Monomers, carbohydrates and lipidsAQA A-Level Biology: Topic test

1. 1
   A biochemist completely hydrolyses a sample of glycogen, a sample of RNA and a sample of a protein, each in a separate tube, and then identifies the products in each tube.

   (a)

   Which row shows the monomers released from glycogen, RNA and the protein?

   [1 mark]

   • AGlucose, amino acids, nucleotides
   • BNucleotides, glucose, amino acids
   • CGlucose, nucleotides, amino acids
   • DAmino acids, nucleotides, glucose

   (b)

   Which statement is true of all three hydrolysis reactions?

   [1 mark]

   • AA water molecule is used for each bond broken
   • BA water molecule is released for each bond broken
   • CEach reaction breaks peptide bonds
   • DEach reaction releases the same monomer

   (c)

   A molecule of glycogen contains 2400 glucose units. State the number of water molecules needed to hydrolyse it completely to glucose, and name the type of bond broken.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Milk contains the disaccharide lactose. Some people cannot digest lactose, so dairy products made for them have had the lactose hydrolysed before sale.

   (a)

   Which monosaccharides are produced when lactose is hydrolysed?

   [1 mark]

   • AGlucose and fructose
   • BGlucose and galactose
   • CTwo molecules of glucose
   • DGalactose and fructose

   (b)

   Which bond is broken when lactose is hydrolysed?

   [1 mark]

   • APeptide
   • BEster
   • CPhosphodiester
   • DGlycosidic

   (c)

   Lactose is a reducing sugar. State what is meant by a reducing sugar and give the observation when lactose is heated with Benedict's reagent.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A potato tuber stores carbohydrate as starch, whereas the stem tissue of a celery plant contains cellulose but no starch. A student adds iodine in potassium iodide solution to a thin slice of each tissue.

   (a)

   Predict the result for each tissue and explain the result for the celery.

   [3 marks]

   (b)

   Explain how the structure and properties of starch make it suitable for storage in the potato cells.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A meal of sweet potato and olive oil is eaten. Sweet potato contains starch and sucrose, and olive oil is mainly triglyceride. The products of digestion are absorbed, and some are used by body cells to build storage molecules.

   (a)

   Describe how the starch and triglyceride in the meal are broken down, and how the absorbed products are rebuilt into storage molecules.

   [6 marks]

   (b)

   Describe how you would test a sample of the sweet potato for starch, reducing sugar and non-reducing sugar, and how you would test the olive oil for lipid.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Linoleic acid, CH₃(CH₂)₄CH=CHCH₂CH=CH(CH₂)₇COOH, and stearic acid, CH₃(CH₂)₁₆COOH, are both fatty acids found in triglycerides.

   (a)

   Which statement about these fatty acids is correct?

   [1 mark]

   • ABoth are saturated
   • BLinoleic acid is saturated and stearic acid is unsaturated
   • CBoth are unsaturated
   • DLinoleic acid is unsaturated and stearic acid is saturated

   (b)

   Which group of a fatty acid forms an ester bond with glycerol?

   [1 mark]

   • AAmine group
   • BCarboxyl group
   • CMethyl group
   • DPhosphate group

   (c)

   A triglyceride made from three molecules of linoleic acid is liquid at room temperature, but a triglyceride made from three molecules of stearic acid is solid. Explain this difference.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Sucrose is a disaccharide with the molecular formula C₁₂H₂₂O₁₁. Relative atomic masses: C = 12, H = 1, O = 16.

   (a)

   What is the relative molecular mass of sucrose?

   [1 mark]

   • A342
   • B360
   • C324
   • D180

   (b)

   Which monosaccharides are produced when sucrose is hydrolysed?

   [1 mark]

   • ATwo molecules of glucose
   • BGlucose and galactose
   • CGlucose and fructose
   • DGalactose and fructose

   (c)

   Sucrose gives no colour change when heated with Benedict's reagent, but gives a brick-red precipitate if it is first boiled with dilute acid. Explain these results.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Adipose cells store energy as triglyceride. A mouse adipose cell holds 2.5 × 10⁹ molecules of triglyceride, each made from one molecule of glycerol and three molecules of fatty acid, in a droplet that does not mix with the cytoplasm.

   (a)

   Explain how a triglyceride molecule is formed from glycerol and fatty acids.

   [3 marks]

   (b)

   Calculate the number of water molecules eliminated when all the triglyceride molecules in the cell were made. Explain why the triglyceride forms a separate droplet rather than dissolving in the cytoplasm.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Plant cells are surrounded by a cell wall made mainly of cellulose and by a cell-surface membrane built from a phospholipid bilayer. A student states that both of these structures are made of polymers built by condensation reactions.

   (a)

   Discuss how far the student's statement is correct.

   [6 marks]

   (b)

   Explain why phospholipids form a bilayer in water and why this bilayer is a barrier to water-soluble ions.

   [6 marks]

   Total for question 8: 12 marks

End of questions