Carbohydrates: AQA A Level Biology Exam Questions by Topic

AQACarbohydratesExam Questions
Written by Tom Whitburn

Carbohydrates is one of the first topics you meet in AQA A Level Biology (section 3.1.2), and it keeps coming back: on AS Paper 1, on A-level Paper 1, and as the background to questions on digestion, respiration and plant cells. You need the monosaccharides and the disaccharides they make, how condensation forms a glycosidic bond, the difference between α-glucose and β-glucose, how starch, glycogen and cellulose are built, why their structures suit their jobs, and the biochemical tests for sugars and starch.

Most marks here are lost on precise wording rather than on big ideas. Students write "broken down" when the mark scheme wants "hydrolysed", say that starch is compact without saying it is coiled, list the features of starch and cellulose without comparing them, or describe the Benedict's test without the word "heat". The examiners' reports for these questions say so again and again.

Work through the questions in order: recall first, then new molecules you haven't seen before, then the food tests. Answer each on paper before you open the mark scheme. If you'd like help with this topic, my AQA masterclasses run online on Tuesdays at 5.15pm for Year 12 and on Wednesdays or Thursdays at 6.30pm for Year 13.

What these questions cover (AQA 3.1.2)

The questions test these statements from the AQA AS and A-level Biology specification (7401/7402):

  • Monomers: Monosaccharides are the monomers from which larger carbohydrates are made. Glucose, galactose and fructose are common monosaccharides.
  • Glycosidic bonds: A condensation reaction between two monosaccharides forms a glycosidic bond.
  • Disaccharides: Disaccharides are formed by the condensation of two monosaccharides.
  • Maltose: maltose is a disaccharide formed by condensation of two glucose molecules
  • Sucrose: sucrose is a disaccharide formed by condensation of a glucose molecule and a fructose molecule
  • Lactose: lactose is a disaccharide formed by condensation of a glucose molecule and a galactose molecule.
  • Isomers: Glucose has two isomers, α-glucose and β-glucose.
  • Polysaccharides: Polysaccharides are formed by the condensation of many glucose units.
  • Glycogen and starch: Glycogen and starch are formed by the condensation of α-glucose.
  • Cellulose: Cellulose is formed by the condensation of β-glucose.
  • Structure and functions: The basic structure and functions of glycogen, starch and cellulose.
  • Structure related to function: The relationship of structure to function of these substances in animal cells and plant cells.
  • Biochemical tests: Biochemical tests using Benedict's solution for reducing sugars and non-reducing sugars and iodine/potassium iodide for starch.

How to use this page

Write your answer on paper before you open the mark scheme. Allow about 1.3 minutes per mark: the 18 questions are worth 42 marks, so about 55 minutes in total. Then mark your answer strictly against the mark scheme and read the examiners' tip underneath it. If you'd rather print the questions, download the PDF pack.

Monosaccharides and disaccharides (3.1.2)

Question 1

A level Paper 1 · June 2024 · Q04.1 · 1 mark · Easy

Count the different monosaccharides in a mixture of sucrose, glucose and fructose.

Question text: molasses contains sucrose, glucose and fructose; give the number of different types of monosaccharides present.
Show the mark scheme Mark scheme: 2 (glucose and fructose).

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From the examiners' report: Three-quarters of students got this. Sucrose is a disaccharide, so it doesn't count: only glucose and fructose are monosaccharides.

Question 2

A level Paper 1 · November 2020 · Q07.1 · 2 marks · Easy

Give the formula of a disaccharide made from two galactose molecules and say how it forms.

Question text: alpha-gal is made of two galactose molecules, formula C6H12O6; give its formula and describe how it forms, with lines for Formula and Description.
Show the mark scheme Mark scheme: C12H22O11; condensation reaction or a glycosidic bond. Reject any other named reaction or bond, or the addition of water.

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From the examiners' report: Nearly everyone knew condensation and the glycosidic bond, but only 49% got the formula. Add the two formulae, then take away one H2O.

