Required practical 1

Question 1

Investigation into the effect of a named variable on the rate of an enzyme-controlled reaction.

Give examples of variables that could affect the rate of an enzyme controlled reaction

Answer 1

● Enzyme concentration / volume
● Substrate concentration / volume
● Temperature of solution
● pH of solution
● Inhibitor concentration

Any one of these can be the independent variable
and need to be varied (eg. by preparing a dilution
series of varying concentrations). All others (except
inhibitors) would be control variables so would
need to be kept constant.

Question 2

Describe how temperature can be
controlled. (2)

Answer 2

● Use a thermostatically controlled water bath
● Monitor using a thermometer at regular intervals
and add hot / cold water if temperature fluctuates

Question 3

Describe how pH can be controlled. (2)

Answer 3

● Use a buffer solution
● Monitor using a pH meter at regular intervals

Question 4

Why were enzyme & substrate solutions left in
water baths for 10 mins before mixing? (1)

Answer 4

● So solutions equilibrate / reach the temperature of
the water bath

Question 5

Describe a control experiment. (2)

Answer 5

● Use denatured enzymes (eg. by boiling)
● Everything else same as experiment, eg. same conc.
/ volume of substrate (at start) and enzyme, same
type / volume of buffer solution, same temperature

Question 6

Describe how the rate of an enzyme-controlled reaction can be measured

Answer 6

● Measure time taken for reaction to reach a set point,
eg. concentration / volume / mass / colour of
substrate or product
○ Rate of reaction = 1 / time; example units = s
-1
● Measure concentration / volume / mass / colour of
substrate or product at regular intervals (or using a
continuous data logger) throughout reaction
○ Plot on a graph with time on the x axis and
whatever is being measured on the y axis
○ Draw a tangent at t = 0 (or any other time for
rate at a particular point)
○ Initial rate of reaction = change in y / change in
x; example units = cm3 s
-1

Question 7

Suggest a safety risk and explain
how to reduce this risk. (2)

Answer 7

● Handling enzymes may cause an allergic reaction
● Avoid contact with skin by wearing gloves and eye protection

Question 8

Explain why using a colorimeter to
measure colour change is better
than comparison to colour
standards. (2)

Answer 8

Not subjective
● More accurate

Question 9

Explain a procedure that could be
used to stop each reaction. (1)

Answer 9

● Boil / add strong acid / alkali → denature enzyme
● Put in ice → lower kinetic energy so no enzyme-substrate
complexes form
● Add high concentration of inhibitor → no enzyme-substrate
complexes form

Question 10

Describe how processed data can be presented as a graph

Answer 10

1. Label x-axis with ‘[named independent variable] / [units]’ and y-axis with ‘rate of reaction / [units]’
2. Plot a linear scale on each axis that will allow the graph to occupy over half available space
3. Mark data points with crosses
4. Join data points by straight lines if intermediate values are not known
   OR draw a line / curve of best fit if there is a clear trend

Question 11

Explain why the rate of reaction decreases over time throughout each
experiment

Answer 11

● Initial rate is highest as substrate concentration not limiting / many
enzyme-substrate complexes form
● Reaction slows as substrate used up and often stops as there is no substrate left