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O Level Biology: Enzymes MCQs — Biology

18 multiple-choice questions on O 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. In the amylase–starch investigation, the end point is reached when

  1. Benedict's turns brick-red
  2. bubbles stop forming
  3. iodine turns blue-black
  4. iodine no longer turns blue-black

Answer: D) iodine no longer turns blue-black

Explanation: Starch has been broken down when iodine stays orange-brown. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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2. The substance an enzyme acts on is called the

  1. active site
  2. catalyst
  3. substrate
  4. product

Answer: C) substrate

Explanation: The enzyme acts on its substrate. O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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3. The optimum temperature for most human enzymes is about

  1. 37 °C
  2. 60 °C
  3. 0 °C
  4. 100 °C

Answer: A) 37 °C

Explanation: Human body temperature is about 37 °C. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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4. A catalyst

  1. is used up in the reaction
  2. speeds up a reaction and is not changed by it
  3. is always a carbohydrate
  4. slows down a reaction

Answer: B) speeds up a reaction and is not changed by it

Explanation: Definition of a catalyst. O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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5. Above the optimum temperature an enzyme's activity falls because

  1. the enzyme is killed
  2. there is too much kinetic energy for collisions
  3. the active site changes shape and the enzyme is denatured
  4. the substrate is denatured

Answer: C) the active site changes shape and the enzyme is denatured

Explanation: Denaturation changes the active site shape. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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6. Catalase activity can be measured by

  1. counting cells
  2. collecting the oxygen produced
  3. testing for starch
  4. using Benedict's solution

Answer: B) collecting the oxygen produced

Explanation: Oxygen is a product of catalase. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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7. Which should be kept constant when investigating the effect of temperature on amylase?

  1. Concentration of starch
  2. Temperature
  3. Time taken for starch to disappear
  4. Rate of reaction

Answer: A) Concentration of starch

Explanation: Only the independent variable (temperature) changes. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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8. Catalase breaks down hydrogen peroxide into

  1. glucose and oxygen
  2. carbon dioxide and water
  3. amino acids
  4. water and oxygen

Answer: D) water and oxygen

Explanation: 2H₂O₂ → 2H₂O + O₂. O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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9. The part of the enzyme where the substrate binds is the

  1. active site
  2. cell membrane
  3. nucleus
  4. product

Answer: A) active site

Explanation: The substrate fits into the active site. O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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10. How can you keep the pH constant in an enzyme experiment?

  1. Add more enzyme
  2. Use a buffer solution
  3. Use a water bath
  4. Add iodine

Answer: B) Use a buffer solution

Explanation: Buffers keep pH constant. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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11. An enzyme cooled from 37 °C to 5 °C and then warmed back to 37 °C will

  1. work faster than before
  2. work again normally
  3. stay denatured
  4. be destroyed

Answer: B) work again normally

Explanation: Cold inactivates but does not denature enzymes. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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12. Pepsin works best in the stomach because its optimum pH is about

  1. 2
  2. 9
  3. 7
  4. 12

Answer: A) 2

Explanation: Stomach acid gives pH about 2. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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13. At a low temperature an enzyme works slowly because

  1. there is no substrate
  2. there are fewer effective collisions due to low kinetic energy
  3. the active site is destroyed
  4. it is denatured

Answer: B) there are fewer effective collisions due to low kinetic energy

Explanation: Low kinetic energy means fewer collisions. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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14. A graph of enzyme activity against pH is a curve with a peak. The peak shows the

  1. denaturing pH
  2. optimum pH
  3. substrate concentration
  4. active site

Answer: B) optimum pH

Explanation: Fastest rate = optimum pH. O Level Biology · Topic 5 Enzymes · 5.2 Effects of temperature and pH

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15. Amylase breaks down starch but not protein. This is because

  1. protein does not fit amylase's active site
  2. amylase is denatured by protein
  3. protein is too small
  4. protein is not a food

Answer: A) protein does not fit amylase's active site

Explanation: Enzymes are specific to their substrate's shape. O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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16. What forms when a substrate binds to an enzyme?

  1. A denatured enzyme
  2. An enzyme–substrate complex
  3. A new enzyme
  4. A product only

Answer: B) An enzyme–substrate complex

Explanation: The enzyme–substrate complex forms before products are released. O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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17. An enzyme is best described as

  1. a hormone carried in the blood
  2. a carbohydrate that speeds up reactions
  3. a lipid that is used up in reactions
  4. a protein that acts as a biological catalyst

Answer: D) a protein that acts as a biological catalyst

Explanation: Enzymes are proteins and biological catalysts. O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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18. Enzymes are specific because

  1. they work at any temperature
  2. they are used up
  3. the active site is complementary in shape to one substrate
  4. they are made of protein

Answer: C) the active site is complementary in shape to one substrate

Explanation: Specificity depends on shape (lock and key). O Level Biology · Topic 5 Enzymes · 5.1 Enzyme action

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