Question 3

AS Paper 1 · November 2020 · Q01.2 · 1 mark · Hard

Work out the number of C, H and O atoms in a trisaccharide made of three hexoses.

Question text: raffinose is a trisaccharide of galactose, glucose and fructose, each C6H12O6; give the numbers of carbon, hydrogen and oxygen atoms.
Show the mark scheme Mark scheme: C = 18, H = 32, O = 16. Only these answers accepted.

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From the examiners' report: Surprisingly few students got this mark. Three monosaccharides are joined by two glycosidic bonds, so two molecules of water are removed.

Question 4

A level Paper 1 · June 2022 · Q02.2 · 1 mark

Name a second disaccharide made from two glucose molecules.

Question text: trehalose is a disaccharide formed from two glucose molecules; name another disaccharide formed from two glucose molecules.
Show the mark scheme Mark scheme: maltose. Reject maltase.

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From the examiners' report: 85% got this. Check the spelling: "maltase" is the enzyme and is rejected.

Question 5

A level Paper 1 · June 2025 · Q03.1 · 3 marks · Easy

Tick which features are found in glycogen, sucrose and DNA.

Table with columns glycogen, sucrose and DNA and rows: contains glucose, galactose, fructose, phosphate; formed by condensation reactions; is a polymer.
Show the mark scheme Mark scheme table: glycogen contains glucose, is formed by condensation and is a polymer; sucrose contains glucose and fructose and is formed by condensation; DNA contains phosphate, is formed by condensation and is a polymer.

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From the examiners' report: About 45% got all three columns right. Work down each column: sucrose is glucose plus fructose, and it is not a polymer.

Question 6

A level Paper 1 · June 2018 · Q10.3 · 4 marks · Medium

Describe how lactose is formed and where it is added to a polypeptide in the cell.

Question text: a glycoprotein in mucus has lactose attached; describe how lactose is formed and where in the cell it is attached to a polypeptide.
Show the mark scheme Mark scheme: glucose and galactose; joined by condensation; by a glycosidic bond; added to the polypeptide in the Golgi apparatus.

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From the examiners' report: Most knew glucose, galactose and condensation, but some left out the glycosidic bond. Fewer knew the Golgi apparatus; answer "where in the cell", not where on the polypeptide.

Question 7

AS Paper 2 · June 2022 · Q02.1 · 1 mark · Medium

Give the term for the different structures of α-glucose and β-glucose.

Question text: what term describes the different structures of alpha-glucose and beta-glucose?
Show the mark scheme Mark scheme: isomer(ism). Ignore "structural".

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From the examiners' report: Only just over 40% scored, and 14% didn't try. α-glucose and β-glucose are isomers.

Polysaccharides: starch, glycogen and cellulose (3.1.2)

Question 8

AS Paper 2 · June 2023 · Q03.1 · 2 marks · Medium

Explain why polymers of galactose are called polysaccharides.

Question text: galacto-oligosaccharides (GOS) are polymers of galactose; explain why GOS are described as polysaccharides.
Show the mark scheme Mark scheme, any two: galactose is a monosaccharide; a polysaccharide is a carbohydrate polymer; many galactose monomers joined by condensation or by glycosidic bonds.

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From the examiners' report: Over two-thirds scored at least 1 mark. Marks were lost for vague talk of polymers and monomers; name galactose and the glycosidic bonds.

Question 9

AS Paper 2 · June 2023 · Q03.2 · 2 marks

Give two differences between the structures of GOS and lactose.

Question text: give two differences between the structures of GOS and lactose, with two numbered answer lines.
Show the mark scheme Mark scheme, any two: lactose contains glucose and galactose but GOS only galactose; lactose is a disaccharide and GOS a polysaccharide; lactose has one glycosidic bond and GOS many.

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From the examiners' report: Many students were confused about lactose, even though they were told GOS is a polymer of galactose. Write each difference as a comparison of both molecules.

