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A Level Biology: Enzymes MCQs — Biology

20 multiple-choice questions on A Level Biology: Enzymes from Biology, each with the correct answer and, where available, a teacher's explanation. Read through them to revise the chapter, then practise the same questions one by one to test yourself.

1. An advantage of immobilised enzymes is that

  1. they never denature
  2. the reaction is always faster
  3. the enzyme can be reused and does not contaminate the product
  4. they need no substrate

Answer: C) the enzyme can be reused and does not contaminate the product

Explanation: Beads are easily separated and reused. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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2. A competitive inhibitor

  1. permanently denatures the enzyme
  2. increases Vmax
  3. binds to the active site and its effect is reduced by more substrate
  4. binds elsewhere and changes the active site

Answer: C) binds to the active site and its effect is reduced by more substrate

Explanation: Competitive inhibitors compete for the active site. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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3. pH changes affect enzymes mainly by

  1. adding phosphate groups
  2. altering ionic and hydrogen bonds in the tertiary structure
  3. changing the primary structure
  4. breaking peptide bonds

Answer: B) altering ionic and hydrogen bonds in the tertiary structure

Explanation: H⁺ ions affect charges on R groups. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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4. With excess substrate, doubling the enzyme concentration will

  1. denature the enzyme
  2. not change the rate
  3. halve the rate
  4. double the rate

Answer: D) double the rate

Explanation: More active sites, more reactions. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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5. Enzymes are immobilised in alginate beads by dripping a mixture of enzyme and sodium alginate into

  1. hydrochloric acid
  2. calcium chloride solution
  3. distilled water
  4. sodium hydroxide

Answer: B) calcium chloride solution

Explanation: Calcium ions set the alginate. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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6. In the amylase investigation, the end point is when samples

  1. no longer turn blue-black with iodine
  2. turn brick-red
  3. turn blue-black
  4. become cloudy

Answer: A) no longer turn blue-black with iodine

Explanation: Starch has been hydrolysed. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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7. The progress of a catalase reaction can be followed by measuring

  1. the colour with Benedict's
  2. the disappearance of starch
  3. the formation of glucose
  4. the volume of oxygen produced

Answer: D) the volume of oxygen produced

Explanation: Catalase produces oxygen. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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8. Increasing temperature above the optimum reduces rate because

  1. bonds holding the tertiary structure break, changing the active site
  2. molecules move more slowly
  3. substrate is used up
  4. the enzyme becomes a substrate

Answer: A) bonds holding the tertiary structure break, changing the active site

Explanation: Denaturation. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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9. Buffers are used in enzyme experiments to

  1. keep pH constant
  2. provide substrate
  3. keep temperature constant
  4. inhibit the enzyme

Answer: A) keep pH constant

Explanation: Buffers resist pH changes. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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10. The induced-fit hypothesis states that

  1. the active site changes shape slightly when the substrate binds
  2. enzymes act on many substrates
  3. the substrate changes the enzyme permanently
  4. the active site is rigid

Answer: A) the active site changes shape slightly when the substrate binds

Explanation: The active site moulds around the substrate. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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11. Km is the substrate concentration at which

  1. the rate is Vmax
  2. the enzyme denatures
  3. the rate is half of Vmax
  4. the rate is zero

Answer: C) the rate is half of Vmax

Explanation: Definition of the Michaelis–Menten constant. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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12. Enzymes increase the rate of reactions by

  1. raising the temperature
  2. lowering the activation energy
  3. supplying ATP
  4. increasing the energy of products

Answer: B) lowering the activation energy

Explanation: Enzymes lower activation energy. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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13. A colorimeter measures

  1. the volume of gas
  2. the pH of a solution
  3. the absorbance or transmission of light through a solution
  4. the mass of a precipitate

Answer: C) the absorbance or transmission of light through a solution

Explanation: It measures colour intensity. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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14. Enzymes are globular proteins because they

  1. are insoluble fibres
  2. are soluble and have a specific 3D shape
  3. have no tertiary structure
  4. are carbohydrates

Answer: B) are soluble and have a specific 3D shape

Explanation: The tertiary structure forms the active site. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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15. An enzyme that acts outside the cell that produced it is

  1. extracellular
  2. allosteric
  3. immobilised
  4. intracellular

Answer: A) extracellular

Explanation: Extracellular enzymes are secreted. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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16. A competitive inhibitor affects Vmax and Km as follows:

  1. both increased
  2. both decreased
  3. Vmax decreased, Km unchanged
  4. Vmax unchanged, Km increased

Answer: D) Vmax unchanged, Km increased

Explanation: More substrate overcomes it but more is needed for ½Vmax. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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17. An enzyme with a low Km has

  1. a low Vmax
  2. no active site
  3. a high affinity for its substrate
  4. a low affinity for its substrate

Answer: C) a high affinity for its substrate

Explanation: It reaches half Vmax at a low substrate concentration. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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18. Why is the initial rate used to compare enzyme reactions?

  1. Product inhibits the enzyme only at the start
  2. Enzymes denature at the start
  3. The rate is fastest when substrate concentration is highest at the start
  4. The colorimeter only works at time zero

Answer: C) The rate is fastest when substrate concentration is highest at the start

Explanation: Substrate is used up later, slowing rate. AS & A Level Biology · Topic 3 Enzymes · 3.1 Mode of action of enzymes

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19. A non-competitive inhibitor

  1. reduces Vmax and is not overcome by more substrate
  2. increases Km only
  3. binds to the active site
  4. has no effect at high substrate concentrations

Answer: A) reduces Vmax and is not overcome by more substrate

Explanation: It changes the shape of the active site. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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20. At high substrate concentrations, increasing substrate further has no effect on rate because

  1. all active sites are occupied
  2. the enzyme is denatured
  3. pH has changed
  4. the substrate is inhibited

Answer: A) all active sites are occupied

Explanation: The enzyme is saturated. AS & A Level Biology · Topic 3 Enzymes · 3.2 Factors that affect enzyme action

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