required practical 1: investigation into the effect of a named variable on the rate of an enzyme-controlled reaction

Question 1

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

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 to denature enzyme
-put in ice to lower kinetic energy so no enzyme-substrate complexes form
-add high concentration of inhibitor so there is no enzyme-substrate complexes formed

Question 10

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

Answer 10

-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