Practise › Questions › Factors affecting enzyme rate: temperature, pH and concentration

Factors affecting enzyme rate: temperature, pH and concentration questions

Initial rate and why the rate of an enzyme-catalysed reaction falls during a run, the effect of temperature including Q10 and denaturation, the effect of pH, saturation with substrate, the effect of enzyme concentration, and how each curve should be drawn and interpreted.

6 original questions · 19 marks · the factors affecting enzyme rate: temperature, ph and concentration notes · Enzymes and metabolic control

Every question here is written for this library rather than taken from a past paper. Write your answer out before opening a mark scheme: the schemes award marks point by point, and the points are much easier to see when you have something of your own to compare against.

• 4 marks · Explain

  Explain why the rate of an enzyme-catalysed reaction climbs gradually as the temperature is raised towards the optimum, and then falls steeply above it.

  Mark scheme

  1. B1 raising the temperature gives the molecules more kinetic energy, so substrate and active site collide more often and with more energy
  2. B1 more of those collisions are successful, so more enzyme-substrate complexes form and the rate roughly doubles for each 10 °C rise
  3. B1 above the optimum the increased vibration breaks the hydrogen bonds and ionic bonds holding the tertiary structure, so the enzyme denatures
  4. B1 the active site is then no longer complementary to the substrate, and denaturation proceeds far faster than a doubling per 10 °C, which is why the fall is steeper than the climb

• 4 marks · Suggest

  A student collects the oxygen released by catalase every thirty seconds for ten minutes and finds that the rate falls steadily throughout the run. Suggest three reasons for the fall, and suggest what she should measure instead if she wants to compare different temperatures fairly.

  Mark scheme

  1. B1 substrate is being used up, so collisions between hydrogen peroxide and active sites become less frequent
  2. B1 product accumulates, and for many enzymes it competes for the active site or binds elsewhere and slows the reaction
  3. B1 a fraction of the enzyme denatures over a long warm run
  4. B1 she should compare initial rates, taken from the gradient of the tangent at time zero or from 1/time to a fixed early end point

• 3 marks · Explain

  Explain how a change in pH away from the optimum reduces the rate of an enzyme-catalysed reaction.

  Mark scheme

  1. B1 a change in hydrogen ion concentration protonates or deprotonates the side chains, altering the charge they carry
  2. B1 ionic bonds between those side chains fail and hydrogen bonds are disrupted, so the tertiary structure shifts
  3. B1 the active site is a product of that structure, so its shape changes and the substrate is no longer complementary to it

• 3 marks · Calculate

  An enzyme has an initial rate of 0.8 µmol per minute at 10 °C and 1.8 µmol per minute at 20 °C. Calculate the Q10 for this enzyme, and calculate the rate you would predict for it at 30 °C.

• 3 marks · Describe

  Describe the shape of a graph of initial rate against substrate concentration, and describe what is limiting the rate in each part of that graph.

• 2 marks · Give

  Give the optimum pH of pepsin and the optimum pH of trypsin, naming where in the gut each of these enzymes works.

Practise factors affecting enzyme rate: temperature, ph and concentration one question at a time

The player marks nothing for you. It shows one question, waits, then shows the scheme so you can mark yourself, and brings a question back sooner when it went badly.