IGCSE Biology Biomolecules & enzymes topical questions

Complete the table about biological molecules.

1. (a)Complete the elements column for all three rows.[2]
2. (b)Complete the “built from” column for all three rows.[2]

Match each food test to its reagent and its positive result.

1. (a)State the reagent used to test for starch, and the positive result.[2]
2. (b)State the reagent used to test for vitamin C, and the positive result.[2]
3. (c)Name the condition Benedict’s solution must be given for a result to appear.[1]

A drink was tested with four food tests. The results are shown.

1. (a)State which nutrient the drink contains, using the results.[2]
2. (b)State which two nutrients the drink does NOT contain, using the results.[2]

Three solid foods are tested. The results are shown in the table.

1. (a)State which large molecules food X contains, and how you know from the table.[2]
2. (b)State which molecule food Y contains, and how you know.[1]
3. (c)Food Z gave a negative result for every test in the table. Name the molecule whose presence DCPIP would reveal, and give the positive result.[2]

A student investigates the effect of temperature on the enzyme amylase, which digests starch. Identical tubes of starch and amylase are kept at different temperatures. Every minute a drop is removed and tested with iodine. The time taken for iodine to stop turning blue-black is recorded.

1. (a)Calculate the rate of reaction at 30 °C, in s⁻¹, using rate = 1 ÷ time. Give your answer to 3 decimal places.[2]
2. (b)Describe the effect of temperature on the rate between 20 °C and 40 °C.[1]
3. (c)Suggest why the time is longer at 50 °C than at 40 °C.[2]
4. (d)State the term for the temperature at which this enzyme works fastest in these results.[1]

Explain enzyme action using the terms active site, complementary, enzyme-substrate complex, substrate and product.

1. (a)Describe what happens when a substrate and its enzyme meet.[3]
2. (b)Explain why each enzyme acts on only one substrate.[2]

The activity of an enzyme was measured across a range of pH values.

1. (a)State the optimum pH for this enzyme.[1]
2. (b)Describe the effect of pH on the enzyme’s activity in these results.[2]
3. (c)Explain the low rate at pH 10.[2]

Trypsin, a protein-digesting enzyme, was mixed with a protein suspension at different pH values. The time taken to clear the suspension was recorded.

1. (a)Calculate the rate of digestion at pH 8, in minutes⁻¹. Give your answer to two significant figures.[2]
2. (b)State the optimum pH for trypsin in this experiment.[1]
3. (c)Describe and explain the trend in rate between pH 6 and pH 9.[2]
4. (d)Explain the result at pH 4.[1]

A sample of double-stranded DNA contains 22% adenine. (Supplement)

1. (a)State the percentage of thymine in the sample. Give a reason.[2]
2. (b)Calculate the percentage of guanine. Show your working.[2]
3. (c)One strand of this DNA reads A–G–T–C–C–A. Write the sequence of the complementary strand.[1]
4. (d)Name the shape formed by the two coiled strands.[1]

100 g of a breakfast cereal contains 68 g carbohydrate, 12 g protein and 9 g fat.

1. (a)Name the chemical elements present in each of the three molecules listed.[1]
2. (b)Calculate the mass of the cereal that is none of these three molecules. Give your answer as a percentage of 100 g.[2]
3. (c)A test on the cereal with iodine solution gives a blue-black colour. Name the large molecule this reveals.[1]
4. (d)The label says “no added sugar”, yet a heated Benedict’s test on the cereal gives a small positive result. Explain why.[2]

One strand of a DNA molecule has the base sequence A–C–C–T–G–A. (Supplement)

1. (a)State the name of the shape formed by the two strands of a DNA molecule.[1]
2. (b)Write the base sequence of the complementary strand.[2]
3. (c)In a sample of DNA, 18% of the bases are adenine. Explain what percentage must be thymine.[2]
4. (d)State what holds the two strands together.[1]

A cook prepares pineapple in a jelly dessert. Fresh pineapple contains an enzyme that digests gelatine protein, so the jelly stays liquid. Canned pineapple is heated before canning, and jelly made with it sets normally.

1. (a)Explain why fresh pineapple stops the jelly from setting, using the terms enzyme, substrate and active site.[3]
2. (b)Explain why canned pineapple does not have this effect.[2]
3. (c)A student suggests cooking the fresh pineapple at 60 °C for “a bit longer than needed to be safe” would work better than the standard short heating. Explain why excessive heating is unnecessary to stop the enzyme working.[2]

The activity of an enzyme was measured at different temperatures. The results are shown.

1. (a)State the optimum temperature from the data.[1]
2. (b)Describe the trend in activity between 10 °C and 40 °C.[1]
3. (c)Explain the increase in activity up to 40 °C. (Supplement)[2]
4. (d)Explain the fall in activity above 50 °C. (Supplement)[2]
5. (e)A tube of this enzyme is heated to 60 °C and then cooled back to 40 °C. State and explain whether its activity returns to 60 units per minute.[1]

Amylase digests starch but has no effect on proteins such as gelatine.

1. (a)Define the term active site.[1]
2. (b)Explain why amylase digests starch but not gelatine. (Supplement)[2]
3. (c)State what happens to the enzyme after the reaction.[1]
4. (d)Name the products of starch digestion by amylase.[1]
5. (e)Explain the term complementary as used for enzymes and substrates.[1]