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Biology journal Cambridge IGCSE 0610
Contains the answers

Answers · Enzymes

Enzymes — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. CR1 · Return

Model answer. a. Enzymes are proteins that act as biological catalysts in all metabolic reactions.

A full answer includes

  • I chose a.

Also accept

  • a

Watch for

  • “Enzymes are used up in reactions.” — Enzymes are catalysts: they are not changed by the reaction.

2. CR2 · Return

Model answer. The substrate fits into the active site of the enzyme because their shapes are complementary. Together they form an enzyme–substrate complex. The substrate is changed into products. Each enzyme acts only on its own substrate, or a few very similar ones: enzymes are specific.

A full answer includes

  • g1: active site.
  • g2: complementary.
  • g3: complex.
  • g4: products.
  • g5: specific.

Also accept

  • active site; complementary; complex; products; specific

Watch for

  • “Every enzyme fits every substrate.” — Only a substrate of complementary shape fits; that is why enzymes are specific.

3. CR3 · Return

Model answer. Organisms cannot heat themselves enough to make their reactions fast. Without enzymes, reactions such as respiration and digestion would be far too slow to sustain life. Enzymes catalyse them, giving the reaction rate needed to keep the organism alive.

A full answer includes

  • Without enzymes, metabolic reactions such as respiration and digestion would be far too slow at the organism’s temperature.
  • Enzymes speed them up to a rate that keeps the organism alive.

Also accept

  • reactions too slow without enzymes; enzymes give the rate needed for life

Watch for

  • “Enzymes are a store of energy.” — Enzymes speed up reactions; they are not an energy store.

4. TR1 · Return

Model answer. c. Above the optimum temperature the active site changes shape and the enzyme is denatured; cooling it does not undo this.

A full answer includes

  • I chose c.

Also accept

  • c

Watch for

  • “Cold denatures enzymes.” — Cold slows enzymes down but does not change their shape; they work again when warmed.

5. TR2 · Return

Model answer. As the temperature rises towards the optimum, the molecules have more kinetic energy, so there are more effective collisions. Above the optimum the active site changes shape and the enzyme is denatured.

A full answer includes

  • g1: kinetic.
  • g2: effective collisions (collisions).
  • g3: shape.
  • g4: denatured.

Also accept

  • kinetic; effective collisions; shape; denatured

Watch for

  • “Enzymes are killed above the optimum.” — Enzymes are denatured, not killed.

6. PR1 · Return

Model answer. b. The pH at which the enzyme works fastest.

A full answer includes

  • I chose b.

Also accept

  • b

Watch for

  • “The optimum pH is always 7.” — Each enzyme has its own optimum pH.

7. PR2 · Return

Model answer. Away from its optimum pH, the active site of an enzyme changes shape, so the substrate fits less well and fewer enzyme–substrate complexes form. At an extreme pH the enzyme is denatured.

A full answer includes

  • g1: shape.
  • g2: fits.
  • g3: complexes (enzyme–substrate complexes).
  • g4: denatured.

Also accept

  • shape; fits; complexes; denatured

Watch for

  • “A change of pH kills the enzyme.” — Enzymes are not alive; they are denatured.