3.1.3 Lipids

State two main types of lipid. (2 marks)

- Triglycerides.

- Phospholipids.

Explain why lipids are not considered polymers. (2 marks)

- They are not made from a small repeating unit (monomers).

- They are macromolecules (made of two or more different molecules).

Describe the structure of a fatty acid (RCOOH). (2 marks)

- A variable hydrocarbon chain, which can be saturated or unsaturated.

- A carboxyl (-COOH) group.

Explain the difference between saturated and unsaturated fatty acids. (2 marks)

- Saturated fatty acids have no C=C double bonds, so each carbon is fully bonded to hydrogen.

- Unsaturated fatty acids have one or more C=C double bonds, which cause a bend or kink in the chain.

Describe how triglycerides are formed. (4 marks)

- One glycerol molecule bonds with three fatty acid molecules.

- This involves three condensation reactions.

- Three water molecules are released.

- Three ester bonds are produced.

Draw a diagram for the formation of a triglyceride. (3 marks)

Explain how the structure of triglycerides relates to their properties. (4 marks)

- They have many C-H bonds in their hydrocarbon chains, releasing large amounts of energy when respired.

- Their fatty acid chains are non-polar and hydrophobic, making them insoluble in water.

- Insolubility means they do not affect the water potential of cells.

- Can form droplets for energy storage or waterproofing.

Describe how a phospholipid is formed. (3 marks)

- One glycerol, two fatty acids and a phosphate group.

- Joined together by an ester bond.

- During a condensation reaction.

Draw a diagram of a phospholipid molecule (3 marks)

Explain how the structure of phospholipids is linked to their function. (3 marks)

- They form a bilayer in cell membranes.

- The phosphate head is hydrophilic and faces the water inside and outside the cell.

- The fatty acid tails are hydrophobic and face away from water, towards the inside of the bilayer.

Draw a diagram of the phospholipid bilayer. (3 marks)

Describe the test for lipids. (2 marks)

1. Add ethanol and shake to dissolve the lipid, then add water.

2. A positive result is shown by a white, milky emulsion.

State the two methods that can be used to hydrolyse lipids. (2 marks)

- Heat with acid or alkali.

- Use lipase at its optimum temperature and pH.

State the structural similarities between triglycerides and phospholipids. (4 marks)

- Both have a glycerol backbone.

- Both contain ester bonds.

- Both contain the elements C, H, O.

- Both are formed by condensation reactions.

State the structural differences between triglycerides and phospholipids. (3 marks)

- Phospholipids contain two fatty acids whereas, triglycerides contain 3 fatty acids.

- Phospholipids contain one phosphate group whereas, triglycerides do not contain a phosphate group.

- Phospholipids have a hydrophilic head and a hydrophobic tail whereas, triglycerides are hydrophobic.

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