3.1.3 Lipids

Question 1

What type of bond is made from a condensation reaction between glycerol and a fatty acid molecule?

Answer 1

An ester bond

Question 2

Describe how you would test for lipids.

Answer 2

• Shake the test substance with ethanol for about a minute so that it dissolves, then pour the solution into water.
• Any lipid will show up as a milky emulsion.
• The more lipid there is, the more noticeable the milky colour will be.

Question 3

Triglycerides have a hydrophobic tail. Explain how this feature of a lipid is important for its function. [2 marks]

Answer 3

• The hydrophobic tails force them to clump together in the cytoplasm as insoluble droplets.
• This means they can be stored in cells, as a source of energy, without affecting the cell’s water potential.

Question 4

Cell membranes contain phospholipids. Describe the structure of a phospholipid. [3 marks]

Answer 4

• Two fatty acid molecules…
• …and a phosphate group…
• …attached to one glycerol molecule.

Question 5

Cell membranes contain phospholipids. Explain the difference between a saturated fatty acid and an unsaturated fatty acid. [2 marks]

Answer 5

• Saturated fatty acids don’t have any double bonds between their carbon atoms.
• Unsaturated fatty acids have one or more double bonds between their carbon atoms.

Question 6

Give the functions of a lipid.

Answer 6

Lipids…
…form parts of cell membranes.
…are insoluble in water
…act as minor energy supply