cholesterol.
 Vitamin D is made from cholesterol.
 Cholesterol is made mainly in the liver. Too much in bile can form gall stones. Too much in
blood can lead to atherosclerosis.
 FCH = Familial Hypercholesterolaemia. This is a genetic disorder where cells manufacture
and secrete cholesterol even though there is already sufficient amounts in the blood. This
happens because the cells lack a cell surface receptor that detects levels of cholesterol in
the blood.