Biological Molecules - Lipids

Question 1

What are lipids?

Answer 1

• 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

Question 2

What are triglycerides?

Answer 2

• Lipids made of one molecule of glycerol and three fatty acids joined by ester bonds formed in a condensation reactions
• Over 70 different types of fatty acids which vary in cahin length, presence and number of double bonds

Question 3

What are saturated lipids?

Answer 3

• Lipids that don’t contain any carbon-carbon double bonds
• E.g. animal fats

Question 4

What are unsaturated lipids?

Answer 4

• Lipids that contain carbon-carbon double bonds
• The presence of a double bond means that the molecule is able to bend
• As a result unsaturated fats cannot pack together as tightly and are therefore liquid at room temperature

Question 5

How is the structure of triglycerides related to their properties?

Answer 5

• High ratio of energy storing carbon-hydrogen bonds to carbon atoms and therefore 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 an important source of water for organisms to live in dry environments

Question 6

What are phospholipids?

Answer 6

• One of the fatty acids of a triglyceride is substituted by a phosphate containing group
• Phosphate heads are hydrophilic (attracts water) and the tails are hydrophobic (repels water) and as a result phospholipids form micelles when they are in contact with water
• The molecule is therefore known as polar

Question 7

How are phospholipids structure related to their properties?

Answer 7

• 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 glycolipids with carbohydrates which are important on the cell surface membrane for cell recognition

Question 8

State and explain the biochemical test for the presence of lipids?

Answer 8

• An emulsion test
• Take a complementary grease free test tube and add 2cm3 of the sample to be tested and 5cm3 of ethanol
• Shake the test tube thoroughly to dissolve all the lipid in the solution
• Add 5cm3 of water and shake gently
• A cloudy-white colour indicates the presence of a lipid