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LipidsAQA A-Level Biology: Revision notes

Section 1

Two groups of lipid

Triglycerides and phospholipids are two groups of lipid. Both contain glycerol and fatty acids, but the two groups have different structures and so different properties.

Lipids are insoluble in water and soluble in ethanol, which is why the emulsion test works.

Key termslipidtriglyceridephospholipid

Section 2

Fatty acids: saturated and unsaturated

A fatty acid has the formula R–COOH: a carboxyl group and a hydrocarbon R group.

  • Saturated: no C=C double bonds in the R group. Every carbon is bonded to the maximum number of hydrogens, for example palmitic acid, CH₃(CH₂)₁₄COOH.
  • Unsaturated: one or more C=C double bonds, for example oleic acid, CH₃(CH₂)₇CH=CH(CH₂)₇COOH.

To recognise them, look for C=C in the chain.

Key termssaturatedunsaturated
Exam tip

Saturated = no double bonds. 'Fully saturated with hydrogen.'

Section 3

Forming a triglyceride

A triglyceride forms by condensation of one glycerol and three fatty acids. Each reaction between the –OH of glycerol and the –COOH of a fatty acid forms an ester bond and eliminates one water molecule, so three waters are released.

Worked example: glycerol 92 and three palmitic acids 3 × 256 = 768. Total 860 minus 3 × 18 = 54 gives 806.

Key termsester bond

Section 4

Phospholipids

In a phospholipid one fatty acid of a triglyceride is replaced by a phosphate-containing group. The molecule has two parts:

  • a hydrophilic head (phosphate group), attracted to water
  • two hydrophobic tails (fatty acids), repelled by water.

In water, phospholipids arrange themselves with the heads facing the water and the tails facing away, forming a bilayer, as in the cell surface membrane.

Key termshydrophilichydrophobic

Section 5

Properties related to structure

Triglycerides:

  • High ratio of C–H bonds to carbon atoms, so a high energy store per gram.
  • High ratio of H to O, so a source of metabolic water.
  • Hydrophobic, so insoluble and no effect on water potential (osmosis).
  • Also insulate and protect organs.

Phospholipids:

  • Hydrophilic head and hydrophobic tails, so they form bilayers (membranes) and separate watery compartments.
Key termsmetabolic water
Common mistake

Do not say a triglyceride has a 'phosphate head'. Only a phospholipid has a phosphate-containing group.

Section 6

Emulsion test for lipids

  1. Shake the sample with ethanol so that any lipid dissolves.
  2. Pour the solution into water.
  3. A cloudy white emulsion shows that lipid is present. If the mixture stays clear, there is no lipid.

The lipid is insoluble in water, so it forms tiny droplets that scatter light.

Key termsemulsion test

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Lipids

  1. Fatty acids have the general formula R–COOH, where R is a hydrocarbon chain. Palmitic acid is CH₃(CH₂)₁₄COOH. Oleic acid is CH₃(CH₂)₇CH=CH(CH₂)₇COOH.
    Explain why oleic acid is described as an unsaturated fatty acid.2 marks
  2. A triglyceride is made from one molecule of glycerol and three molecules of palmitic acid. To the nearest whole number the relative formula masses are: glycerol 92, palmitic acid 256 and water 18.
    Describe how the emulsion test could be used to show that a sample contains a triglyceride.2 marks
  3. Phospholipids and triglycerides both contain fatty acids joined to glycerol. In a phospholipid one fatty acid is replaced by a phosphate-containing group. Phospholipids form the cell surface membrane, whereas triglycerides are stored in fat cells.
    Describe how the structure of a phospholipid differs from the structure of a triglyceride.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).