Enzymes — final check key

1. CK1 · Final check

Model answer. Sucrose is the substrate. Its shape is complementary to the active site of sucrase, so it fits into the active site and forms an enzyme–substrate complex. While it is held there, the sucrose is broken down into glucose and fructose, the products. The products leave the active site, and the sucrase, unchanged, can bind another sucrose molecule.

A full answer includes

• Sucrose (the substrate) has a shape complementary to the active site of sucrase.
• Sucrose fits into the active site, forming an enzyme–substrate complex.
• Sucrose is broken down into glucose and fructose (the products), which leave the active site.
• Sucrase is unchanged and can be used again.

Also accept

• substrate fits complementary active site; complex forms; products glucose and fructose leave; enzyme unchanged

Watch for

• “The sucrase turns into glucose and fructose.” — The substrate, sucrose, becomes the products; the enzyme is unchanged.

2. CK2 · Final check

Model answer. (a) A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction. (b) At the temperatures organisms live at, metabolic reactions such as digestion and respiration would happen far too slowly without catalysts. Enzymes speed them up to the rate needed to keep the organism alive.

A full answer includes

• (a) A substance that increases the rate of a chemical reaction and is not changed by the reaction.
• (b) Without enzymes the reactions of metabolism (digestion, respiration, growth) would be far too slow at body temperature to sustain life; enzymes make them fast enough.

Also accept

• (a) speeds up a reaction, unchanged; (b) reactions too slow without them to sustain life

Watch for

• “Enzymes give organisms energy.” — Enzymes speed up reactions such as respiration; they are not a source of energy themselves.

3. CK3 · Final check

Model answer. Enzymes are specific. The active site of the protease has a shape complementary to its substrate, protein, so protein fits and forms an enzyme–substrate complex and is broken down. Starch has a different shape, does not fit the active site, forms no complex and is not broken down.

A full answer includes

• Enzymes are specific.
• The active site of the protease has a shape complementary to protein, which fits and forms an enzyme–substrate complex.
• Starch has a different shape, so it does not fit the active site and is not broken down.

Also accept

• specific; active site complementary to protein; starch does not fit

Watch for

• “Starch is too hard for a protease.” — The reason is shape: starch does not fit the protease’s active site.

4. CK4 · Final check

Model answer. Enzymes are proteins. They act as biological catalysts, speeding up reactions without being used up, and they are involved in all the metabolic reactions of an organism, both building up and breaking down substances.

A full answer includes

• Enzymes are proteins.
• They act as biological catalysts: they speed up the reactions without being changed.
• They are involved in all metabolic reactions (building up and breaking down substances).

Also accept

• proteins; biological catalysts; all metabolic reactions

Watch for

• “Enzymes are only found in the gut.” — Enzymes are involved in all metabolic reactions, in every cell.

5. TK1 · Final check

Model answer. (a) Relative rate = 1000 ÷ 120 = 8.3. (b) 35 °C, which had the shortest time. (c) As the temperature rises from 15 °C to 35 °C the rate increases; it is highest at 35 °C, the best tested temperature, near the optimum. Above it the rate falls. At 60 °C there is no end point within 5 minutes: the rate is below 1000 ÷ 300 = 3.3 and too low to measure, most likely because the amylase is denatured.

Checked automatically

• (a) Relative rate at 25 °C (no unit): 8.3 (2 significant figures; within ±0.001)
  • Common wrong answer 0.12 : That is 120 ÷ 1000. Divide 1000 by 120.
  • Common wrong answer 8.33 : Round to one decimal place: 8.3.
• (b) Best tested temperature: 35 °C (within ±0.1 °C)

A full answer includes

• (a) 1000 ÷ 120 = 8.3.
• (b) 35 °C.
• (c) The rate increases from 15 °C to a peak near 35 °C (the optimum), then decreases; at 60 °C there is no end point within 5 minutes, so the rate is too low to measure there (most likely the amylase is denatured).

Also accept

• (a) 8.3; (b) 35 °C; (c) rises to an optimum, then falls; no end point at 60 °C (rate too low to measure)

Watch for

• “The best temperature is the highest one tested.” — The best temperature is where the time is shortest, here 35 °C.
• “No end point means the rate is zero.” — It means the end point was not reached in the time you watched: the rate is too low to measure, not necessarily zero.

6. TK2 · Final check

Model answer. Above the optimum the enzyme molecules vibrate so much that bonds holding the protein in shape break, and the active site changes shape. The substrate fits less well or no longer fits, so fewer effective collisions produce enzyme–substrate complexes and the rate falls. An enzyme whose active site has lost its shape for good is denatured.

A full answer includes

• Above the optimum, bonds holding the enzyme’s shape break and the active site changes shape.
• The substrate fits less well or no longer fits.
• Fewer enzyme–substrate complexes form, so the rate falls.
• When the shape is lost for good, the enzyme is denatured.

Also accept

• active site changes shape; substrate no longer fits; fewer complexes; denatured

Watch for

• “The enzyme is used up faster when it is hot.” — Enzymes are not used up; their active site changes shape.

7. TK3 · Final check

Model answer. Take samples more often, for example every 10 seconds, so the end-point time is known more precisely (now it lies anywhere between 30 s and 60 s). Repeat the run at 40 °C at least three times and calculate a mean, so one odd result has less effect and the result is more reliable. (Testing 35 °C and 45 °C would help to find the optimum, but it does not improve the result at 40 °C.)

A full answer includes

• Sample more often (for example every 10 seconds), so the end point is found more precisely.
• Repeat the run at 40 °C (at least three times) and calculate a mean, so the result is more reliable.

Also accept

• sample more often; repeat at 40 °C and take a mean

Watch for

• “Use a bigger tile.” — The size of the tile does not change how precisely the time is measured; the time between samples does.
• “Test more temperatures.” — That helps to find the optimum, but it does not make the 40 °C result more precise or more reliable.

8. PK1 · Final check

Model answer. The stomach protease has its optimum near pH 2, where its active site has the shape complementary to protein: protein fits and many enzyme–substrate complexes form, so the rate is high. At pH 7, far from its optimum, the bonds holding its shape change, the active site changes shape and the enzyme is denatured: protein no longer fits, almost no complexes form and the rate is very low.

A full answer includes

• pH 2 is the optimum of this protease: its active site has the shape that fits protein, so many enzyme–substrate complexes form.
• At pH 7, far from its optimum, bonds holding its shape change and the active site changes shape.
• Protein no longer fits (the enzyme is denatured), so almost no complexes form.

Also accept

• optimum pH 2: active site fits; pH 7: shape changed, denatured, no fit

Watch for

• “Proteases only work in acid because acid digests protein by itself.” — The enzyme’s active site has the right shape only near its optimum pH.

9. PK2 · Final check

Model answer. (a) Relative rate = 1000 ÷ 90 = 11.1. (b) pH 7, which had the shortest time to the end point.

Checked automatically

• (a) Relative rate at pH 6 (no unit): 11.1 (3 significant figures; within ±0.001)
  • Common wrong answer 0.09 : That is 90 ÷ 1000. Divide 1000 by 90.
  • Common wrong answer 11.11 : Round to one decimal place: 11.1.
• (b) Best tested pH: 7 (within ±0.1)
  • Common wrong answer 5 : At pH 5 the time was longest, so the rate was lowest.

A full answer includes

• (a) 1000 ÷ 90 = 11.1.
• (b) pH 7.

Also accept

• (a) 11.1; (b) pH 7

Watch for

• “The best pH is the one with the longest time.” — The longest time means the slowest reaction.