Rate Calculations for Enzyme Activity (Edexcel GCSE Biology: Combined Science)

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Rate Calculations for Enzyme Activity

  • Rate calculations are important in determining how fast an enzyme is working (i.e. the rate of reaction)
  • To perform a rate calculation, use the following formula:

rate = change ÷ time

    • Change = the change in the substance being measured
      • E.g. the amount of substrate used up in the reaction or the amount of product formed
    • Time = the time taken for that change to occur

Worked example

Amylase catalyses the breakdown of starch into maltose. 15 grams of starch were added to a solution containing amylase. It took 2 hours for all the starch to be broken down.

Calculate the rate of reaction.

Step 1: write out the equation for calculating the rate of enzyme activity

rate = change ÷ time

In this case: rate = amount of substrate used ÷ time

Step 2: substitute in the known values and calculate the rate

rate = 15 g ÷ 2 hours

rate = 7.5 g / hr or 7.5 g hr⁻¹

Worked example

The enzyme catalase catalyses the breakdown of hydrogen peroxide into water and oxygen. In one experiment, a student found that 45 cm³ of oxygen was released in 5 minutes.

Calculate the rate of reaction.

Step 1: write out the equation for calculating the rate of enzyme activity

rate = change ÷ time

In this case: rate = amount of product formed ÷ time

Step 2: substitute in the known values and calculate the rate

rate = 45 cm³ ÷ 5 minutes

rate = 9 cm³ / min or 9 cm³ min⁻¹

  • In some situations you may not be told how much something has changed during a reaction; instead, you may only be told the time taken for the reaction to occur
  • In this case you can still calculate the rate of reaction by using the following (slightly different) formula:

rate = 1 ÷ time

Worked example

A student adds a set volume of starch solution to a set volume of amylase solution at a range of different pH values. At each pH, the student times how long it takes for the amylase to break down all of the starch. At pH 6 the time taken for amylase to break down all of the starch was 50 seconds.

Calculate the rate of reaction at pH 6.

Step 1: write out the equation for calculating the rate of enzyme activity

rate = 1 ÷ time

Step 2: substitute in the known values and calculate the rate

rate = 1 ÷ 50 seconds

rate = 0.02 s⁻¹

The units for the calculation above are in s⁻¹ because rate is given per unit time.

Examiner Tip

In an exam you could be asked to plot the reaction rates (from an enzyme catalysed reaction) on a graph. However, using the equation 'rate = 1 ÷ time' often gives small numbers that are difficult to plot on a graph. In these cases, you can also use the equation:

rate = 1000 ÷ time

This equation give you bigger numbers that are easier to plot on a graph. So, for the calculation in the worked example above, you would get:

rate = 1000 ÷ 50 seconds

rate = 20 s⁻¹

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Alistair

Author: Alistair

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.