AQA AS Biology Unit 1: Lipids

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  • Created on: 22-08-13 16:22
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Lipids
Lipids are a varied group of substances that share the following characteristics:
- They contain carbon, hydrogen and oxygen
- The proportion of oxygen to carbon and hydrogen is smaller than in carbohydrates
- They are insoluble in water
- They are soluble in organic solvents such as alcohols and acetone
Functions of Lipids
Energy source ­ provide more energy than carbohydrates and proteins
Waterproofing ­ both plants and insects have waxy cuticles that conserve water, while
mammals produce an oily secretion from the sebaceous gland in the skin
Insulation ­ fats conduct heat slowly and so are useful insulators when stored in the body.
Protection ­ fat is often stored around organs to protect them
Saturated
No double bonds between carbon atoms
Unsaturated
One or more double bond between carbon atoms
Mono-unsaturated
The double bonds cause the
One double bond between carbon atoms molecule to bend. They cannot
therefore pack together closely
making them liquid at room
Poly-unsaturated
temperature, i.e. they are oils
More than one double bond between carbon atoms

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Triglycerides
The main group of lipids are triglycerides (fats and oils), phospholipids and waxes.
Triglycerides are so called because they have 3 fatty acids combined with glycerol
Tri = 3
Glyceride = glycerol molecules
Glycerol is an alcohol containing 3 carbon atoms with an ­OH group.
A fatty acid is a molecule containing a long chain of hydrogen and carbon atoms
called a hydrocarbon.…read more

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Phospholipids
A phospholipid is a lipid in which one of the fatty acids is replaced by a phosphate
molecule.
The phosphate is hydrophilic ­ it attracts water
The phosphate group ionise which gives it a negative charge making it polar and so
attracts the water.
The fatty acid chain carries no charge and does not attract water molecules ­
hydrophobic.
Phospholipids therefore have one end attracted to water and the other repels water.
This property of phospholipids enables them to form cell membranes.…read more

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Test for Lipids
1. Clean test tube
2. Place sample in test tube
3. Add 5cm³ of ethanol
4. Shake tube until dissolved
5. Add 5cm³ of water
6. Shake gently
Results: cloudy ­ white suspension
The cloudy colour is due to any lipid in the sample being finely dispersed in the water to
form an emulsion.
Light passes through the emulsion and is refracted as it passes from oil to water
making it cloudy.…read more

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Sadam Hussain

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