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Lipids
Lipids are biological molecules made of carbon, hydrogen and oxygen which are only soluble in organic solvents such as alcohols. The main lipid types are triglycerides and phospholipids.
Triglycerides
Triglycerides are lipids made of one molecule of glycerol and three fatty acids joined by ester bonds formed in condensation reactions. There are many different types of fatty acids (over 70 different types) which vary in chain length, presence and number of double bonds.The presence of double bonds changes the name of the lipid, they can either be saturated or unsaturated. These are explained below:
•Saturated lipids such as those found in animal fats – saturated
lipids don’t contain any carbon-carbon double bonds.
•Unsaturated lipids which can be found in plants – unsaturated lipids contain a double bond.The presence of a double bond means that the molecule is able to bend. As aresult unsaturated fats cannot pack together as tightly and aretherefore liquid at room temperature.Triglyceride structure related to their properties
•High ratio of energy storing carbon-hydrogen bonds to carbon
atoms and therefore they are an excellent energy store.
• A low mass to energy ratio meaning that they are a good storage molecule, with a lot of
energy being stored in a small volume. This is beneficial for animals as it is less mass to move
around.
• Being large and non-polar lipids are insoluble in water and therefore their storage does not
affect the water potential of cells.
• A high ratio of hydrogen-oxygen atoms means that triglycerides release water when they are
oxidised and therefore provide and important source of water for organisms to live in dry
environments.
Phospholipids
In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group. Phosphate heads are hydrophilic (loves water) and the tails are hydrophobic (hates water) and as a result phospholipids form micelles when they are in contact with water. The molecule is therefore known as polar. Phospholipids structure related to their properties
• In an aqueous environment being polar means a bilayer can be formed.
• The hydrophilic heads of the phospholipids can be used to hold at the surface of the cell
surface membrane.
• Their structure allows them to form glycolipds with carbohydrates which are important on the
cell surface membrane for cell recognition.
Emulsion Test
An emulsion test can be used to test for the presence of lipids. This is carried out as follows:
1. Take a completely grease free test tube and add 2cm3 of the sample to be tested and
5cm3 of ethanol.
2. Shake the test tube thoroughly to dissolve all the lipid in the solution.
3. Add 5cm3 of water and shake gently.
4. A cloudy-white colour indicates the presence of a lipid.
5. As a control repeat the experiment using water as the sample, the final solution
should remain clear
These are my revision notes I would also do bit of your own revision as these are very condensed Hope this alright 👍