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3.1.3 Lipids

AQA A-level Biology 3.1.3 practice on Lipids, with free MCQs, clues and worked explanations drawn from the Biological molecules section of specification 7402.

Specification route
3.1.3
Question bank
22 questions
Course stage
AS / Year 12

Sample questions

QUESTION 1 · 3.1.3 · LEVEL 3

Which structural feature differentiates an unsaturated fatty acid from a saturated fatty acid?

  • The presence of at least one carbon-carbon double bond ($C=C$) in the hydrocarbon chain.
  • The substitution of a fatty acid tail with a phosphate-containing group.
  • The presence of multiple carboxyl ($COOH$) groups along the hydrocarbon chain.
  • A larger number of hydrogen atoms attached to the carbon skeleton.
Show clue

'Unsaturated' refers to the carbon chain not holding its maximum possible number of hydrogen atoms.

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Answer: The presence of at least one carbon-carbon double bond ($C=C$) in the hydrocarbon chain.

The R-group of a fatty acid may be saturated (only single bonds) or unsaturated (contains one or more carbon-carbon double bonds, which cause kinks in the chain).

QUESTION 2 · 3.1.3 · LEVEL 3

A student wants to perform the emulsion test for lipids. What is the correct sequence of steps?

  • Shake the sample with ethanol to dissolve the lipid, then pour the mixture into water to observe a milky emulsion.
  • Add water to the sample, heat to boiling, then add ethanol to observe a colour change.
  • Dissolve the sample in dilute acid, neutralise, and shake with water until an emulsion forms.
  • Add Sudan III stain to the sample and observe a red layer separating at the top.
Show clue

Lipids are soluble in organic solvents but not in water.

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Answer: Shake the sample with ethanol to dissolve the lipid, then pour the mixture into water to observe a milky emulsion.

The emulsion test involves dissolving the lipid in ethanol (since lipids are not soluble in water) and then pouring the solution into water. The lipid precipitates, forming a milky-white emulsion.

QUESTION 3 · 3.1.3 · LEVEL 4

Why do phospholipids form a bilayer in an aqueous environment?

  • They have a hydrophilic phosphate head and two hydrophobic fatty acid tails.
  • They are highly non-polar and repel water molecules entirely.
  • The three fatty acid tails intertwine to exclude water from the molecule.
  • They contain cholesterol which forces them into a double-layered structure.
Show clue

The substitution of one fatty acid for a phosphate group changes how the molecule interacts with water.

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Answer: They have a hydrophilic phosphate head and two hydrophobic fatty acid tails.

In phospholipids, one of the fatty acids is substituted by a phosphate-containing group. This phosphate head is hydrophilic (attracts water), while the remaining two fatty acid tails are hydrophobic (repel water), leading to the formation of a bilayer in water.

QUESTION 4 · 3.1.3 · LEVEL 2

What are the components that condense to form a single triglyceride molecule?

  • One molecule of glycerol and three molecules of fatty acid
  • Three molecules of glycerol and one molecule of fatty acid
  • One molecule of glycerol, two molecules of fatty acid, and a phosphate group
  • Three molecules of glycerol and three molecules of fatty acid
Show clue

The prefix 'tri-' indicates the number of fatty acid tails.

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Answer: One molecule of glycerol and three molecules of fatty acid

Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid.

QUESTION 5 · 3.1.3 · LEVEL 1

Which bond is formed by a condensation reaction between glycerol and a fatty acid ($RCOOH$)?

  • Ester bond
  • Glycosidic bond
  • Peptide bond
  • Phosphodiester bond
Show clue

This bond links the hydroxyl group of glycerol with the carboxyl group of the fatty acid.

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Answer: Ester bond

A condensation reaction between glycerol and a fatty acid (RCOOH) forms an ester bond.

QUESTION 6 · 3.1.3 · LEVEL 2

How does the structure of a phospholipid differ from that of a triglyceride?

  • One of the fatty acids of a triglyceride is substituted by a phosphate-containing group
  • The glycerol molecule is substituted by a phosphate-containing group
  • All three fatty acids are substituted by phosphate-containing groups
  • A phospholipid contains two glycerol molecules instead of one
Show clue

Phospholipids form bilayers because they have a hydrophilic 'head' and two hydrophobic 'tails'.

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Answer: One of the fatty acids of a triglyceride is substituted by a phosphate-containing group

In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group.

QUESTION 7 · 3.1.3 · LEVEL 3

How many molecules of water are eliminated during the formation of a single triglyceride molecule?

  • Three
  • One
  • Two
  • Four
Show clue

One water molecule is eliminated for every ester bond formed.

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Answer: Three

Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid. Because three fatty acids bond to one glycerol, three condensation reactions occur, eliminating three water molecules.

QUESTION 8 · 3.1.3 · LEVEL 3

Phospholipids have different properties compared to triglycerides. How is this related to their structure?

  • The phosphate group makes one end of the molecule hydrophilic, while the two fatty acid tails are hydrophobic.
  • The phosphate group provides three strong bonds that make the molecule entirely rigid in water.
  • Phospholipids lack a glycerol backbone, allowing them to dissolve completely in water.
  • The presence of three saturated fatty acids in a phospholipid repels all water molecules.
Show clue

Think about how cell membranes are structured (bilayers).

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Answer: The phosphate group makes one end of the molecule hydrophilic, while the two fatty acid tails are hydrophobic.

The different properties of triglycerides and phospholipids related to their different structures. In phospholipids, one fatty acid is substituted by a phosphate-containing group, which is hydrophilic and alters how the molecule interacts with water compared to fully hydrophobic triglycerides.

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