These eight questions go from enzyme action to a planned experiment. Write each answer on paper before you open the explanation.
The lessons are in metabolism and enzymes. Note your slips in the mistake log and paper-error review.
Questions
1. Name the part of the enzyme where the substrate binds and state why only one substrate fits.
Answer
The active site. Its shape is complementary to the substrate, so only a molecule with the matching shape can bind.
2. State what happens to an enzyme after it has converted a substrate into products.
Answer
The enzyme is released unchanged. It can bind another substrate molecule, so it is reusable.
3. The rate of an enzyme reaction is 6 units at 20 °C and 12 units at 30 °C. Explain the increase.
Answer
At a higher temperature the particles have more kinetic energy and move faster. Enzyme and substrate collide more often, so more enzyme-substrate complexes form each second and the rate rises by 6 units.
4. Explain why the rate of the same enzyme is 1 unit at 60 °C.
Answer
At 60 °C the heat breaks bonds that hold the enzyme’s shape. The active site changes, so the substrate no longer fits and the enzyme is denatured. The rate falls to a very low value.
5. An enzyme has rates 2, 9 and 3 units at pH 4, 6 and 8. State the optimum pH and explain the lower values.
Answer
The optimum pH is 6, where the rate is highest (9 units). At pH 4 and pH 8 the shape of the active site is altered, so the substrate fits less well and fewer reactions occur.
6. With excess substrate, a reaction has a rate of 5 units. The enzyme concentration is doubled and the rate becomes 10 units. Name the limiting factor before the change.
Answer
Enzyme concentration. Substrate was in excess, so every active site was busy, and doubling the enzyme doubled the active sites, which doubled the rate.
7. A graph of rate against substrate concentration becomes flat above 6 units of substrate. Explain the flat section.
Answer
All active sites are occupied by substrate, so substrate is no longer limiting. Enzyme concentration now sets the maximum rate, and extra substrate molecules must wait for a free active site.
8. Plan a fair test of the effect of pH on the rate at which amylase breaks down starch. Name the manipulated, responding and two fixed variables, and say how you would make the results reliable.
Answer
Manipulated variable: pH, using buffer solutions at pH 4, 6, 8 and 10.
Responding variable: the time taken for starch to disappear, tested with iodine solution at regular intervals.
Fixed variables (any two): temperature, volume and concentration of starch, volume and concentration of amylase.
Reliability: repeat each pH three times and calculate a mean time. Do not delete an unusual result automatically. Check it by repeating the trial.
If you got these wrong
- Questions 1 and 2: read explaining enzyme action.
- Questions 3, 4 and 5: read interpreting temperature and pH graphs.
- Questions 6 and 7: read distinguishing limiting factors.
- Question 8: read planning controlled enzyme comparisons.
For data with odd readings, continue to interpreting enzymes and experimental rates. If you want a teacher to read your answers and sharpen the wording, see online one-to-one Biology tuition or the one-hour trial class (from RM50).