Question 10

A level Paper 1 · June 2017 · Q02.1 · 2 marks

Describe the structure of glycogen.

Question text: cells lining the uterus contain a high concentration of glycogen in early pregnancy; describe the structure of glycogen.
Show the mark scheme Mark scheme: polysaccharide or polymer of α-glucose; joined by glycosidic bonds or branched structure.

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From the examiners' report: There's no examiners' report for this paper. The mark scheme wants α-glucose by name, then glycosidic bonds or branching.

Question 11

A level Paper 1 · June 2022 · Q09.2 · 6 marks · Medium

Compare and contrast the structures of starch and cellulose (6 marks).

Question text: compare and contrast the structure of starch and the structure of cellulose, 6 marks.
Show the mark scheme Mark scheme: both polysaccharides of glucose; both have glycosidic bonds; both contain C, H and O; starch α-glucose and cellulose β-glucose; starch helical and cellulose straight; starch branched and cellulose unbranched; cellulose forms microfibrils. Statements must be comparative.

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From the examiners' report: A quarter scored 5 or 6. Lists of features that weren't linked into comparisons scored nothing, and long answers about function didn't answer the question.

Question 12

AS Paper 1 · June 2025 · Q07.2 · 3 marks · Medium

Explain three features of starch that make it a good storage molecule.

Question text: in plant storage tissues, carbohydrate from the phloem is converted into starch; explain three features of starch that make it a good storage molecule.
Show the mark scheme Mark scheme, any three: insoluble so doesn't affect water potential; coiled or helical so compact; many α-glucose for respiration; branched so fast hydrolysis; large so can't leave the cell.

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From the examiners' report: About 25% got all three. Each mark needs a feature and its explanation; many said "compact" without linking it to the coiled or helical shape.

Applying it to new molecules (3.1.2)

Question 13

AS Paper 1 · June 2018 · Q02.2 · 2 marks

Give one similarity and one difference between lactulose and lactose.

Question text: lactulose is a disaccharide of galactose and fructose; give one similarity and one difference between the structures of lactulose and lactose.
Show the mark scheme Mark scheme: similarity, both contain galactose or a glycosidic bond; difference, lactulose contains fructose whereas lactose contains glucose.

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From the examiners' report: Students did well on the similarity but often lost the difference by writing only "lactulose contains fructose". State both sides of the difference.

Question 14

A level Paper 1 · June 2023 · Q02.1 · 3 marks · Easy

Use a diagram of chitin to describe three ways it is similar to cellulose.

Question text with Figure 1: the chitin monomer, a beta-glucose with one OH replaced by N(Ac), and a chain of four monomers joined by oxygen bridges with alternate monomers inverted; describe three ways chitin is similar to cellulose.
Show the mark scheme Mark scheme, any three: alternate monomers flipped or rotated; joined by glycosidic bonds; straight, unbranched chains. Also 1-4 linkages or β-glucose; reject 1-6.

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From the examiners' report: About 40% got full marks. Say "monomers", not "molecules", and don't add hydrogen bonds or 1-6 bonds, which aren't in the figure.

Biochemical tests for sugars and starch (3.1.2)

Question 15

A level Paper 1 · June 2024 · Q04.2 · 3 marks

Describe the test for reducing sugars and explain the result for molasses.

Question text: a student tests a clear sample of molasses for reducing sugars; describe the test and explain the expected result, with lines for each.
Show the mark scheme Mark scheme: heat with Benedict's; red colour or precipitate; because glucose or fructose are reducing sugars. Reject heating with acid, or saying sucrose is detected.

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From the examiners' report: Most described the test well, but some missed "heat" or wrote only "water bath". Few explained the result: glucose and fructose give the positive result, not sucrose.

Question 16

AS Paper 1 · November 2020 · Q01.3 · 3 marks

Describe a test to show that a solution contains a non-reducing sugar.

Question text: the reducing sugar test is negative with raffinose solution; describe a biochemical test to show it contains a non-reducing sugar.
Show the mark scheme Mark scheme: heat with acid and neutralise; heat with Benedict's; red precipitate or colour.

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From the examiners' report: Many scored one mark but few wrote the steps clearly and in order. Acid and heat, neutralise, then heat with Benedict's again.

Question 17

AS Paper 2 · June 2022 · Q02.2 · 1 mark · Hard

Say how colorimeter readings show which fruit juice has more sugar.

Question text: Benedict's test on two fruit juices followed by a colorimeter; describe how the colorimeter results identify the juice with the higher sugar content.
Show the mark scheme Mark scheme: higher absorbance, or lower transmission, means more sugar.

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From the examiners' report: Only 20% scored. Too many described the red colour of a positive test; the question is about what the colorimeter reads.

Question 18

AS Paper 2 · June 2022 · Q02.3 · 2 marks · Medium

Give two variables to control in a Benedict's test.

Question text: the student controlled variables in the Benedict's test; give two variables, with two numbered lines.
Show the mark scheme Mark scheme, any two: volume of Benedict's; concentration of Benedict's; volume of juice; temperature of the water bath; heating time.

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From the examiners' report: 43% got both marks. "Amount" isn't accepted at A level: say volume, concentration, temperature or time.

Where students lose marks (from AQA examiners' reports and mark schemes)

Each point below is followed by wording that gained marks.

  • Hydrolysed, not broken down: Explaining why yeast respires maltose more slowly than glucose, the most common reason for losing a mark was writing that maltose is "broken down" instead of hydrolysed.

    ✔ Gains credit: "Maltose is a disaccharide, so it has to be hydrolysed into glucose first" (or the glycosidic bond has to be broken).

  • Take the water out of the formula: Almost everyone knew that two galactose molecules join by condensation, but only 49% gave the right formula for the disaccharide. Many said water is lost and then didn't take it away.

    ✔ Gains credit: "C12H22O11, formed by a condensation reaction, with a glycosidic bond."

  • State the difference in full: Asked for a difference between lactulose and lactose, many wrote only "lactulose contains fructose". The mark scheme says a difference must be stated, not implied.

    ✔ Gains credit: "Lactulose contains fructose, whereas lactose contains glucose."

  • Helix, so compact: In starch questions many students said starch is compact but didn't link this to its shape. Each mark needs the feature and the reason.

    ✔ Gains credit: "Coiled/helical, so compact"; "insoluble, so doesn't affect water potential"; "branched, so more ends for fast hydrolysis".

  • Link each comparison: In "compare and contrast" questions on starch and cellulose, answers that listed the structures separately scored nothing, and some confused cellulose microfibrils with muscle myofibrils.

    ✔ Gains credit: "Starch has α-glucose and cellulose has β-glucose"; "starch is branched and cellulose is unbranched"; "both contain glycosidic bonds".

  • No "strong hydrogen bonds": Students often explained the strength of cellulose by "strong hydrogen bonds", which the mark scheme penalises. Each hydrogen bond is weak; it's the large number of them that matters.

    ✔ Gains credit: "Long, straight chains of β-glucose joined by hydrogen bonding to form microfibrils, which give strength."

  • Heat it with Benedict's: Some answers left out heating, or said only "use a water bath" with no sign that it was hot. Others described the non-reducing sugar test when asked about reducing sugars, or said sucrose would be detected.

    ✔ Gains credit: "Heat with Benedict's solution; red precipitate, because glucose and fructose are reducing sugars."

  • Read the absorbance: Only 20% scored when asked how a colorimeter shows which juice has more sugar. Most described the red colour of a positive Benedict's test instead of the colorimeter reading.

    ✔ Gains credit: "Higher absorbance (or lower transmission) means more reducing sugar."

Download the questions

Download the PDF pack (18 questions, mark schemes at the end)

Keep practising

Questions, mark schemes and examiners' report extracts © AQA.

Tom Whitburn
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